unique_constraint on 2 fields not being recognized

Question

I'am trying to do 2 fields unique constraint in Ecto Postgres. So far I managed to do:

In Migration:

create unique_index(:presences, [:event_id, :user_id], name: :special_event_user)

Changeset:

 def changeset(presence, params \\ :empty) do
    presence
      |> cast(params, @required_fields, @optional_fields)
      |> foreign_key_constraint(:user_id)
      |> foreign_key_constraint(:event_id)
      |> unique_constraint(:event_id, name: :special_event_user)
  end

Have also tried:

|> unique_constraint(:special_event_user)

But I keep getting:

If you would like to convert this constraint into an error, please
call unique_constraint/3 in your changeset and define the proper
constraint name. The changeset has not defined any constraint.

Which I thought I already did. Any suggestions?

Edit:

Action:

def assign(conn, %{"event" => event_id}) do
    case Integer.parse(event_id) do
      {val, _ } ->
          case Repo.one(User.unique_user(User, get_session(conn, :login))) do
            nil -> conn
              |> put_flash(:error, "Błąd bazy danych")
              |> redirect(to: "/event")
            result ->
              presence = %Presence{ user_id: result.id, event_id: val, state: Presence.get_assigned } |> Repo.insert!
              conn
                |> put_flash(:info, "Zostałeś zapisany na wydarzenie")
                |> redirect(to: "/event")
          end
    end
  end

Egzample presence from above code when there is no error:

%Kpsz.Model.Presence{__meta__: #Ecto.Schema.Metadata<:loaded>,
 event: #Ecto.Association.NotLoaded<association :event is not loaded>,
 event_id: 1, id: 1, inserted_at: #Ecto.DateTime<2015-12-14T15:45:39Z>,
 state: 1, updated_at: #Ecto.DateTime<2015-12-14T15:45:39Z>,
 user: #Ecto.Association.NotLoaded<association :user is not loaded>, user_id: 1}

Edit: Now I'm getting:

constraint error when attempting to insert model:

    * unique: special_event_user

If you would like to convert this constraint into an error, please
call unique_constraint/3 in your changeset and define the proper
constraint name. The changeset defined the following constraints:

    * unique: presences_special_event_user_index
    * foreign_key: presences_event_id_fkey
    * foreign_key: presences_user_id_fkey

Answer 1

You are not going through your changeset function.

presence =
  %Presence{ user_id: result.id, event_id: val, state: Presence.get_assigned }
  |> Repo.insert!

Should be:

presence =
  Presence.changeset(%Presence{}, %{user_id: result.id, event_id: val, state: Presence.get_assigned})
  |> Repo.insert!