How to write Elixir macro similar to Ecto's field/2?

Question

I'm learning Elixir and came across such situation:
I have an Ecto schema and I want to make a function like "get_by" that takes a column name and it's value as an arguments like this: get_by(:id, 7) So the working version of the function would be like this:

def get_by(column, value) do
  Repo.all(
    from(
      r in __MODULE__,
      where: field(r, ^column) == ^value,
    )
  )
end

I know this is fully functional but I was wondering how the field macro works.
The original code is too hard to read for me. I was trying to play with AST in macro, but nothing seems to work. The best I had was this:

defmacro magic(var, {:^, _, [{column, _, _}]}) do
  dot = {:., [], [var, column]}
  {dot, [], []}
end

But this returns r.column instead of the atom bound to column variable.
How the macro should be written to return r.id?

If you look at the source code for Ecto.Query.API.field/2, it's just a placeholder: github.com/elixir-ecto/ecto/blob/v3.2.5/lib/ecto/query/… The query macro looks for calls to it and treats them as desired. — Brett Beatty

Aleksei Matiushkin Aleksei Matiushkin · Accepted Answer · 2019-11-15T17:32:03

If you check the source code for Ecto.Query.API.field/2, you’ll see that the explicit call to this function (it’s not a macro btw) raises.

That is because it makes sense only inside Ecto.Query.from/2 macro.

What you want is still possible to some extent; not with a dot notation (AFAICT, but with an Access)

defmodule M do
  defmacro magic(a1, {:^, _, [a2]}) do
    quote do: unquote(a1)[unquote(a2)]
  end
end
import M
{r, column} = {%{id: 42}, :id}
magic(r, ^column)
#⇒ 42

I was unable to get quote do: unquote(a1).unquote(a2) work without absolutely nasty tricks like inplace eval.

To better understand macros, you probably should clarify for yourself what AST is available where.

I highly recommend Metaprogramming Elixir by Chris McCord.

How to write Elixir macro similar to Ecto's field/2?

1 Answers