How to use Ecto's has_many and cast_assoc

Question

I'm new to Elixir and Ecto and I would need some help with Ecto's has_many and cast_assoc. Can't understand the basics, like how am I to create a new model with assoc one.

Here's my Has_Model:

defmodule Example.Has_Model do
  use Ecto.Schema
  import Ecto.Changeset
  alias Example.Repo
  alias Example.Has_Model

  schema "has_models" do
    has_many :belong_models, Example.Belong_Model
    field :name, string
    timestamps
  end

  def changeset(has_model, params \\ %{}) do
    has_model
    |> cast(params, [:name])
  end
end

and here's Belong_Model:

defmodule Example.Belong_Model do
  use Ecto.Schema
  import Ecto.Changeset
  alias Example.Repo
  alias Example.Belong_Model

  schema "belong_models" do
    belongs_to :has_model, Example.Has_Model
    field :name, string
    timestamps
  end

  def changeset(belong_model, params \\ %{}) do
    belong_model
    |> cast(params, [:name])
    |> cast_assoc(:has_model, required: true)
  end
end

Now, what I try to do is first to create a Has_Model

iex()> changeset = Example.Has_Model.changeset(%Has_Model, %{name: "I have"})
iex()> Example.Repo.insert(changeset)

This work ok.

Then I'd like to create a new Belong_Model and use that previously created Has_Model with it:

iex()> has = Example.Has_Model |> Example.Repo.get_by(name: "I have")
iex()> changeset = Example.Belong_Model.changeset(%Belong_Model, %{name: "I belong", belongs_to: has})

And this is where it fails:

** (Ecto.CastError) expected params to be a map, got: `%Example.Has_Model{__meta__: #Ecto.Schema.Metadata<:loaded, "has_models">, id: 1, inserted_at: #Ecto.DateTime<2016-10-04 19:39:38>, name: "I have", updated_at: #Ecto.DateTime<2016-10-04 19:39:38>, belong_models: #Ecto.Association.NotLoaded<association :belong_models is not loaded>}`
    (ecto) lib/ecto/changeset.ex:345: Ecto.Changeset.do_cast/4
    (example) lib/example/models/has_model.ex:15: Example.Has_Model.changeset/2
    (ecto) lib/ecto/changeset/relation.ex:99: Ecto.Changeset.Relation.do_cast/5
    (ecto) lib/ecto/changeset/relation.ex:235: Ecto.Changeset.Relation.single_change/5
    (ecto) lib/ecto/changeset.ex:571: Ecto.Changeset.cast_relation/4
    (example) lib/example/models/belong_model.ex:16: Example.Belong_Model.changeset/2

What's happening and what's the solution? I've tried to turn these parameters around the whole day but I just don't get it.

Answer 1

I think what you are looking for is build_assoc/3

iex()> has = Example.Has_Model |> Example.Repo.get_by(name: "I have")
iex()> belong_assoc = Ecto.build_assoc(has, :belong_models, has_model_id: has.id, name: "I belong")
iex()> Repo.insert!(belong_assoc)

And cast_assoc/3 should be use like this:

iex()> changeset = Example.Belong_Model.changeset(%Belong_Model, %{name: "I belong", has_model: %{name: "I have"})
iex()> Repo.insert!(changeset)

Note:

The parameters for the given association will be retrieved from changeset.params and the changeset function in the association module will be invoked