Calling my own predicate in "where" in Ecto

Question

I want to be able to call a function or my predicated in "where" of Ecto. I'm trying this:

items = where(Item, [x], Item.my_predicate?(x))

# items = where(Item, [x], Item.my_predicate?(x) == true)

It throws an exception:

`Item.my_predicate?(x)` is not a valid query expression

How to fix it?

Answer 1

Of course it is possible however you cannot use functions but macros.

Example:

defmodule Foo do
  defmacro concat_ws(joiner, columns) do
    params_list = "?" |> List.duplicate(Enum.count(columns)) |> Enum.join(",")

    quote do
      fragment(unquote("concat_ws(?," <> params_list <>" )", joiner, unquote_splicing(columns))
    end
  end
end

items = where(Item, [x], Foo.concat_ws(" ", [x.a, x.b]) == "a b")

---

Example without fragment:

defmodule Foo do
  defmacro eql(a, b) do
    quote do: not (is_nil(a) or is_nil(b)) and a == b 
  end
end

But still macro result need to be valid query AST, so nothing fancy can land there.