loopback/lib/persisted-model.js

/*!
 * Module Dependencies.
 */

var runtime = require('./runtime');
var assert = require('assert');
var async = require('async');
var deprecated = require('depd')('loopback');
var debug = require('debug')('loopback:persisted-model');
var PassThrough = require('stream').PassThrough;
var utils = require('./utils');

module.exports = function(registry) {
  var Model = registry.getModel('Model');

  /**
   * Extends Model with basic query and CRUD support.
   *
   * **Change Event**
   *
   * Listen for model changes using the `change` event.
   *
   * ```js
   * MyPersistedModel.on('changed', function(obj) {
   *    console.log(obj) // => the changed model
   * });
   * ```
   *
   * @class PersistedModel
   */

  var PersistedModel = Model.extend('PersistedModel');

  /*!
   * Setup the `PersistedModel` constructor.
   */

  PersistedModel.setup = function setupPersistedModel() {
    // call Model.setup first
    Model.setup.call(this);

    var PersistedModel = this;

    // enable change tracking (usually for replication)
    if (this.settings.trackChanges) {
      PersistedModel._defineChangeModel();
      PersistedModel.once('dataSourceAttached', function() {
        PersistedModel.enableChangeTracking();
      });
    } else if (this.settings.enableRemoteReplication) {
      PersistedModel._defineChangeModel();
    }

    PersistedModel.setupRemoting();
  };

  /*!
   * Throw an error telling the user that the method is not available and why.
   */

  function throwNotAttached(modelName, methodName) {
    throw new Error(
      'Cannot call ' + modelName + '.' + methodName + '().' +
      ' The ' + methodName + ' method has not been setup.' +
      ' The PersistedModel has not been correctly attached to a DataSource!'
    );
  }

  /*!
   * Convert null callbacks to 404 error objects.
   * @param  {HttpContext} ctx
   * @param  {Function} cb
   */

  function convertNullToNotFoundError(ctx, cb) {
    if (ctx.result !== null) return cb();

    var modelName = ctx.method.sharedClass.name;
    var id = ctx.getArgByName('id');
    var msg = 'Unknown "' + modelName + '" id "' + id + '".';
    var error = new Error(msg);
    error.statusCode = error.status = 404;
    error.code = 'MODEL_NOT_FOUND';
    cb(error);
  }

  /**
   * Create new instance of Model, and save to database.
   *
   * @param {Object}|[{Object}] data Optional data argument.  Can be either a single model instance or an array of instances.
   *
   * @callback {Function} callback Callback function called with `cb(err, obj)` signature.
   * @param {Error} err Error object; see [Error object](http://docs.strongloop.com/display/LB/Error+object).
   * @param {Object} models Model instances or null.
   */

  PersistedModel.create = function(data, callback) {
    throwNotAttached(this.modelName, 'create');
  };

  /**
   * Update or insert a model instance
   * @param {Object} data The model instance data to insert.
   * @callback {Function} callback Callback function called with `cb(err, obj)` signature.
   * @param {Error} err Error object; see [Error object](http://docs.strongloop.com/display/LB/Error+object).
   * @param {Object} model Updated model instance.
   */

  PersistedModel.upsert = PersistedModel.updateOrCreate = function upsert(data, callback) {
    throwNotAttached(this.modelName, 'upsert');
  };

  /**
   * Find one record matching the optional `where` filter. The same as `find`, but limited to one object.
   * Returns an object, not collection.
   * If not found, create the object using data provided as second argument.
   *
   * @param {Object} where Where clause, such as `{test: 'me'}`
   * <br/>see [Where filter](http://docs.strongloop.com/display/public/LB/Where+filter).
   * @param {Object} data Data to insert if object matching the `where` filter is not found.
   * @callback {Function} callback Callback function called with `cb(err, instance)` arguments.  Required.
   * @param {Error} err Error object; see [Error object](http://docs.strongloop.com/display/LB/Error+object).
   * @param {Object} instance Model instance matching the `where` filter, if found.
   */

  PersistedModel.findOrCreate = function findOrCreate(query, data, callback) {
    throwNotAttached(this.modelName, 'findOrCreate');
  };

  PersistedModel.findOrCreate._delegate = true;

  /**
   * Check whether a model instance exists in database.
   *
   * @param {id} id Identifier of object (primary key value).
   *
   * @callback {Function} callback Callback function called with `(err, exists)` arguments.  Required.
   * @param {Error} err Error object; see [Error object](http://docs.strongloop.com/display/LB/Error+object).
   * @param {Boolean} exists True if the instance with the specified ID exists; false otherwise.
   */

  PersistedModel.exists = function exists(id, cb) {
    throwNotAttached(this.modelName, 'exists');
  };

  /**
   * Find object by ID with an optional filter for include/fields.
   *
   * @param {*} id Primary key value
   * @options {Object} [filter] Optional Filter JSON object; see below.
   * @property {String|Object|Array} fields Identify fields to include in return result.
   * <br/>See [Fields filter](http://docs.strongloop.com/display/LB/Fields+filter).
   * @property {String|Object|Array} include  See PersistedModel.include documentation.
   * <br/>See [Include filter](http://docs.strongloop.com/display/LB/Include+filter).
   * @callback {Function} callback Callback function called with `(err, instance)` arguments.  Required.
   * @param {Error} err Error object; see [Error object](http://docs.strongloop.com/display/LB/Error+object).
   * @param {Object} instance Model instance matching the specified ID or null if no instance matches.
   */

  PersistedModel.findById = function find(id, filter, cb) {
    throwNotAttached(this.modelName, 'findById');
  };

  /**
   * Find all model instances that match `filter` specification.
   * See [Querying models](http://docs.strongloop.com/display/LB/Querying+models).
   *
   * @options {Object} [filter] Optional Filter JSON object; see below.
   * @property {String|Object|Array} fields Identify fields to include in return result.
   * <br/>See [Fields filter](http://docs.strongloop.com/display/LB/Fields+filter).
   * @property {String|Object|Array} include  See PersistedModel.include documentation.
   * <br/>See [Include filter](http://docs.strongloop.com/display/LB/Include+filter).
   * @property {Number} limit Maximum number of instances to return.
   * <br/>See [Limit filter](http://docs.strongloop.com/display/LB/Limit+filter).
   * @property {String} order Sort order: either "ASC" for ascending or "DESC" for descending.
   * <br/>See [Order filter](http://docs.strongloop.com/display/LB/Order+filter).
   * @property {Number} skip Number of results to skip.
   * <br/>See [Skip filter](http://docs.strongloop.com/display/LB/Skip+filter).
   * @property {Object} where Where clause, like
   * ```
   * { where: { key: val, key2: {gt: 'val2'}, ...} }
   * ```
   * <br/>See [Where filter](http://docs.strongloop.com/display/LB/Where+filter).
   *
   * @callback {Function} callback Callback function called with `(err, returned-instances)` arguments.    Required.
   * @param {Error} err Error object; see [Error object](http://docs.strongloop.com/display/LB/Error+object).
   * @param {Array} models Model instances matching the filter, or null if none found.
   */

  PersistedModel.find = function find(filter, cb) {
    throwNotAttached(this.modelName, 'find');
  };

  /**
   * Find one model instance that matches `filter` specification.
   * Same as `find`, but limited to one result;
   * Returns object, not collection.
   *
   * @options {Object} [filter] Optional Filter JSON object; see below.
   * @property {String|Object|Array} fields Identify fields to include in return result.
   * <br/>See [Fields filter](http://docs.strongloop.com/display/LB/Fields+filter).
   * @property {String|Object|Array} include  See PersistedModel.include documentation.
   * <br/>See [Include filter](http://docs.strongloop.com/display/LB/Include+filter).
   * @property {String} order Sort order: either "ASC" for ascending or "DESC" for descending.
   * <br/>See [Order filter](http://docs.strongloop.com/display/LB/Order+filter).
   * @property {Number} skip Number of results to skip.
   * <br/>See [Skip filter](http://docs.strongloop.com/display/LB/Skip+filter).
   * @property {Object} where Where clause, like
   * ```
   * {where: { key: val, key2: {gt: 'val2'}, ...} }
   * ```
   * <br/>See [Where filter](http://docs.strongloop.com/display/LB/Where+filter).
   *
   * @callback {Function} callback Callback function called with `(err, returned-instance)` arguments.  Required.
   * @param {Error} err Error object; see [Error object](http://docs.strongloop.com/display/LB/Error+object).
   * @param {Array} model First model instance that matches the filter or null if none found.
   */

  PersistedModel.findOne = function findOne(filter, cb) {
    throwNotAttached(this.modelName, 'findOne');
  };

  /**
   * Destroy all model instances that match the optional `where` specification.
   *
   * @param {Object} [where] Optional where filter, like:
   * ```
   * {key: val, key2: {gt: 'val2'}, ...}
   * ```
   * <br/>See [Where filter](http://docs.strongloop.com/display/LB/Where+filter).
   *
   * @callback {Function} callback Optional callback function called with `(err, info)` arguments.
   * @param {Error} err Error object; see [Error object](http://docs.strongloop.com/display/LB/Error+object).
   * @param {Object} info Additional information about the command outcome.
   * @param {Number} info.count Number of instances (rows, documents) destroyed.
   */

  PersistedModel.destroyAll = function destroyAll(where, cb) {
    throwNotAttached(this.modelName, 'destroyAll');
  };

  /**
   * Alias for `destroyAll`
   */
  PersistedModel.remove = PersistedModel.destroyAll;

  /**
   * Alias for `destroyAll`
   */
  PersistedModel.deleteAll = PersistedModel.destroyAll;

  /**
   * Update multiple instances that match the where clause.
   *
   * Example:
   *
   *```js
   * Employee.updateAll({managerId: 'x001'}, {managerId: 'x002'}, function(err, info) {
   *     ...
   * });
   * ```
   *
   * @param {Object} [where] Optional `where` filter, like
   * ```
   * { key: val, key2: {gt: 'val2'}, ...}
   * ```
   * <br/>see [Where filter](http://docs.strongloop.com/display/public/LB/Where+filter).
   * @param {Object} data Object containing data to replace matching instances, if any.
   *
   * @callback {Function} callback Callback function called with `(err, info)` arguments.  Required.
   * @param {Error} err Error object; see [Error object](http://docs.strongloop.com/display/LB/Error+object).
   * @param {Object} info Additional information about the command outcome.
   * @param {Number} info.count Number of instances (rows, documents) updated.
   *
   */
  PersistedModel.updateAll = function updateAll(where, data, cb) {
    throwNotAttached(this.modelName, 'updateAll');
  };

  /**
   * Alias for updateAll.
   */
  PersistedModel.update = PersistedModel.updateAll;

  /**
   * Destroy model instance with the specified ID.
   * @param {*} id The ID value of model instance to delete.
   * @callback {Function} callback Callback function called with `(err)` arguments.  Required.
   * @param {Error} err Error object; see [Error object](http://docs.strongloop.com/display/LB/Error+object).
   */
  PersistedModel.destroyById = function deleteById(id, cb) {
    throwNotAttached(this.modelName, 'deleteById');
  };

  /**
   * Alias for destroyById.
   */
  PersistedModel.removeById = PersistedModel.destroyById;

  /**
   * Alias for destroyById.
   */
  PersistedModel.deleteById = PersistedModel.destroyById;

  /**
   * Return the number of records that match the optional "where" filter.
   * @param {Object} [where] Optional where filter, like
   * ```
   * { key: val, key2: {gt: 'val2'}, ...}
   * ```
   * <br/>See [Where filter](http://docs.strongloop.com/display/LB/Where+filter).
   * @callback {Function} callback Callback function called with `(err, count)` arguments.  Required.
   * @param {Error} err Error object; see [Error object](http://docs.strongloop.com/display/LB/Error+object).
   * @param {Number} count Number of instances updated.
   */

  PersistedModel.count = function(where, cb) {
    throwNotAttached(this.modelName, 'count');
  };

  /**
   * Save model instance. If the instance doesn't have an ID, then calls [create](#persistedmodelcreatedata-cb) instead.
   * Triggers: validate, save, update, or create.
   * @options {Object} [options] See below.
   * @property {Boolean} validate Perform validation before saving.  Default is true.
   * @property {Boolean} throws If true, throw a validation error; WARNING: This can crash Node.
   * If false, report the error via callback.  Default is false.
   * @callback {Function} callback Optional callback function called with `(err, obj)` arguments.
   * @param {Error} err Error object; see [Error object](http://docs.strongloop.com/display/LB/Error+object).
   * @param {Object} instance Model instance saved or created.
   */

  PersistedModel.prototype.save = function(options, callback) {
    var Model = this.constructor;

    if (typeof options == 'function') {
      callback = options;
      options = {};
    }

    callback = callback || function() {
    };
    options = options || {};

    if (!('validate' in options)) {
      options.validate = true;
    }
    if (!('throws' in options)) {
      options.throws = false;
    }

    var inst = this;
    var data = inst.toObject(true);
    var id = this.getId();

    if (!id) {
      return Model.create(this, callback);
    }

    // validate first
    if (!options.validate) {
      return save();
    }

    inst.isValid(function(valid) {
      if (valid) {
        save();
      } else {
        var err = new Model.ValidationError(inst);
        // throws option is dangerous for async usage
        if (options.throws) {
          throw err;
        }
        callback(err, inst);
      }
    });

    // then save
    function save() {
      inst.trigger('save', function(saveDone) {
        inst.trigger('update', function(updateDone) {
          Model.upsert(inst, function(err) {
            inst._initProperties(data);
            updateDone.call(inst, function() {
              saveDone.call(inst, function() {
                callback(err, inst);
              });
            });
          });
        }, data);
      }, data);
    }
  };

  /**
   * Determine if the data model is new.
   * @returns {Boolean} Returns true if the data model is new; false otherwise.
   */

  PersistedModel.prototype.isNewRecord = function() {
    throwNotAttached(this.constructor.modelName, 'isNewRecord');
  };

  /**
   * Deletes the model from persistence.
   * Triggers `destroy` hook (async) before and after destroying object.
   * @param {Function} callback Callback function.
   */

  PersistedModel.prototype.destroy = function(cb) {
    throwNotAttached(this.constructor.modelName, 'destroy');
  };

  /**
   * Alias for destroy.
   * @header PersistedModel.remove
   */
  PersistedModel.prototype.remove = PersistedModel.prototype.destroy;

  /**
   * Alias for destroy.
   * @header PersistedModel.delete
   */
  PersistedModel.prototype.delete = PersistedModel.prototype.destroy;

  PersistedModel.prototype.destroy._delegate = true;

  /**
   * Update a single attribute.
   * Equivalent to `updateAttributes({name: 'value'}, cb)`
   *
   * @param {String} name Name of property.
   * @param {Mixed} value Value of property.
   * @callback {Function} callback Callback function called with `(err, instance)` arguments.  Required.
   * @param {Error} err Error object; see [Error object](http://docs.strongloop.com/display/LB/Error+object).
   * @param {Object} instance Updated instance.
   */

  PersistedModel.prototype.updateAttribute = function updateAttribute(name, value, callback) {
    throwNotAttached(this.constructor.modelName, 'updateAttribute');
  };

  /**
   * Update set of attributes.  Performs validation before updating.
   *
   * Triggers: `validation`, `save` and `update` hooks
   * @param {Object} data Dta to update.
   * @callback {Function} callback Callback function called with `(err, instance)` arguments.  Required.
   * @param {Error} err Error object; see [Error object](http://docs.strongloop.com/display/LB/Error+object).
   * @param {Object} instance Updated instance.
   */

  PersistedModel.prototype.updateAttributes = function updateAttributes(data, cb) {
    throwNotAttached(this.modelName, 'updateAttributes');
  };

  /**
   * Reload object from persistence.  Requires `id` member of `object` to be able to call `find`.
   * @callback {Function} callback Callback function called with `(err, instance)` arguments.  Required.
   * @param {Error} err Error object; see [Error object](http://docs.strongloop.com/display/LB/Error+object).
   * @param {Object} instance Model instance.
   */

  PersistedModel.prototype.reload = function reload(callback) {
    throwNotAttached(this.constructor.modelName, 'reload');
  };

  /**
   * Set the correct `id` property for the `PersistedModel`. Uses the `setId` method if the model is attached to
   * connector that defines it.  Otherwise, uses the default lookup.
   * Override this method to handle complex IDs.
   *
   * @param {*} val The `id` value. Will be converted to the type that the `id` property specifies.
   */

  PersistedModel.prototype.setId = function(val) {
    var ds = this.getDataSource();
    this[this.getIdName()] = val;
  };

  /**
   * Get the `id` value for the `PersistedModel`.
   *
   * @returns {*} The `id` value
   */

  PersistedModel.prototype.getId = function() {
    var data = this.toObject();
    if (!data) return;
    return data[this.getIdName()];
  };

  /**
   * Get the `id` property name of the constructor.
   *
   * @returns {String} The `id` property name
   */

  PersistedModel.prototype.getIdName = function() {
    return this.constructor.getIdName();
  };

  /**
   * Get the `id` property name of the constructor.
   *
   * @returns {String} The `id` property name
   */

  PersistedModel.getIdName = function() {
    var Model = this;
    var ds = Model.getDataSource();

    if (ds.idName) {
      return ds.idName(Model.modelName);
    } else {
      return 'id';
    }
  };

  PersistedModel.setupRemoting = function() {
    var PersistedModel = this;
    var typeName = PersistedModel.modelName;
    var options = PersistedModel.settings;

    function setRemoting(scope, name, options) {
      var fn = scope[name];
      fn._delegate = true;
      options.isStatic = scope === PersistedModel;
      PersistedModel.remoteMethod(name, options);
    }

    setRemoting(PersistedModel, 'create', {
      description: 'Create a new instance of the model and persist it into the data source.',
      accessType: 'WRITE',
      accepts: {arg: 'data', type: 'object', description: 'Model instance data', http: {source: 'body'}},
      returns: {arg: 'data', type: typeName, root: true},
      http: {verb: 'post', path: '/'}
    });

    setRemoting(PersistedModel, 'upsert', {
      aliases: ['updateOrCreate'],
      description: 'Update an existing model instance or insert a new one into the data source.',
      accessType: 'WRITE',
      accepts: {arg: 'data', type: 'object', description: 'Model instance data', http: {source: 'body'}},
      returns: {arg: 'data', type: typeName, root: true},
      http: {verb: 'put', path: '/'}
    });

    setRemoting(PersistedModel, 'exists', {
      description: 'Check whether a model instance exists in the data source.',
      accessType: 'READ',
      accepts: {arg: 'id', type: 'any', description: 'Model id', required: true},
      returns: {arg: 'exists', type: 'boolean'},
      http: [
        {verb: 'get', path: '/:id/exists'},
        {verb: 'head', path: '/:id'}
      ],
      rest: {
        // After hook to map exists to 200/404 for HEAD
        after: function(ctx, cb) {
          if (ctx.req.method === 'GET') {
            // For GET, return {exists: true|false} as is
            return cb();
          }
          if (!ctx.result.exists) {
            var modelName = ctx.method.sharedClass.name;
            var id = ctx.getArgByName('id');
            var msg = 'Unknown "' + modelName + '" id "' + id + '".';
            var error = new Error(msg);
            error.statusCode = error.status = 404;
            error.code = 'MODEL_NOT_FOUND';
            cb(error);
          } else {
            cb();
          }
        }
      }
    });

    setRemoting(PersistedModel, 'findById', {
      description: 'Find a model instance by id from the data source.',
      accessType: 'READ',
      accepts: [
        { arg: 'id', type: 'any', description: 'Model id', required: true,
          http: {source: 'path'}},
        { arg: 'filter', type: 'object',
          description: 'Filter defining fields and include'}
      ],
      returns: {arg: 'data', type: typeName, root: true},
      http: {verb: 'get', path: '/:id'},
      rest: {after: convertNullToNotFoundError}
    });

    setRemoting(PersistedModel, 'find', {
      description: 'Find all instances of the model matched by filter from the data source.',
      accessType: 'READ',
      accepts: {arg: 'filter', type: 'object', description: 'Filter defining fields, where, include, order, offset, and limit'},
      returns: {arg: 'data', type: [typeName], root: true},
      http: {verb: 'get', path: '/'}
    });

    setRemoting(PersistedModel, 'findOne', {
      description: 'Find first instance of the model matched by filter from the data source.',
      accessType: 'READ',
      accepts: {arg: 'filter', type: 'object', description: 'Filter defining fields, where, include, order, offset, and limit'},
      returns: {arg: 'data', type: typeName, root: true},
      http: {verb: 'get', path: '/findOne'},
      rest: {after: convertNullToNotFoundError}
    });

    setRemoting(PersistedModel, 'destroyAll', {
      description: 'Delete all matching records.',
      accessType: 'WRITE',
      accepts: {arg: 'where', type: 'object', description: 'filter.where object'},
      http: {verb: 'del', path: '/'},
      shared: false
    });

    setRemoting(PersistedModel, 'updateAll', {
      aliases: ['update'],
      description: 'Update instances of the model matched by where from the data source.',
      accessType: 'WRITE',
      accepts: [
        {arg: 'where', type: 'object', http: {source: 'query'},
          description: 'Criteria to match model instances'},
        {arg: 'data', type: 'object', http: {source: 'body'},
          description: 'An object of model property name/value pairs'},
      ],
      http: {verb: 'post', path: '/update'}
    });

    setRemoting(PersistedModel, 'deleteById', {
      aliases: ['destroyById', 'removeById'],
      description: 'Delete a model instance by id from the data source.',
      accessType: 'WRITE',
      accepts: {arg: 'id', type: 'any', description: 'Model id', required: true,
                http: {source: 'path'}},
      http: {verb: 'del', path: '/:id'}
    });

    setRemoting(PersistedModel, 'count', {
      description: 'Count instances of the model matched by where from the data source.',
      accessType: 'READ',
      accepts: {arg: 'where', type: 'object', description: 'Criteria to match model instances'},
      returns: {arg: 'count', type: 'number'},
      http: {verb: 'get', path: '/count'}
    });

    setRemoting(PersistedModel.prototype, 'updateAttributes', {
      description: 'Update attributes for a model instance and persist it into the data source.',
      accessType: 'WRITE',
      accepts: {arg: 'data', type: 'object', http: {source: 'body'}, description: 'An object of model property name/value pairs'},
      returns: {arg: 'data', type: typeName, root: true},
      http: {verb: 'put', path: '/'}
    });

    if (options.trackChanges || options.enableRemoteReplication) {
      setRemoting(PersistedModel, 'diff', {
        description: 'Get a set of deltas and conflicts since the given checkpoint.',
        accessType: 'READ',
        accepts: [
          {arg: 'since', type: 'number', description: 'Find deltas since this checkpoint'},
          {arg: 'remoteChanges', type: 'array', description: 'an array of change objects',
           http: {source: 'body'}}
        ],
        returns: {arg: 'result', type: 'object', root: true},
        http: {verb: 'post', path: '/diff'}
      });

      setRemoting(PersistedModel, 'changes', {
        description: 'Get the changes to a model since a given checkpoint.' +
          'Provide a filter object to reduce the number of results returned.',
        accessType: 'READ',
        accepts: [
          {arg: 'since', type: 'number', description: 'Only return changes since this checkpoint'},
          {arg: 'filter', type: 'object', description: 'Only include changes that match this filter'}
        ],
        returns: {arg: 'changes', type: 'array', root: true},
        http: {verb: 'get', path: '/changes'}
      });

      setRemoting(PersistedModel, 'checkpoint', {
        description: 'Create a checkpoint.',
        // The replication algorithm needs to create a source checkpoint,
        // even though it is otherwise not making any source changes.
        // We need to allow this method for users that don't have full
        // WRITE permissions.
        accessType: 'REPLICATE',
        returns: {arg: 'checkpoint', type: 'object', root: true},
        http: {verb: 'post', path: '/checkpoint'}
      });

      setRemoting(PersistedModel, 'currentCheckpoint', {
        description: 'Get the current checkpoint.',
        accessType: 'READ',
        returns: {arg: 'checkpoint', type: 'object', root: true},
        http: {verb: 'get', path: '/checkpoint'}
      });

      setRemoting(PersistedModel, 'createUpdates', {
        description: 'Create an update list from a delta list.',
        // This operation is read-only, it does not change any local data.
        // It is called by the replication algorithm to compile a list
        // of changes to apply on the target.
        accessType: 'READ',
        accepts: {arg: 'deltas', type: 'array', http: {source: 'body'}},
        returns: {arg: 'updates', type: 'array', root: true},
        http: {verb: 'post', path: '/create-updates'}
      });

      setRemoting(PersistedModel, 'bulkUpdate', {
        description: 'Run multiple updates at once. Note: this is not atomic.',
        accessType: 'WRITE',
        accepts: {arg: 'updates', type: 'array'},
        http: {verb: 'post', path: '/bulk-update'}
      });

      setRemoting(PersistedModel, 'findLastChange', {
        description: 'Get the most recent change record for this instance.',
        accessType: 'READ',
        accepts: {
          arg: 'id', type: 'any', required: true, http: { source: 'path' },
          description: 'Model id'
        },
        returns: { arg: 'result', type: this.Change.modelName, root: true },
        http: { verb: 'get', path: '/:id/changes/last' }
      });

      setRemoting(PersistedModel, 'updateLastChange', {
        description: [
          'Update the properties of the most recent change record',
          'kept for this instance.'
        ],
        accessType: 'WRITE',
        accepts: [
          {
            arg: 'id', type: 'any', required: true, http: { source: 'path' },
            description: 'Model id'
          },
          {
            arg: 'data', type: 'object', http: {source: 'body'},
            description: 'An object of Change property name/value pairs'
          },
        ],
        returns: { arg: 'result', type: this.Change.modelName, root: true },
        http: { verb: 'put', path: '/:id/changes/last' }
      });
    }

    if (options.trackChanges) {
      // Deprecated (legacy) exports kept for backwards compatibility
      // TODO(bajtos) Hide these two exports in LoopBack 3.0
      setRemoting(PersistedModel, 'rectifyAllChanges', {
        description: 'Rectify all Model changes.',
        accessType: 'WRITE',
        http: {verb: 'post', path: '/rectify-all'}
      });

      setRemoting(PersistedModel, 'rectifyChange', {
        description: 'Tell loopback that a change to the model with the given id has occurred.',
        accessType: 'WRITE',
        accepts: {arg: 'id', type: 'any', http: {source: 'path'}},
        http: {verb: 'post', path: '/:id/rectify-change'}
      });
    }

    setRemoting(PersistedModel, 'createChangeStream', {
      description: 'Create a change stream.',
      accessType: 'READ',
      http: [
        {verb: 'post', path: '/change-stream'},
        {verb: 'get', path: '/change-stream'}
      ],
      accepts: {
        arg: 'options',
        type: 'object'
      },
      returns: {
        arg: 'changes',
        type: 'ReadableStream',
        json: true
      }
    });
  };

  /**
   * Get a set of deltas and conflicts since the given checkpoint.
   *
   * See [Change.diff()](#change-diff) for details.
   *
   * @param  {Number}  since  Find deltas since this checkpoint.
   * @param  {Array}  remoteChanges  An array of change objects.
   * @callback {Function} callback Callback function called with `(err, result)` arguments.  Required.
   * @param {Error} err Error object; see [Error object](http://docs.strongloop.com/display/LB/Error+object).
   * @param {Object} result Object with `deltas` and `conflicts` properties; see [Change.diff()](#change-diff) for details.
   */

  PersistedModel.diff = function(since, remoteChanges, callback) {
    var Change = this.getChangeModel();
    Change.diff(this.modelName, since, remoteChanges, callback);
  };

  /**
   * Get the changes to a model since the specified checkpoint. Provide a filter object
   * to reduce the number of results returned.
   * @param  {Number}   since    Return only changes since this checkpoint.
   * @param  {Object}   filter   Include only changes that match this filter, the same as for [#persistedmodel-find](find()).
   * @callback {Function} callback Callback function called with `(err, changes)` arguments.  Required.
   * @param {Error} err Error object; see [Error object](http://docs.strongloop.com/display/LB/Error+object).
   * @param {Array} changes An array of [Change](#change) objects.
   */

  PersistedModel.changes = function(since, filter, callback) {
    if (typeof since === 'function') {
      filter = {};
      callback = since;
      since = -1;
    }
    if (typeof filter === 'function') {
      callback = filter;
      since = -1;
      filter = {};
    }

    var idName = this.dataSource.idName(this.modelName);
    var Change = this.getChangeModel();
    var model = this;

    filter = filter || {};
    filter.fields = {};
    filter.where = filter.where || {};
    filter.fields[idName] = true;

    // TODO(ritch) this whole thing could be optimized a bit more
    Change.find({ where: {
      checkpoint: { gte: since },
      modelName: this.modelName
    }}, function(err, changes) {
      if (err) return callback(err);
      if (!Array.isArray(changes) || changes.length === 0) return callback(null, []);
      var ids = changes.map(function(change) {
        return change.getModelId();
      });
      filter.where[idName] = {inq: ids};
      model.find(filter, function(err, models) {
        if (err) return callback(err);
        var modelIds = models.map(function(m) {
          return m[idName].toString();
        });
        callback(null, changes.filter(function(ch) {
          if (ch.type() === Change.DELETE) return true;
          return modelIds.indexOf(ch.modelId) > -1;
        }));
      });
    });
  };

  /**
   * Create a checkpoint.
   *
   * @param  {Function} callback
   */

  PersistedModel.checkpoint = function(cb) {
    var Checkpoint = this.getChangeModel().getCheckpointModel();
    this.getSourceId(function(err, sourceId) {
      if (err) return cb(err);
      Checkpoint.create({
        sourceId: sourceId
      }, cb);
    });
  };

  /**
   * Get the current checkpoint ID.
   *
   * @callback {Function} callback Callback function called with `(err, currentCheckpointId)` arguments.  Required.
   * @param {Error} err Error object; see [Error object](http://docs.strongloop.com/display/LB/Error+object).
   * @param {Number} currentCheckpointId Current checkpoint ID.
   */

  PersistedModel.currentCheckpoint = function(cb) {
    var Checkpoint = this.getChangeModel().getCheckpointModel();
    Checkpoint.current(cb);
  };

  /**
   * Replicate changes since the given checkpoint to the given target model.
   *
   * @param  {Number}   [since]  Since this checkpoint
   * @param  {Model}    targetModel  Target this model class
   * @param  {Object} [options]
   * @param {Object} [options.filter] Replicate models that match this filter
   * @callback {Function} [callback] Callback function called with `(err, conflicts)` arguments.
   * @param {Error} err Error object; see [Error object](http://docs.strongloop.com/display/LB/Error+object).
   * @param {Conflict[]} conflicts A list of changes that could not be replicated due to conflicts.
   * @param {Object] checkpoints The new checkpoints to use as the "since"
   * argument for the next replication.
   */

  PersistedModel.replicate = function(since, targetModel, options, callback) {
    var lastArg = arguments[arguments.length - 1];

    if (typeof lastArg === 'function' && arguments.length > 1) {
      callback = lastArg;
    }

    if (typeof since === 'function' && since.modelName) {
      targetModel = since;
      since = -1;
    }

    if (typeof since !== 'object') {
      since = { source: since, target: since };
    }

    options = options || {};

    var sourceModel = this;
    callback = callback || utils.createPromiseCallback();

    debug('replicating %s since %s to %s since %s',
      sourceModel.modelName,
      since.source,
      targetModel.modelName,
      since.target);
    if (options.filter) {
      debug('\twith filter %j', options.filter);
    }

    // In order to avoid a race condition between the replication and
    // other clients modifying the data, we must create the new target
    // checkpoint as the first step of the replication process.
    // As a side-effect of that, the replicated changes are associated
    // with the new target checkpoint. This is actually desired behaviour,
    // because that way clients replicating *from* the target model
    // since the new checkpoint will pick these changes up.
    // However, it increases the likelihood of (false) conflicts being detected.
    // In order to prevent that, we run the replication multiple times,
    // until no changes were replicated, but at most MAX_ATTEMPTS times
    // to prevent starvation. In most cases, the second run will find no changes
    // to replicate and we are done.
    var MAX_ATTEMPTS = 3;

    run(1, since);
    return callback.promise;

    function run(attempt, since) {
      debug('\titeration #%s', attempt);
      tryReplicate(sourceModel, targetModel, since, options, next);

      function next(err, conflicts, cps, updates) {
        var finished = err || conflicts.length ||
          !updates || updates.length === 0 ||
          attempt >= MAX_ATTEMPTS;

        if (finished)
          return callback(err, conflicts, cps);
        run(attempt + 1, cps);
      }
    }
  };

  function tryReplicate(sourceModel, targetModel, since, options, callback) {
    var Change = sourceModel.getChangeModel();
    var TargetChange = targetModel.getChangeModel();
    var changeTrackingEnabled = Change && TargetChange;

    assert(
      changeTrackingEnabled,
      'You must enable change tracking before replicating'
    );

    var diff;
    var updates;
    var newSourceCp, newTargetCp;

    var tasks = [
      checkpoints,
      getSourceChanges,
      getDiffFromTarget,
      createSourceUpdates,
      bulkUpdate
    ];

    async.waterfall(tasks, done);

    function getSourceChanges(cb) {
      sourceModel.changes(since.source, options.filter, debug.enabled ? log : cb);

      function log(err, result) {
        if (err) return cb(err);
        debug('\tusing source changes');
        result.forEach(function(it) { debug('\t\t%j', it); });
        cb(err, result);
      }
    }

    function getDiffFromTarget(sourceChanges, cb) {
      targetModel.diff(since.target, sourceChanges, debug.enabled ? log : cb);

      function log(err, result) {
        if (err) return cb(err);
        if (result.conflicts && result.conflicts.length) {
          debug('\tdiff conflicts');
          result.conflicts.forEach(function(d) { debug('\t\t%j', d); });
        }
        if (result.deltas && result.deltas.length) {
          debug('\tdiff deltas');
          result.deltas.forEach(function(it) { debug('\t\t%j', it); });
        }
        cb(err, result);
      }
    }

    function createSourceUpdates(_diff, cb) {
      diff = _diff;
      diff.conflicts = diff.conflicts || [];
      if (diff && diff.deltas && diff.deltas.length) {
        debug('\tbuilding a list of updates');
        sourceModel.createUpdates(diff.deltas, cb);
      } else {
        // nothing to replicate
        done();
      }
    }

    function bulkUpdate(_updates, cb) {
      debug('\tstarting bulk update');
      updates = _updates;
      targetModel.bulkUpdate(updates, function(err) {
        var conflicts = err && err.details && err.details.conflicts;
        if (conflicts && err.statusCode == 409) {
          diff.conflicts = conflicts;
          // filter out updates that were not applied
          updates = updates.filter(function(u) {
            return conflicts
              .filter(function(d) { return d.modelId === u.change.modelId; })
              .length === 0;
          });
          return cb();
        }
        cb(err);
      });
    }

    function checkpoints() {
      var cb = arguments[arguments.length - 1];
      sourceModel.checkpoint(function(err, source) {
        if (err) return cb(err);
        newSourceCp = source.seq;
        targetModel.checkpoint(function(err, target) {
          if (err) return cb(err);
          newTargetCp = target.seq;
          debug('\tcreated checkpoints');
          debug('\t\t%s for source model %s', newSourceCp, sourceModel.modelName);
          debug('\t\t%s for target model %s', newTargetCp, targetModel.modelName);
          cb();
        });
      });
    }

    function done(err) {
      if (err) return callback(err);

      debug('\treplication finished');
      debug('\t\t%s conflict(s) detected', diff.conflicts.length);
      debug('\t\t%s change(s) applied', updates ? updates.length : 0);
      debug('\t\tnew checkpoints: { source: %j, target: %j }',
        newSourceCp, newTargetCp);

      var conflicts = diff.conflicts.map(function(change) {
        return new Change.Conflict(
          change.modelId, sourceModel, targetModel
        );
      });

      if (conflicts.length) {
        sourceModel.emit('conflicts', conflicts);
      }

      if (callback) {
        var newCheckpoints = { source: newSourceCp, target: newTargetCp };
        callback(null, conflicts, newCheckpoints, updates);
      }
    }
  }

  /**
   * Create an update list (for `Model.bulkUpdate()`) from a delta list
   * (result of `Change.diff()`).
   *
   * @param  {Array}    deltas
   * @param  {Function} callback
   */

  PersistedModel.createUpdates = function(deltas, cb) {
    var Change = this.getChangeModel();
    var updates = [];
    var Model = this;
    var tasks = [];

    deltas.forEach(function(change) {
      change = new Change(change);
      var type = change.type();
      var update = {type: type, change: change};
      switch (type) {
        case Change.CREATE:
        case Change.UPDATE:
          tasks.push(function(cb) {
            Model.findById(change.modelId, function(err, inst) {
              if (err) return cb(err);
              if (!inst) {
                return cb &&
                  cb(new Error('Missing data for change: ' + change.modelId));
              }
              if (inst.toObject) {
                update.data = inst.toObject();
              } else {
                update.data = inst;
              }
              updates.push(update);
              cb();
            });
          });
          break;
        case Change.DELETE:
          updates.push(update);
          break;
      }
    });

    async.parallel(tasks, function(err) {
      if (err) return cb(err);
      cb(null, updates);
    });
  };

  /**
   * Apply an update list.
   *
   * **Note: this is not atomic**
   *
   * @param  {Array} updates An updates list, usually from [createUpdates()](#persistedmodel-createupdates).
   * @param  {Function} callback Callback function.
   */

  PersistedModel.bulkUpdate = function(updates, callback) {
    var tasks = [];
    var Model = this;
    var Change = this.getChangeModel();
    var conflicts = [];

    buildLookupOfAffectedModelData(Model, updates, function(err, currentMap) {
      if (err) return callback(err);

      updates.forEach(function(update) {
        var id = update.change.modelId;
        var current = currentMap[id];
        switch (update.type) {
          case Change.UPDATE:
            tasks.push(function(cb) {
              applyUpdate(Model, id, current, update.data, update.change, conflicts, cb);
            });
            break;

          case Change.CREATE:
            tasks.push(function(cb) {
              applyCreate(Model, id, current, update.data, update.change, conflicts, cb);
            });
            break;
          case Change.DELETE:
            tasks.push(function(cb) {
              applyDelete(Model, id, current, update.change, conflicts, cb);
            });
            break;
        }
      });

      async.parallel(tasks, function(err) {
        if (err) return callback(err);
        if (conflicts.length) {
          err = new Error('Conflict');
          err.statusCode = 409;
          err.details = { conflicts: conflicts };
          return callback(err);
        }
        callback();
      });
    });
  };

  function buildLookupOfAffectedModelData(Model, updates, callback) {
    var idName = Model.dataSource.idName(Model.modelName);
    var affectedIds = updates.map(function(u) { return u.change.modelId; });
    var whereAffected = {};
    whereAffected[idName] = { inq: affectedIds };
    Model.find({ where: whereAffected }, function(err, affectedList) {
      if (err) return callback(err);
      var dataLookup = {};
      affectedList.forEach(function(it) {
        dataLookup[it[idName]] = it;
      });
      callback(null, dataLookup);
    });
  }

  function applyUpdate(Model, id, current, data, change, conflicts, cb) {
    var Change = Model.getChangeModel();
    var rev = current ?  Change.revisionForInst(current) : null;

    if (rev !== change.prev) {
      debug('Detected non-rectified change of %s %j',
        Model.modelName, id);
      debug('\tExpected revision: %s', change.rev);
      debug('\tActual revision:   %s', rev);
      conflicts.push(change);
      return Change.rectifyModelChanges(Model.modelName, [id], cb);
    }

    // TODO(bajtos) modify `data` so that it instructs
    // the connector to remove any properties included in "inst"
    // but not included in `data`
    // See https://github.com/strongloop/loopback/issues/1215

    Model.updateAll(current.toObject(), data, function(err, result) {
      if (err) return cb(err);

      var count = result && result.count;
      switch (count) {
        case 1:
          // The happy path, exactly one record was updated
          return cb();

        case 0:
          debug('UpdateAll detected non-rectified change of %s %j',
            Model.modelName, id);
          conflicts.push(change);
          // NOTE(bajtos) updateAll triggers change rectification
          // for all model instances, even when no records were updated,
          // thus we don't need to rectify explicitly ourselves
          return cb();

        case undefined:
        case null:
          return cb(new Error(
            'Cannot apply bulk updates, ' +
            'the connector does not correctly report ' +
            'the number of updated records.'));

        default:
          debug('%s.updateAll modified unexpected number of instances: %j',
            Model.modelName, count);
          return cb(new Error(
            'Bulk update failed, the connector has modified unexpected ' +
            'number of records: ' + JSON.stringify(count)));
      }
    });
  }

  function applyCreate(Model, id, current, data, change, conflicts, cb) {
    Model.create(data, function(createErr) {
      if (!createErr) return cb();

      // We don't have a reliable way how to detect the situation
      // where he model was not create because of a duplicate id
      // The workaround is to query the DB to check if the model already exists
      Model.findById(id, function(findErr, inst) {
        if (findErr || !inst) {
          // There isn't any instance with the same id, thus there isn't
          // any conflict and we just report back the original error.
          return cb(createErr);
        }

        return conflict();
      });
    });

    function conflict() {
      // The instance already exists - report a conflict
      debug('Detected non-rectified new instance of %s %j',
        Model.modelName, id);
      conflicts.push(change);

      var Change = Model.getChangeModel();
      return Change.rectifyModelChanges(Model.modelName, [id], cb);
    }
  }

  function applyDelete(Model, id, current, change, conflicts, cb) {
    if (!current) {
      // The instance was either already deleted or not created at all,
      // we are done.
      return cb();
    }

    var Change = Model.getChangeModel();
    var rev = Change.revisionForInst(current);
    if (rev !== change.prev) {
      debug('Detected non-rectified change of %s %j',
        Model.modelName, id);
      debug('\tExpected revision: %s', change.rev);
      debug('\tActual revision:   %s', rev);
      conflicts.push(change);
      return Change.rectifyModelChanges(Model.modelName, [id], cb);
    }

    Model.deleteAll(current.toObject(), function(err, result) {
      if (err) return cb(err);

      var count = result && result.count;
      switch (count) {
        case 1:
          // The happy path, exactly one record was updated
          return cb();

        case 0:
          debug('DeleteAll detected non-rectified change of %s %j',
            Model.modelName, id);
          conflicts.push(change);
          // NOTE(bajtos) deleteAll triggers change rectification
          // for all model instances, even when no records were updated,
          // thus we don't need to rectify explicitly ourselves
          return cb();

        case undefined:
        case null:
          return cb(new Error(
            'Cannot apply bulk updates, ' +
            'the connector does not correctly report ' +
            'the number of deleted records.'));

        default:
          debug('%s.deleteAll modified unexpected number of instances: %j',
            Model.modelName, count);
          return cb(new Error(
            'Bulk update failed, the connector has deleted unexpected ' +
            'number of records: ' + JSON.stringify(count)));
      }
    });
  }

  /**
   * Get the `Change` model.
   * Throws an error if the change model is not correctly setup.
   * @return {Change}
   */

  PersistedModel.getChangeModel = function() {
    var changeModel = this.Change;
    var isSetup = changeModel && changeModel.dataSource;

    assert(isSetup, 'Cannot get a setup Change model for ' + this.modelName);

    return changeModel;
  };

  /**
   * Get the source identifier for this model or dataSource.
   *
   * @callback {Function} callback Callback function called with `(err, id)` arguments.
   * @param {Error} err Error object; see [Error object](http://docs.strongloop.com/display/LB/Error+object).
   * @param {String} sourceId Source identifier for the model or dataSource.
   */

  PersistedModel.getSourceId = function(cb) {
    var dataSource = this.dataSource;
    if (!dataSource) {
      this.once('dataSourceAttached', this.getSourceId.bind(this, cb));
    }
    assert(
      dataSource.connector.name,
      'Model.getSourceId: cannot get id without dataSource.connector.name'
    );
    var id = [dataSource.connector.name, this.modelName].join('-');
    cb(null, id);
  };

  /**
   * Enable the tracking of changes made to the model. Usually for replication.
   */

  PersistedModel.enableChangeTracking = function() {
    var Model = this;
    var Change = this.Change || this._defineChangeModel();
    var cleanupInterval = Model.settings.changeCleanupInterval || 30000;

    assert(this.dataSource, 'Cannot enableChangeTracking(): ' + this.modelName +
      ' is not attached to a dataSource');

    var idName = this.getIdName();
    var idProp = this.definition.properties[idName];
    var idType = idProp && idProp.type;
    var idDefn = idProp && idProp.defaultFn;
    if (idType !== String || !(idDefn === 'uuid' || idDefn === 'guid')) {
      deprecated('The model ' + this.modelName + ' is tracking changes, ' +
        'which requries a string id with GUID/UUID default value.');
    }

    Model.observe('after save', rectifyOnSave);

    Model.observe('after delete', rectifyOnDelete);

    if (runtime.isServer) {
      // initial cleanup
      cleanup();

      // cleanup
      setInterval(cleanup, cleanupInterval);
    }

    function cleanup() {
      Model.rectifyAllChanges(function(err) {
        if (err) {
          Model.handleChangeError(err, 'cleanup');
        }
      });
    }
  };

  function rectifyOnSave(ctx, next) {
    if (ctx.instance) {
      ctx.Model.rectifyChange(ctx.instance.getId(), reportErrorAndNext);
    } else {
      ctx.Model.rectifyAllChanges(reportErrorAndNext);
    }

    function reportErrorAndNext(err) {
      if (err) {
        ctx.Model.handleChangeError(err, 'after save');
      }
      next();
    }
  }

  function rectifyOnDelete(ctx, next) {
    var id = ctx.instance ? ctx.instance.getId() :
      getIdFromWhereByModelId(ctx.Model, ctx.where);

    if (id) {
      ctx.Model.rectifyChange(id, reportErrorAndNext);
    } else {
      ctx.Model.rectifyAllChanges(reportErrorAndNext);
    }

    function reportErrorAndNext(err) {
      if (err) {
        ctx.Model.handleChangeError(err, 'after delete');
      }
      next();
    }
  }

  function getIdFromWhereByModelId(Model, where) {
    var whereKeys = Object.keys(where);
    if (whereKeys.length != 1) return undefined;

    var idName = Model.getIdName();
    if (whereKeys[0] !== idName) return undefined;

    var id = where[idName];
    // TODO(bajtos) support object values that are not LB conditions
    if (typeof id === 'string' || typeof id === 'number') {
      return id;
    }
    return undefined;
  }

  PersistedModel._defineChangeModel = function() {
    var BaseChangeModel = this.registry.getModel('Change');
    assert(BaseChangeModel,
      'Change model must be defined before enabling change replication');

    this.Change = BaseChangeModel.extend(this.modelName + '-change',
      {},
      {
        trackModel: this
      }
    );

    if (this.dataSource) {
      attachRelatedModels(this);
    }

    // We have to attach related model whenever the datasource changes,
    // this is a workaround for autoAttach called by loopback.createModel
    var self = this;
    this.on('dataSourceAttached', function() {
      attachRelatedModels(self);
    });

    return this.Change;

    function attachRelatedModels(self) {
      self.Change.attachTo(self.dataSource);
      self.Change.getCheckpointModel().attachTo(self.dataSource);
    }
  };

  PersistedModel.rectifyAllChanges = function(callback) {
    this.getChangeModel().rectifyAll(callback);
  };

  /**
   * Handle a change error. Override this method in a subclassing model to customize
   * change error handling.
   *
   * @param {Error} err Error object; see [Error object](http://docs.strongloop.com/display/LB/Error+object).
   */

  PersistedModel.handleChangeError = function(err, operationName) {
    if (!err) return;
    this.emit('error', err, operationName);
  };

  /**
   * Specify that a change to the model with the given ID has occurred.
   *
   * @param {*} id The ID of the model that has changed.
   * @callback {Function} callback
   * @param {Error} err
   */

  PersistedModel.rectifyChange = function(id, callback) {
    var Change = this.getChangeModel();
    Change.rectifyModelChanges(this.modelName, [id], callback);
  };

  PersistedModel.findLastChange = function(id, cb) {
    var Change = this.getChangeModel();
    Change.findOne({ where: { modelId: id } }, cb);
  };

  PersistedModel.updateLastChange = function(id, data, cb) {
    var self = this;
    this.findLastChange(id, function(err, inst) {
      if (err) return cb(err);
      if (!inst) {
        err = new Error('No change record found for ' +
          self.modelName + ' with id ' + id);
        err.statusCode = 404;
        return cb(err);
      }

      inst.updateAttributes(data, cb);
    });
  };

  /**
   * Create a change stream. [See here for more info](http://docs.strongloop.com/pages/viewpage.action?pageId=6721710)
   *
   * @param {Object} options
   * @param {Object} options.where Only changes to models matching this where filter will be included in the `ChangeStream`.
   * @callback {Function} callback
   * @param {Error} err
   * @param {ChangeStream} changes
   */

  PersistedModel.createChangeStream = function(options, cb) {
    if (typeof options === 'function') {
      cb = options;
      options = undefined;
    }

    var idName = this.getIdName();
    var Model = this;
    var changes = new PassThrough({objectMode: true});
    var writeable = true;

    changes.destroy = function() {
      changes.removeAllListeners('error');
      changes.removeAllListeners('end');
      writeable = false;
      changes = null;
    };

    changes.on('error', function() {
      writeable = false;
    });
    changes.on('end', function() {
      writeable = false;
    });

    process.nextTick(function() {
      cb(null, changes);
    });

    Model.observe('after save', createChangeHandler('save'));
    Model.observe('after delete', createChangeHandler('delete'));

    function createChangeHandler(type) {
      return function(ctx, next) {
        // since it might have set to null via destroy
        if (!changes) {
          return next();
        }

        var where = ctx.where;
        var data = ctx.instance || ctx.data;
        var whereId = where && where[idName];

        // the data includes the id
        // or the where includes the id
        var target;

        if (data && (data[idName] || data[idName] === 0)) {
          target = data[idName];
        } else if (where && (where[idName] || where[idName] === 0)) {
          target = where[idName];
        }

        var hasTarget = target === 0 || !!target;

        var change = {
          target: target,
          where: where,
          data: data
        };

        switch (type) {
          case 'save':
            if (ctx.isNewInstance === undefined) {
              change.type = hasTarget ? 'update' : 'create';
            } else {
              change.type = ctx.isNewInstance ? 'create' : 'update';
            }

            break;
          case 'delete':
            change.type = 'remove';
            break;
        }

        // TODO(ritch) this is ugly... maybe a ReadableStream would be better
        if (writeable) {
          changes.write(change);
        }

        next();
      };
    }
  };

  PersistedModel.setup();

  return PersistedModel;
};