While merging relations preserve context for joins

Fixes #3002. Also see #5494.

```
class Comment < ActiveRecord::Base
  belongs_to :post
end

class Author < ActiveRecord::Base
  has_many :posts
end

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

`Comment.joins(:post).merge(Post.joins(:author).merge(Author.where(:name => "Joe Blogs"))).all` would
fail with `ActiveRecord::ConfigurationError: Association named 'author' was not found on Comment`.

It is failing because `all` is being called on relation which looks like this after all the merging:
`{:joins=>[:post, :author], :where=>[#<Arel::Nodes::Equality: ....}`. In this relation all the context that
`Post` was joined with `Author` is lost and hence the error that `author` was not found on `Comment`.

Ths solution is to build JoinAssociation when two relations with join information are being merged. And later
while building the arel use the  previously built `JoinAssociation` record in `JoinDependency#graft` to
build the right from clause.

Thanks to Jared Armstrong (https://github.com/armstrjare) for most of the work. I ported it to make it
compatible with new code base.

1 files changed, 10 insertions, 1 deletions

diff --git a/activerecord/test/cases/relation_test.rb b/activerecord/test/cases/relation_test.rb
index 9ca980fdf6..1967d8716a 100644
--- a/activerecord/test/cases/relation_test.rb
+++ b/activerecord/test/cases/relation_test.rb
@@ -1,10 +1,12 @@
 require "cases/helper"
 require 'models/post'
 require 'models/comment'
+require 'models/author'
+require 'models/rating'
 
 module ActiveRecord
   class RelationTest < ActiveRecord::TestCase
-    fixtures :posts, :comments
+    fixtures :posts, :comments, :authors
 
     class FakeKlass < Struct.new(:table_name, :name)
     end
@@ -176,6 +178,13 @@ module ActiveRecord
       relation.merge!(where: ['foo = ?', 'bar'])
       assert_equal ['foo = bar'], relation.where_values
     end
+
+    def test_relation_merging_with_merged_joins
+      special_comments_with_ratings = SpecialComment.joins(:ratings)
+      posts_with_special_comments_with_ratings = Post.group("posts.id").joins(:special_comments).merge(special_comments_with_ratings)
+      assert_equal 3, authors(:david).posts.merge(posts_with_special_comments_with_ratings).to_a.length
+    end
+
   end
 
   class RelationMutationTest < ActiveSupport::TestCase