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          


        

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)