How to get distinct polymorphic association

Question

I'm trying to get show a list of polymorphic relations without having any duplicates.

I have a StoreViews table with a polymorphic field called viewable (so there's a viewable_id and viewable_type column in my table). Now I want to display views with each polymorphic relation showing up just once, without duplicates.

@views = StoreView.
    .distinct(:viewable_id)
    .distinct(:viewable_type)
    .order("created_at DESC")
    .limit(10)

So if there's two records in StoreViews, both with the same viewable relation, @views should only return the most recent one. However, this is not the case.

Answer 1

ORDER BY items must appear in the select list if SELECT DISTINCT is specified. There are several ways to work around this issue.

In this example using an aggregate function should work:

@views = StoreView
           .select('DISTINCT viewable_type, viewable_id, MAX(created_at)')
           .group(:viewable_type, :viewable_id)
           .order('MAX(created_at) DESC')
           .limit(10)