one-to-many.md

One-to-Many Associations

A one-to-many association states that a model can be associated with many other models. To build this association a virtual attribute is added to a model using the collection property. In a one-to-many association one side must have a collection attribute and the other side must contain a model attribute. This allows the many side to know which records it needs to get when a populate is used.

Because you may want a model to have multiple one-to-many associations on another model a via key is needed on the collection attribute. This states which model attribute on the one side of the association is used to populate the records.

// A user may have many pets
var User = Waterline.Collection.extend({

  identity: 'user',
  connection: 'local-postgresql',

  attributes: {
    firstName: 'string',
    lastName: 'string',

    // Add a reference to Pets
    pets: {
      collection: 'pet',
      via: 'owner'
    }
  }
});

// A pet may only belong to a single user
var Pet = Waterline.Collection.extend({

  identity: 'pet',
  connection: 'local-postgresql',

  attributes: {
    breed: 'string',
    type: 'string',
    name: 'string',

    // Add a reference to User
    owner: {
      model: 'user'
    }
  }
});

Now that the pets and users know about each other, they can be associated. To do this we can create or update a pet with the user's primary key for the owner value.

Pet.create({
  breed: 'labrador',
  type: 'dog',
  name: 'fido',

  // Set the User's Primary Key to associate the Pet with the User.
  owner: 123
})
.exec(function(err, pet) {});

Now that the Pet is associated with the User, all the pets belonging to a specific user can be populated by using the populate method.

User.find()
.populate('pets')
.exec(function(err, users) {
  if(err) // handle error

  // The users object would look something like the following
  // [{
  //   id: 123,
  //   firstName: 'Foo',
  //   lastName: 'Bar',
  //   pets: [{
  //     id: 1,
  //     breed: 'labrador',
  //     type: 'dog',
  //     name: 'fido',
  //     user: 123
  //   }]
  // }]
});