I appreciated your feedback and ended up going another way, but just to see if what I wanted was technically possible, I made it work in dev by overriding Account.__schema__/2 to support associations that don’t show up in the struct.

  defoverridable __schema__: 2

  def __schema__(:association, :my_assoc) do
    %Ecto.Association.Has{
      cardinality: :one,
      field: :my_assoc,
      owner: __MODULE__,
      related: MyApp.MyContext.MySchema,
      owner_key: :id,
      related_key: :account_id,
      on_cast: nil,
      queryable: MyApp.MyContext.MySchema,
      on_delete: :nothing,
      on_replace: :raise,
      where: [],
      unique: true,
      defaults: [],
      relationship: :child,
      ordered: false,
      preload_order: []
    }
  end

  def __schema__(arg1, arg2) do
    super(arg1, arg2)
  end

I have no plans to use this in production, but it does seem to work with Ecto.assoc(account, :my_assoc), Ecto.build_assoc(account, :my_assoc), and join: m in assoc(a, :my_assoc).