Ecto: Unit Test for Validating Association Declarations

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I’m sharing this because it may be of general interest. I was doing some refactoring of code and removed some automatic aliases when doing use MyApp.Schema, which broke some of my unit tests…but not all of them (I don’t have 100% coverage; imagine that).

The error raised was UndefinedFunctionError for __schema__. After some thinking, this is the unit test I came up with to verify this. The only thing to maintain in it is the list of @relations, which are the application Ecto schemas that have belongs_to, has_many, or many_to_many relationships listed in them.

This will ensure that the relationships are not referring to the wrong aliased module, since Ecto does not (cannot?) verify the module specifications a compile-time. Example:

defmodule MyApp.Business do
  use MyApp.Schema

  schema "businesses" do
    has_many :users, User
  end
end

The error is fairly visible, but unless you have a test that exercises the association (via join(Business, :left, [b], assoc(b, :user)), there will be nothing that exposes the missing alias MyApp.User from the module during compilation.

Adding a test like this will help:

defmodule MyApp.ValidEctoRelationsTest do
  @moduledoc """
  Test that belongs_to, has_many, and many_to_many relationships are valid,
  since these things cannot be determined at compile time.
  """

  use ExUnit.Case

  alias MyApp.Repo

  import Ecto.Query

  setup do
    :ok = Ecto.Adapters.SQL.Sandbox.checkout(Repo)
  end

  @relations [
    MyApp.Business,
    MyApp.User
  ]

  for schema <- @relations, association <- schema.__schema__(:associations) do
    test "#{inspect(schema)} has a valid association for #{inspect(association)}" do
      assert_valid_relationship(unquote(schema), unquote(association))
    end
  end

  defp assert_valid_relationship(schema, association) do
    schema
    |> join(:left, [s], assoc(s, ^association))
    |> where(false)
    |> Repo.all()

    assert true
  rescue
    UndefinedFunctionError ->
      %{queryable: module} = schema.__schema__(:association, association)

      flunk("""
      Schema #{inspect(schema)} association #{inspect(association)} is invalid.
      The associated module #{inspect(module)} does not appear to be an Ecto
      schema. Is #{inspect(schema)} missing an alias?
      """)
  end
end
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