Generic composable Ecto query w/ dynamic field name in query expression

Question

I'm trying to allow for passing in a field name and running it in an Ecto query expression dynamically, like so:

def count_distinct(query, field_name) when is_binary(field_name) do
  query
  |> select([x], count(Map.fetch!(x, field_name), :distinct))
end

However, I get this compilation error:

(Ecto.Query.CompileError) `Map.fetch!(x, field_name)` is not a valid query expression

Is there any way to accomplish this?

Answer 1

You need to use field/2 to dynamically generate fields in queries:

query
|> select([x], count(field(x, ^field_name), :distinct))

An example using the other query syntax for completion:

from x in query,
  select: count(field(x, ^field_name), :distinct)