var debug = require('debug')('loopback:connector:transaction');
var uuid = require('node-uuid');
var utils = require('./utils');
var jutil = require('./jutil');
var ObserverMixin = require('./observer');

var Transaction = require('loopback-connector').Transaction;

module.exports = TransactionMixin;

/**
 * TransactionMixin class.  Use to add transaction APIs to a model class.
 *
 * @class TransactionMixin
 */
function TransactionMixin() {
}

/**
 * Begin a new transaction
 * @param {Object|String} [options] Options can be one of the forms:
 * - Object: {isolationLevel: '...', timeout: 1000}
 * - String: isolationLevel
 *
 * Valid values of `isolationLevel` are:
 *
 * - Transaction.READ_COMMITTED = 'READ COMMITTED'; // default
 * - Transaction.READ_UNCOMMITTED = 'READ UNCOMMITTED';
 * - Transaction.SERIALIZABLE = 'SERIALIZABLE';
 * - Transaction.REPEATABLE_READ = 'REPEATABLE READ';
 *
 * @param {Function} cb Callback function. It calls back with (err, transaction).
 * To pass the transaction context to one of the CRUD methods, use the `options`
 * argument with `transaction` property, for example,
 *
 * ```js
 *
 * MyModel.beginTransaction('READ COMMITTED', function(err, tx) {
 *   MyModel.create({x: 1, y: 'a'}, {transaction: tx}, function(err, inst) {
 *     MyModel.find({x: 1}, {transaction: tx}, function(err, results) {
 *       // ...
 *       tx.commit(function(err) {...});
 *     });
 *   });
 * });
 * ```
 *
 * The transaction can be committed or rolled back. If timeout happens, the
 * transaction will be rolled back. Please note a transaction is typically
 * associated with a pooled connection. Committing or rolling back a transaction
 * will release the connection back to the pool.
 *
 * Once the transaction is committed or rolled back, the connection property
 * will be set to null to mark the transaction to be inactive. Trying to commit
 * or rollback an inactive transaction will receive an error from the callback.
 *
 * Please also note that the transaction is only honored with the same data
 * source/connector instance. CRUD methods will not join the current transaction
 * if its model is not attached the same data source.
 *
 */
TransactionMixin.beginTransaction = function(options, cb) {
  cb = cb || utils.createPromiseCallback();
  if (Transaction) {
    var connector = this.getConnector();
    Transaction.begin(connector, options, function(err, transaction) {
      if (err) return cb(err);
      if (transaction) {
        // Set an informational transaction id
        transaction.id = uuid.v1();
      }
      if (options.timeout) {
        setTimeout(function() {
          var context = {
            transaction: transaction,
            operation: 'timeout'
          };
          transaction.notifyObserversOf('timeout', context, function(err) {
            if (!err) {
              transaction.rollback(function() {
                debug('Transaction %s is rolled back due to timeout',
                  transaction.id);
              });
            }
          });
        }, options.timeout);
      }
      cb(err, transaction);
    });
  } else {
    process.nextTick(function() {
      var err = new Error('Transaction is not supported');
      cb(err);
    });
  }
  return cb.promise;
};

// Promisify the transaction apis
if (Transaction) {
  jutil.mixin(Transaction.prototype, ObserverMixin);
  /**
   * Commit a transaction and release it back to the pool
   * @param {Function} cb Callback function
   * @returns {Promise|undefined}
   */
  Transaction.prototype.commit = function(cb) {
    var self = this;
    cb = cb || utils.createPromiseCallback();
    // Report an error if the transaction is not active
    if (!self.connection) {
      process.nextTick(function() {
        cb(new Error('The transaction is not active: ' + self.id));
      });
      return cb.promise;
    }
    var context = {
      transaction: self,
      operation: 'commit'
    };
    self.notifyObserversOf('before commit', context, function(err) {
      if (err) return cb(err);
      self.connector.commit(self.connection, function(err) {
        if (err) return cb(err);
        self.notifyObserversOf('after commit', context, function(err) {
          // Deference the connection to mark the transaction is not active
          // The connection should have been released back the pool
          self.connection = null;
          cb(err);
        });
      });
    });
    return cb.promise;
  };

  /**
   * Rollback a transaction and release it back to the pool
   * @param {Function} cb Callback function
   * @returns {Promise|undefined}
   */
  Transaction.prototype.rollback = function(cb) {
    var self = this;
    cb = cb || utils.createPromiseCallback();
    // Report an error if the transaction is not active
    if (!self.connection) {
      process.nextTick(function() {
        cb(new Error('The transaction is not active: ' + self.id));
      });
      return cb.promise;
    }
    var context = {
      transaction: self,
      operation: 'rollback'
    };
    self.notifyObserversOf('before rollback', context, function(err) {
      if (err) return cb(err);
      self.connector.rollback(self.connection, function(err) {
        if (err) return cb(err);
        self.notifyObserversOf('after rollback', context, function(err) {
          // Deference the connection to mark the transaction is not active
          // The connection should have been released back the pool
          self.connection = null;
          cb(err);
        });
      });
    });
    return cb.promise;
  };

  Transaction.prototype.toJSON = function() {
    return this.id;
  };

  Transaction.prototype.toString = function() {
    return this.id;
  };
}

TransactionMixin.Transaction = Transaction;