I’m trying to create a simple query to select distinct values from a column. If I only use select and distinct I get back a list of unique values.

Users
|> select([u], u.group_id)
|> distinct(true)
|> Repo.all()

Which gives me back a list of ids as expected:

[42, 43, nil]

However, when I add a where clause to exclude the nil value I get back a different result.

Users
|> where([u], not is_nil(u.group_id))
|> select([u], u.group_id)
|> distinct(true)
|> Repo.all()

This returns the value:

'*+'

I would have expected the query to return of a list of integers without a nil value. When I run the generated raw SQL in psql I get back the list as expected.

How can I get back a list of distinct group_ids that does not include the nil value?