Summary

I am currently trying to get a better understanding of elixir macros and in doing so encountered the section about dynamic queries in the ecto documentation. The example, however, leads to an unwanted true and part in the ecto query where and I am trying to have a look at how to solve that.

The trimmed down documentation example

So I tried out the example given and it seems to add a superfluous true and in front of the where clause. I trimmed the example down and put it inside a testcase:

defmodule MyMacroTest do
  use ExUnit.Case, async: true
  import Ecto.Query

  def filter_where(params) do
    Enum.reduce(params, dynamic(true), fn
      {"author", value}, dynamic ->
        dynamic([p], ^dynamic and p.author == ^value)
    end)
  end

  @q from(p in "post")

  test "author" do
    query = %{"author" => "foo"}
    assert inspect(where(@q, ^filter_where(query))) ==
             inspect(where(@q, [p], p.author == ^query["author"]))
  end
end

which fails with

     Assertion with == failed
     code:  assert inspect(where(@q, ^filter_where(query))) == inspect(where(@q, [p], p.author == ^query["author"]))
     left:  "#Ecto.Query<from p0 in \"post\", where: true and p0.author == ^\"foo\">"
     right: "#Ecto.Query<from p0 in \"post\", where: p0.author == ^\"foo\">"

The fancy if statement

Switching out the dynamic(true) within the call to Enum.reduce/3 with nil and using if seems to work

defmodule MyMacro2Test do
  use ExUnit.Case, async: true
  import Ecto.Query

  def filter_where(params) do
    Enum.reduce(params, nil, fn
      {"author", value}, dynamic ->
        if dynamic do
          dynamic([p], ^dynamic and p.author == ^value)
        else
          dynamic([p], p.author == ^value)
        end
    end)
  end

  @q from(p in "post")

  test "author" do
    query = %{"author" => "foo"}
    assert inspect(where(@q, ^filter_where(query))) ==
             inspect(where(@q, [p], p.author == ^query["author"]))
  end
end

Finished in 0.04 seconds (0.04s async, 0.00s sync)
1 test, 0 failures

Even though it works, it is not a thing of beauty and given that the dynamic macro basically outputs AST similar to what I would like, writing another macro appears to be the correct thing to do.

But here is the part I seem to struggle with

I now don’t know how to exactly extract that conditional into a macro. I guess I will need a macro because I need to wrap the dynamic macro and pass through the arguments in the same syntax.
Currently stuck with this version

defmodule MyMacroTest3 do
  use ExUnit.Case, async: true
  import Ecto.Query

  defmacro dynamic_and(dynamic, bindings, expr) do
    quote do
      if unquote(dynamic) do
        dynamic(unquote(bindings), unquote(dynamic) and unquote(expr))
      else
        dynamic(unquote(bindings), unquote(expr))
      end
    end
  end

  def filter_where(params) do
    Enum.reduce(params, nil, fn
      {"author", value}, dynamic ->
        dynamic_and(^dynamic, [p], p.author == ^value)
    end)
  end

  @q from(p in "post")

  test "author" do
    query = %{"author" => "foo"}
    assert inspect(where(@q, ^filter_where(query))) ==
             inspect(where(@q, [p], p.author == ^query["author"]))
  end
end

which gives me a compilation error about a misplaced operator ^dynamic

    The pin operator ^ is supported only inside matches or inside custom macros. Make sure you are inside a match or all necessary macros have been required
    │
 18 │         dynamic_and(^dynamic, [p], p.author == ^value)

I tried multiple variations of pinning and unquote/not unquote but I somehow currently fail to see where to look next for the correct way of solving this.
I also thought that a potential solution could involve returning some AST directly, or maybe it would be better to try to construct an %Ecto.Query.DynamicExpr struct myself.
Also got myself a copy of that Metaprogramming book and I am currently reading it hoping that it might help, and even got some bots involved

Anyway, I would really apreciate any pointers that could be thrown at me and thank you very much in advance!