Phoenix Ecto: foreign key not being inserted

Question

I am inserting a model A that contains a foreign key to another model B.

defmodule MyApp.ModelA do
  use MyApp.Web, :model

  schema "model_a" do
    field :type, :string, null: false
    field :data, :string, null: false
    belongs_to :model_b, MyApp.ModelB
    timestamps()
  end

  @required_fields ~w(type data)
  @optional_fields ~w()

  @doc """
  Builds a changeset based on the `struct` and `params`.
  """
  def changeset(struct, params \\ %{}) do
    struct
    |> cast(params, @required_fields, @optional_fields)
    |> assoc_constraint(:model_b)
  end
end

and the Model B:

defmodule MyApp.ModelB do
  use MyApp.Web, :model

  schema "model_b" do
    field :username, :string
    field :pass, :string
    has_many :model_a, MyApp.ModelA
    timestamps()
  end

  @required_fields ~w(username pass)
  @optional_fields ~w()

  @doc """
  Builds a changeset based on the `struct` and `params`.
  """
  def changeset(struct, params \\ %{}) do
    struct
    |> cast(params, @required_fields, @optional_fields)
    |> cast_assoc(:model_a)
    |> validate_required([])
  end
end

Model B exists, as I can get it via Repo.all(ModelB).

Model A changeset is successfully validated, and I can see the model_b_id value when I print model A changeset struct.

But when inserted, the reference is not inserted. Although I can see it when printing the changeset of model A, in the MySQL log, this field is completely missing, is not in the INSERT query.

I have played a little bit, and if I force this reference field to not be null in the MySQL table, then I get a {"does not exist", []} for this foreign key field when inserting as a Repo.insert(...) response, although the model B exists in database.

Answer 1

I was supremely interested in a response to this question.

I'd lost >7 hours to the exact condition of "I KNOW this row was created, WHY OH WHY is the changeset not letting me store the reference?"

I thought I'd leave a suggestion for anybody with the same frustration to check your database migration references.

I was sure I had triple checked them, but there it was, clear as day after a good nights sleep. I had setup references to wrong table.

Hopefully this saves somebody some time.