Use variable to reference named binding in an Ecto Query

So the answer to this question seems to be that there isn't a way supported by Ecto to do this. @maartenvanvliet solution works nicely, with the downside of relying on internal implementation.

My solution to this problem was to have the function search_field to always search in the last joined table, using the ... syntax described here:

# Searches for the `search_term` in the `field` in the last joined table in `initial_query`.
defp search_field(initial_query, field, search_term) do
  initial_query
  |> or_where(
    [..., t],
    fragment(
      "CAST(? AS varchar) ILIKE ?",
      field(t, ^field),
      ^"%#{search_term}%"
    )
  )
end

So this function would be used like this:

Answer
|> join(:left, [a], q in assoc(a, :question), as: :question)
|> search_field(:text, search_text)
|> join(:left, [a, q], s in assoc(a, :survey), as: :survey)
|> search_field(:title, search_text)

Which, in my opinion, still reads nicely, with the downside of requiring that we are able to change the initial_query.

The trick is to retrieve the position of the named binding in the bindings. The named bindings are stored in the %Ecto.Query{aliases: aliases} field.

def named_binding_position(query, binding) do
  Map.get(query.aliases, binding)
end

def search_field(query, table, field, search_term) do
  position = named_binding_position(query, table)
  query
  |> or_where(
    [{t, position}],
    fragment(
      "CAST(? AS varchar) ILIKE ?",
      field(t, ^field),
      ^"%#{search_term}%"
    )
  )
end

We first lookup the position of the named binding in the query.aliases. Then use this position to build the query.

Now, when we call

Answer
|> join(:left, [a], q in assoc(a, :question), as: :question)
|> join(:left, [a, q], s in assoc(a, :survey), as: :survey)
|> search_field(:question, :text, "bogus")

It should yield something like

#Ecto.Query<from a in Answer,
left_join: q in assoc(a, :question), as: :question,
or_where: fragment("CAST(? AS varchar) ILIKE ?", q.text, ^"%bogus%")>

Of note is that the {t, position} tuples in %Query.aliases to refer to the position of the named binding is an internal implementation and not documented. Therefore, could be subject to change. See https://github.com/elixir-ecto/ecto/issues/2832 for more information