4 files changed, 54 insertions, 0 deletions
diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 3893292cee..a45d912017 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,16 @@
+*   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*
+
 *   Calling `delete_all` on an unloaded `CollectionProxy` no longer
     generates a SQL statement containing each id of the collection:
 
diff --git a/activerecord/lib/active_record/association_relation.rb b/activerecord/lib/active_record/association_relation.rb
index ef9650d482..45f1b07f69 100644
--- a/activerecord/lib/active_record/association_relation.rb
+++ b/activerecord/lib/active_record/association_relation.rb
@@ -17,6 +17,10 @@ module ActiveRecord
       @association.empty?
     end
 
+    def ==(other)
+      other == to_a
+    end
+
     private
 
     def exec_queries
diff --git a/activerecord/lib/active_record/relation.rb b/activerecord/lib/active_record/relation.rb
index d1764a2bb2..709edbee88 100644
--- a/activerecord/lib/active_record/relation.rb
+++ b/activerecord/lib/active_record/relation.rb
@@ -569,6 +569,8 @@ module ActiveRecord
     # Compares two relations for equality.
     def ==(other)
       case other
+      when Associations::CollectionProxy, AssociationRelation
+        self == other.to_a
       when Relation
         other.to_sql == to_sql
       when Array
diff --git a/activerecord/test/cases/base_test.rb b/activerecord/test/cases/base_test.rb
index 4969344763..2e5b8cffa6 100644
--- a/activerecord/test/cases/base_test.rb
+++ b/activerecord/test/cases/base_test.rb
@@ -551,6 +551,41 @@ class BasicsTest < ActiveRecord::TestCase
     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_equality_of_relation_and_association_relation
+    car = Car.create!
+    car.bulbs.build
+    car.save
+
+    assert_equal Bulb.where(car_id: car.id), car.bulbs.includes(:car), 'Relation should be comparable with AssociationRelation'
+    assert_equal car.bulbs.includes(:car), Bulb.where(car_id: car.id), 'AssociationRelation should be comparable with Relation'
+  end
+
+  def test_equality_of_collection_proxy_and_association_relation
+    car = Car.create!
+    car.bulbs.build
+    car.save
+
+    assert_equal car.bulbs, car.bulbs.includes(:car), 'CollectionProxy should be comparable with AssociationRelation'
+    assert_equal car.bulbs.includes(:car), car.bulbs, 'AssociationRelation should be comparable with CollectionProxy'
+  end
+
   def test_hashing
     assert_equal [ Topic.find(1) ], [ Topic.find(2).topic ] & [ Topic.find(1) ]
   end