Elixir Ecto Insert empty belongs_to

First: I've found this topic but it does not work.

I have a relation between two models

Council(1)<->(n)Department

and I want to insert a council without a relation to a department.

I've got this ecto schema:

schema "councils" do
  field :name, :string
  field :description, :string
  belongs_to :department, Db2.Department
  many_to_many :students, Db2.Student, join_through: Db2.StudentCouncil
  many_to_many :periods, Db2.Period, join_through: Db2.StudentCouncil

  timestamps()
end

with this SQL schema:

CREATE TABLE public.councils
(
  id integer NOT NULL DEFAULT nextval('councils_id_seq'::regclass),
  name character varying(255),
  description character varying(255),
  department_id integer,
  inserted_at timestamp without time zone NOT NULL,
  updated_at timestamp without time zone NOT NULL,
  CONSTRAINT councils_pkey PRIMARY KEY (id),
  CONSTRAINT councils_department_id_fkey FOREIGN KEY (department_id)
      REFERENCES public.departments (id) MATCH SIMPLE
      ON UPDATE NO ACTION ON DELETE NO ACTION
)

The documentation says that cast/4 is deprecated, but phoenix created the changeset with cast/3 & validate_required/3 by itself. I only added the department so that I have the following changeset:

def changeset(struct, params \\ %{}) do
  struct
  |> cast(params, [:name, :description, :department_id])
  |> validate_required([:name])
  |> unique_constraint(:name)
  |> assoc_constraint(:department)
end

For the form I'am using the the default phoenix.html form elements. When I submit the form phoenix says the following:

[debug] QUERY OK db=1.2ms queue=0.1ms
SELECT u0."id", u0."uid", u0."is_admin", u0."is_staff", u0."password_hash", u0."inserted_at", u0."updated_at" FROM "users" AS u0 WHERE (u0."id" = $1) [1]
[debug] Processing by Db2.CouncilController.create/2
Parameters: %{"_csrf_token" => "KwpdFCZ/bFN9fwkcXSAYLhRCIzgOAAAAXeoEa4+d1MoT7SvzZpgOdg==", "_utf8" => "✓", "council" => %{"department_id" => "", "description" => "", "name" => "test2"}}
  Pipelines: [:browser]
[debug] QUERY OK db=1.2ms queue=0.1ms


#Ecto.Changeset<action: :insert, changes: %{description: "", name: "test2"},
  errors: [department_id: {"is invalid", [type: :id]}], data: #Db2.Council<>,
  valid?: false>

Solution

The problem is that you're sending an empty string as the department_id instead of nil. Empty strings are not cast to nil automatically:

iex(1)> MyApp.Post.changeset %MyApp.Post{}, %{"title" => "Hello"}
#Ecto.Changeset<action: nil, changes: %{title: "Hello"}, errors: [],
 data: #MyApp.Post<>, valid?: true>
iex(2)> MyApp.Post.changeset %MyApp.Post{}, %{"title" => "Hello", "user_id" => nil}
#Ecto.Changeset<action: nil, changes: %{title: "Hello"}, errors: [],
 data: #MyApp.Post<>, valid?: true>
iex(3)> MyApp.Post.changeset %MyApp.Post{}, %{"title" => "Hello", "user_id" => "1"}
#Ecto.Changeset<action: nil, changes: %{title: "Hello", user_id: 1}, errors: [],
 data: #MyApp.Post<>, valid?: true>
iex(4)> MyApp.Post.changeset %MyApp.Post{}, %{"title" => "Hello", "user_id" => ""}
#Ecto.Changeset<action: nil, changes: %{title: "Hello"},
 errors: [user_id: {"is invalid", [type: :id]}], data: #MyApp.Post<>,
 valid?: false>

You can either explicitly set department_id to nil before calling changeset, or use Phoenix's scrub_params plug, which does the same thing for all empty strings:

# web/controllers/council_controller.ex
plug :scrub_params, "council" when action in [:create]

(Change "council" to the field name and [:create] to the list of actions where you're accepting that field.)