Honour the order of the joining model in a `has_many :through`

association when eager loading.

Previously, eager loading a `has_many :through` association with no
defined order would return the records in the natural order of the
database. Now, these records will be returned in the order that the
joining record is returned, in case there is a defined order there.

2 files changed, 38 insertions, 0 deletions

diff --git a/activerecord/test/cases/associations/eager_test.rb b/activerecord/test/cases/associations/eager_test.rb
index 7d8b933992..79e660c7e5 100644
--- a/activerecord/test/cases/associations/eager_test.rb
+++ b/activerecord/test/cases/associations/eager_test.rb
@@ -743,6 +743,38 @@ class EagerAssociationTest < ActiveRecord::TestCase
     }
   end
 
+  def test_eager_has_many_through_with_order
+    tag = OrderedTag.create(name: 'Foo')
+    post1 = Post.create!(title: 'Beaches', body: "I like beaches!")
+    post2 = Post.create!(title: 'Pools', body: "I like pools!")
+
+    Tagging.create!(taggable_type: 'Post', taggable_id: post1.id, tag: tag)
+    Tagging.create!(taggable_type: 'Post', taggable_id: post2.id, tag: tag)
+
+    tag_with_includes = OrderedTag.includes(:tagged_posts).find(tag.id)
+    assert_equal(tag_with_includes.taggings.map(&:taggable).map(&:title), tag_with_includes.tagged_posts.map(&:title))
+  end
+
+  def test_eager_has_many_through_multiple_with_order
+    tag1 = OrderedTag.create!(name: 'Bar')
+    tag2 = OrderedTag.create!(name: 'Foo')
+
+    post1 = Post.create!(title: 'Beaches', body: "I like beaches!")
+    post2 = Post.create!(title: 'Pools', body: "I like pools!")
+
+    Tagging.create!(taggable: post1, tag: tag1)
+    Tagging.create!(taggable: post2, tag: tag1)
+    Tagging.create!(taggable: post2, tag: tag2)
+    Tagging.create!(taggable: post1, tag: tag2)
+
+    tags_with_includes = OrderedTag.where(id: [tag1, tag2].map(&:id)).includes(:tagged_posts).order(:id).to_a
+    tag1_with_includes = tags_with_includes.first
+    tag2_with_includes = tags_with_includes.last
+
+    assert_equal([post2, post1].map(&:title), tag1_with_includes.tagged_posts.map(&:title))
+    assert_equal([post1, post2].map(&:title), tag2_with_includes.tagged_posts.map(&:title))
+  end
+
   def test_eager_with_default_scope
     developer = EagerDeveloperWithDefaultScope.where(:name => 'David').first
     projects = Project.order(:id).to_a
diff --git a/activerecord/test/models/tag.rb b/activerecord/test/models/tag.rb
index 80d4725f7e..b48b9a2155 100644
--- a/activerecord/test/models/tag.rb
+++ b/activerecord/test/models/tag.rb
@@ -5,3 +5,9 @@ class Tag < ActiveRecord::Base
 
   has_many :tagged_posts, :through => :taggings, :source => 'taggable', :source_type => 'Post'
 end
+
+class OrderedTag < Tag
+  self.table_name = "tags"
+
+  has_many :taggings, -> { order('taggings.id DESC') }, foreign_key: 'tag_id'
+end