The comparison between `Relation` and `CollectionProxy` should be consistent.

Example:

    author.posts == Post.where(author_id: author.id)
    # => true
      Post.where(author_id: author.id) == author.posts
    # => true

Fixes #13506

activerecord/CHANGELOG.md

+*   The comparison between `Relation` and `CollectionProxy` should be consistent.
+
+    Example:
+
+        author.posts == Post.where(author_id: author.id)
+        # => true
+        Post.where(author_id: author.id) == author.posts
+        # => true
+
+    Fixes #13506.
+
+    *Lauro Caetano*
+
 *   Fixed error for aggregate methods (`empty?`, `any?`, `count`) with `select`
     which created invalid SQL.
 

activerecord/lib/active_record/relation.rb

@@ -569,6 +569,8 @@ def uniq_value
     # Compares two relations for equality.
     def ==(other)
       case other
+      when Associations::CollectionProxy
+        self == other.to_a
       when Relation
         other.to_sql == to_sql
       when Array

activerecord/test/cases/base_test.rb

@@ -551,6 +551,23 @@ def test_equality_with_blank_ids
     assert_equal one, two
   end
 
+  def test_equality_of_relation_and_collection_proxy
+    car = Car.create!
+    car.bulbs.build
+    car.save
+
+    assert car.bulbs == Bulb.where(car_id: car.id), 'CollectionProxy should be comparable with Relation'
+    assert Bulb.where(car_id: car.id) == car.bulbs, 'Relation should be comparable with CollectionProxy'
+  end
+
+  def test_equality_of_relation_and_array
+    car = Car.create!
+    car.bulbs.build
+    car.save
+
+    assert Bulb.where(car_id: car.id) == car.bulbs.to_a, 'Relation should be comparable with Array'
+  end
+
   def test_hashing
     assert_equal [ Topic.find(1) ], [ Topic.find(2).topic ] & [ Topic.find(1) ]
   end