# LoopBack Definition Language (LDL)

LoopBack Definition Language (LDL) is simple DSL to define data models in JavaScript or plain JSON. With LoopBack, we often
start with a model definition which describes the structure and types of data. The model establishes common knowledge of
data in LoopBack.

## Describing a model

Let's start with a simple example in plain JSON.

    {
        "id": "number",
        "firstName": "string",
        "lastName": "string"
    }

The model simply defines a `user` model that consists of three properties:

* id - The user id. It's a number.
* firstName - The first name. It's a string.
* lastName - The last name. It's a string.

Each key in the JSON object defines a property in our model which will be cast to its associated type. The simplest form of
a property definition is `propertyName: type`. The key is the name of the property and the value is the type of the property.
We'll cover more advanced form later in this guide.

LDL supports a list of built-in types, including the basic types from JSON:

* String
* Number
* Boolean
* Array
* Object

**Note**: The type name is case-insensitive, i.e., either "Number" or "number" can be used.

The same model can also be described in JavaScript code:

    var UserDefinition = {
        id: Number,
        firstName: String,
        lastName: String
    }

As we can see, the JavaScript version is less verbose as it doesn't require quotes for property names. The types are
described using JavaScript constructors, for example, `Number` for `"Number"`. String literals are also supported.

Now we have the definition of a model, how do we use it in LoopBack Node.js code? It's easy, LoopBack will build a
JavaScript constructor (or class) for you.

## Creating a model constructor

LDL compiles the model definition into a JavaScript constructor using `ModelBuilder.define` APIs. ModelBuilder is the
basic factory to create model constructors.

ModelBuilder.define() method takes the following arguments:

- name: The model name
- properties: An object of property definitions
- options: An object of options, optional


    var ModelBuilder = require('loopback-datasource-juggler').ModelBuilder;

    // Create an instance of the ModelBuilder
    var modelBuilder = new ModelBuilder();

    // Describe the user model
    var UserDefinition = {
        id: Number,
        firstName: String,
        lastName: String
    }

    // Compile the user model definition into a JavaScript constructor
    var User = modelBuilder.define('User', UserDefinition);

    // Create a new instance of User
    var user = new User({id: 1, firstName: 'John', lastName: 'Smith'});

    console.log(user.id); // 1
    console.log(user.firstName); // 'John'
    console.log(user.lastName); // 'Smith'

That's it. Now you have a User constructor representing the user model.

At this point, the constructor only has a set of accessors to model properties. No behaviors have been introduced yet.

## Adding methods to a model constructor

There are a few ways to add methods to a model constructor:

1. Create the model constructor from a data source

        var DataSource = require('loopback-datasource-juggler').DataSource;
        var ds = new DataSource('memory');

        // Compile the user model definition into a JavaScript constructor
        var User = ds.define('User', UserDefinition);

        // Create a new instance of User
        User.create({id: 1, firstName: 'John', lastName: 'Smith'}, function(err, user) {
            console.log(user); // The newly created user instance
            User.findById(1, function(err, user) {
                console.log(user); // The user instance for id 1
                user.firstName = 'John1'; // Change the property
                user.save(function(err, user) {
                    console.log(user); // The modified user instance for id 1
                });
            };
        });


2. Attach the model to a data source

A plain model constructor created from `ModelBuilder` can be attached a `DataSource`.

        var DataSource = require('loopback-datasource-juggler').DataSource;
        var ds = new DataSource('memory');

        User.attachTo(ds); // The CRUD methods will be mixed into the User constructor

3. Manually declare methods to the model constructor

We can add static and prototype methods to a model constructor.

    // Define a static method
    User.greet = function(msg) {
        console.log('Hello ', msg);
    };

    // Define a prototype method
    User.prototype.getFullName = function () {
        return this.firstName + ' ' + this.lastName;
    };

    User.greet('world'); // prints 'Hello world'
    var user = new User({id: 1, firstName: 'John', lastName: 'Smith'});
    console.log(user.getFullName()); // 'John Smith'


## Exploring advanced features

The basic example use `propertyName: type` to describe a property.

Properties can have options in addition to the type. LDL uses a JSON object to describe such properties, for example:

    "id": {"type": "number", "id": true, "doc": "User ID"}

    "firstName": {"type": "string", "required": true, "oracle": {"column": "FIRST_NAME", "type": "VARCHAR(32)"}}

Common options for a property are:

* type: The property type
  - String/Text
  - Number
  - Date
  - Boolean
  - Buffer/Binary
  - Array
  - Any/Object/JSON
  - GeoPoint

* id: Indicate if the property is an `id` of the model. The value can be true, false, or a number
    - true: It's an id
    - false: It's not an id
    - 0: It's not an id
    - 1: It's the first part of the composite id

LDL supports the definition of a composite id that has more than one properties. For example,

    var InventoryDefinition =
    {
        productId: {type: String, id: 1},
        locationId: {type: String, id: 2},
        qty: Number
    }

The composite id is (productId, locationId) for an inventory model.

* doc: Documentation of the property

* default: The default value of the property

Constraints are modeled as options, for example:

* required: Indicate if the property is required
* pattern: A regular expression pattern that a string should match
* min/max: The minimal and maximal value
* length: The maximal length of a string

Format conversions can also be declared as options, for example:

* trim: Trim the string
* lowercase: Convert the string to be lowercase
* uppercase: Convert the string to be uppercase
* format: Format a Date

Data source specific mappings can be added to the property options, for example, to map a property to be a column in
Oracle database table, you can use the following syntax:

    "oracle": {"column": "FIRST_NAME", "type": "VARCHAR", "length": 32}


### Array types

LDL supports array types as follows:

    {emails: [String]}

    or

    {"emails": ["String"]}

    or

    {emails: [{type: String, length: 64}]}

### Object types

#### Embed anonymous types
#### Reference named types

### Advanced example

    var User = modelBuilder.define('User', {
        name: String,
        bio: ModelBuilder.Text,
        approved: Boolean,
        joinedAt: Date,
        age: Number,
        address: {
            street: String,
            city: String,
            state: String,
            zipCode: String,
            country: String
        },
        emails: [{
            label: String,
            email: String
        }],
        friends: [String]
    });

### Model level options

LDL uses a list of options to control the definition of a model.

- strict:
- idInjection:
- Data source specific mappings


### Relations between models

    // setup relationships
    User.hasMany(Post,   {as: 'posts',  foreignKey: 'userId'});

    Post.belongsTo(User, {as: 'author', foreignKey: 'userId'});

    User.hasAndBelongsToMany('groups');

### Extend from a base model
### Mix in model definitions

    var Group = modelBuilder.define('Group', {group: String});

    User.mixin(Group);