Extract join values to merge as an original type

activerecord/CHANGELOG.md

@@ -1,3 +1,19 @@
+*   Fix nesting merge `#joins' with String argument raises `ActiveRecord::ConfigurationError`.
+
+    Example:
+	```ruby
+	# Before
+	User.joins(:comments).merge(
+	  Comment.joins(:article).merge(
+	    Article.joins(
+	      'LEFT OUTER JOIN categories ON articles.id = categories.article_id'
+	    )
+	  )
+	) # => ActiveRecord::ConfigurationError
+	```
+
+    *Takamichi Yoshikawa*
+
 *   Fix has_many :through relation merging failing when dynamic conditions are
     passed as a lambda with an arity of one.
 

activerecord/lib/active_record/relation/merger.rb

@@ -100,7 +100,7 @@ def merge_joins
           join_dependency = ActiveRecord::Associations::JoinDependency.new(other.klass,
                                                                            joins_dependency,
                                                                            [])
-          relation.joins! rest
+          relation.joins!(*rest)
 
           @relation = relation.joins join_dependency
         end

activerecord/test/cases/relation_test.rb

@@ -3,6 +3,7 @@
 require 'models/comment'
 require 'models/author'
 require 'models/rating'
+require 'models/tagging'
 
 module ActiveRecord
   class RelationTest < ActiveRecord::TestCase
@@ -236,6 +237,27 @@ def test_relation_merging_with_merged_joins_as_strings
       assert_equal 3, authors(:david).posts.merge(posts_with_special_comments_with_ratings).count.length
     end
 
+    def test_relation_merging_with_nested_joins_as_strings
+      post = Post.create!(title: "haha", body: "huhu")
+      comment = post.comments.create!(body: "hu")
+      comment.ratings.create!
+
+      join_string = <<-JOIN
+        LEFT OUTER JOIN
+          #{Tagging.quoted_table_name}
+        ON
+          #{Tagging.quoted_table_name}.taggable_id = #{Rating.quoted_table_name}.id AND #{Tagging.quoted_table_name}.taggable_type = 'Rating'
+      JOIN
+
+      relation = Post.where(id: post.id).joins(:comments).merge(
+        Comment.joins(:ratings).merge(
+          Rating.joins(join_string)
+        )
+      )
+
+      assert_equal 1, relation.count
+    end
+
     class EnsureRoundTripTypeCasting < ActiveRecord::Type::Value
       def type
         :string