Make sure that a joins Relation can be merged with has_many :through + association proxy

Closes #11248.

activerecord/test/cases/associations/has_many_through_associations_test.rb

@@ -5,6 +5,7 @@
 require 'models/job'
 require 'models/reader'
 require 'models/comment'
+require 'models/rating'
 require 'models/tag'
 require 'models/tagging'
 require 'models/author'
@@ -616,6 +617,11 @@ def test_has_many_association_through_a_belongs_to_association
     assert_equal post.author.author_favorites, post.author_favorites
   end
 
+  def test_merge_join_association_with_has_many_through_association_proxy
+    author = authors(:mary)
+    assert_nothing_raised { author.comments.ratings.to_sql }
+  end
+
   def test_has_many_association_through_a_has_many_association_with_nonstandard_primary_keys
     assert_equal 2, owners(:blackbeard).toys.count
   end

activerecord/test/models/author.rb

@@ -13,7 +13,11 @@ class Author < ActiveRecord::Base
   has_many :posts_with_extension, :class_name => "Post"
   has_one  :post_about_thinking, -> { where("posts.title like '%thinking%'") }, :class_name => 'Post'
   has_one  :post_about_thinking_with_last_comment, -> { where("posts.title like '%thinking%'").includes(:last_comment) }, :class_name => 'Post'
-  has_many :comments, :through => :posts
+  has_many :comments, through: :posts do
+    def ratings
+      Rating.joins(:comment).merge(self)
+    end
+  end
   has_many :comments_containing_the_letter_e, :through => :posts, :source => :comments
   has_many :comments_with_order_and_conditions, -> { order('comments.body').where("comments.body like 'Thank%'") }, :through => :posts, :source => :comments
   has_many :comments_with_include, -> { includes(:post) }, :through => :posts, :source => :comments