Casting Associations in Phoenix

If you want to cast changes on a struct's associations, you can use the changeset function cast_assoc/3. This allows you to make changes to a parent struct and its associations at the same time.

Example

Lets say you have a struct called Artist and an associated schema called Album. The schema for the Album looks something like this:

def module MyApp.Music.Album do
 use Ecto.Schema
 alias MyApp.Music.Artist

  schema "albums" do
    field :title, :string
    
    belongs_to :artist, Artist
  end

  def changeset(artist, params \\ %{}) do
    artist
    |> cast(params, [:title, :artist_id])
    |> validate_required([:title])
  end
end

and the schema for artist looks like this:

def module MyApp.Music.Artist do
 use Ecto.Schema
 alias MyApp.Music.Album

  schema "albums" do
    field :name, :string
    
    has_many :albums, Album
  end
end

Now, if you wanted to build a changeset for the Artist where you can change the artist's name and the title field on an associated album:

def changeset(artist, params \\ %{}) do
    artist
    |> cast(params, [:name])
    |> cast_assoc(:albums, with: &MyApp.Album.changeset/2)
    |> validate_required([:name])
end