Set a source association when setting a belongs_to association

When a through association that is a belongs_to association is set, we
don't set the source association, which might be confusing, because the
source association is available after saving and reloading the record.

For example:

  class Post < ActiveRecord::Base
    belongs_to :author
    has_many :comments
  end

  class Comment < ActiveRecord::Base
    belongs_to :post
    has_one :post_author, through: :post
  end

  class Author < ActiveRecord::Base
    has_many :posts
  end

The post association in this case is a belongs_to association and also
it's used as a through association. Before this commit the result of
assigning the post to the comment would be the following:

  comment = Comment.new(text: "Just a comment")
  author = Author.new(name: "drogus")
  post = Post.new(title: "A title", author: author)

  comment.post = post

  comment.post_author #=> nil

  comment.save
  comment.reload

  comment.post_author #=> #<Author name: "drogus">

With a fix introduced in the commit, the post_author would be set before
saving the record:

  comment = Comment.new(text: "Just a comment")
  author = Author.new(name: "drogus")
  post = Post.new(title: "A title", author: author)

  comment.post = post

  comment.post_author #=> #<Author name: "drogus">

  comment.save
  comment.reload

  comment.post_author #=> #<Author name: "drogus">

Author: drogus

activerecord/lib/active_record/associations/belongs_to_association.rb

@@ -71,6 +71,15 @@ def replace(record)
           replace_keys(record)
 
           self.target = record
+
+          through_reflection = self.reflection.active_record.reflections.values.find { |r| r.through_reflection && r.through_reflection.name == self.reflection.name }
+          if through_reflection && !through_reflection.collection?
+            through_target = record.send(through_reflection.name)
+            if through_target
+              source_target_name = through_reflection.source_reflection.name
+              owner.send("#{source_target_name}=", through_target)
+            end
+          end
         end
 
         def update_counters(by)

activerecord/test/cases/associations/has_one_through_associations_test.rb

@@ -103,6 +103,21 @@ def test_creating_association_sets_both_parent_ids_for_new
     assert_equal club.id, member.current_membership.club_id
   end
 
+  def test_assigning_a_belongs_to_through_record_also_assigns_the_source_record
+    # setting ids is needed here because of `self.primary_key=` in models
+    minivan = Minivan.new(name: "A minivan", minivan_id: 999)
+    dashboard = Dashboard.new(name: "A dashboard", dashboard_id: 1000)
+    speedometer = Speedometer.new(dashboard: dashboard, speedometer_id: 1001)
+
+    minivan.speedometer = speedometer
+
+    assert_equal dashboard, minivan.dashboard
+
+    minivan.save!
+    minivan.reload
+
+    assert_equal dashboard, minivan.dashboard
+  end
 
   def test_assigning_a_has_one_through_record_also_assigns_the_source_record
     club = Club.new(name: "Da Club")