Bug with passing an Arel::Relation into an ActiveResource predicate.

Suppose you have a fairly complex query that you have built using Arel, and now you want to pass it into a where predicate to get your AR models back (here is a simple example, but could be any number of outer joins, etc):

pt = Post.arel_table
current = pt.where(pt[:created_at].gt(1.day.ago)).project(pt[:id])
current.to_a.size
 => 6
Post.where(:id => current).size
 => 1

This is because ActiveRecord::PredicateBuilder#build_from_hash doesn't test for Arel::Relation and thus defaults to eq() instead of in():

      case value
      when Array, ActiveRecord::Associations::AssociationCollection, ActiveRecord::Relation
        values = value.to_a
        attribute.in(values)
      when Range
        attribute.in(value)
      else
        attribute.eq(value)
      end

Resolution is to modify the line when Range to when Range, Arel::Relation. Patch forthcoming.