diff --git a/activerecord/lib/active_record/associations/join_dependency.rb b/activerecord/lib/active_record/associations/join_dependency.rb
index cd366ac8b72ba..e3d8356f49be4 100644
--- a/activerecord/lib/active_record/associations/join_dependency.rb
+++ b/activerecord/lib/active_record/associations/join_dependency.rb
@@ -109,7 +109,7 @@ def build(associations, parent = nil, join_type = Arel::InnerJoin)
         case associations
         when Symbol, String
           reflection = parent.reflections[associations.to_s.intern] or
-          raise ConfigurationError, "Association named '#{ associations }' was not found; perhaps you misspelled it?"
+          raise ConfigurationError, "Association named '#{ associations }' was not found on #{parent.active_record.name}; perhaps you misspelled it?"
           unless join_association = find_join_association(reflection, parent)
             @reflections << reflection
             join_association = build_join_association(reflection, parent)
diff --git a/activerecord/lib/active_record/associations/join_dependency/join_association.rb b/activerecord/lib/active_record/associations/join_dependency/join_association.rb
index 0d7d28e458597..ea4856408daca 100644
--- a/activerecord/lib/active_record/associations/join_dependency/join_association.rb
+++ b/activerecord/lib/active_record/associations/join_dependency/join_association.rb
@@ -55,7 +55,12 @@ def ==(other)
 
         def find_parent_in(other_join_dependency)
           other_join_dependency.join_parts.detect do |join_part|
-            parent == join_part
+            case parent
+            when JoinBase
+              parent.active_record == join_part.active_record
+            else
+              parent == join_part
+            end
           end
         end
 
diff --git a/activerecord/lib/active_record/relation/merger.rb b/activerecord/lib/active_record/relation/merger.rb
index 3f880ce5e934b..c2f0a82fd3acd 100644
--- a/activerecord/lib/active_record/relation/merger.rb
+++ b/activerecord/lib/active_record/relation/merger.rb
@@ -29,7 +29,7 @@ def other
     end
 
     class Merger
-      attr_reader :relation, :values
+      attr_reader :relation, :other
 
       def initialize(relation, other)
         if other.default_scoped? && other.klass != relation.klass
@@ -37,13 +37,17 @@ def initialize(relation, other)
         end
 
         @relation = relation
-        @values   = other.values
+        @other    = other
+      end
+
+      def values
+        @other.values
       end
 
       def normal_values
         Relation::SINGLE_VALUE_METHODS +
           Relation::MULTI_VALUE_METHODS -
-          [:where, :order, :bind, :reverse_order, :lock, :create_with, :reordering]
+          [:where, :joins, :order, :bind, :reverse_order, :lock, :create_with, :reordering]
       end
 
       def merge
@@ -54,6 +58,7 @@ def merge
 
         merge_multi_values
         merge_single_values
+        merge_joins
 
         relation
       end
@@ -84,6 +89,26 @@ def merge_single_values
         end
       end
 
+      def merge_joins
+        return if values[:joins].blank?
+
+        if other.klass == relation.klass
+          relation.joins!(values[:joins])
+        else
+          joins_to_stash, other_joins = values[:joins].partition { |join|
+            case join
+            when Hash, Symbol, Array
+              true
+            else
+              false
+            end
+          }
+
+          join_dependency = ActiveRecord::Associations::JoinDependency.new(other.klass, joins_to_stash, [])
+          relation.joins!(join_dependency.join_associations + other_joins)
+        end
+      end
+
       def merged_binds
         if values[:bind]
           (relation.bind_values + values[:bind]).uniq(&:first)
diff --git a/activerecord/test/cases/relations_test.rb b/activerecord/test/cases/relations_test.rb
index 3ef357e29747a..48fc82bbc7f6e 100644
--- a/activerecord/test/cases/relations_test.rb
+++ b/activerecord/test/cases/relations_test.rb
@@ -6,6 +6,7 @@
 require 'models/comment'
 require 'models/author'
 require 'models/comment'
+require 'models/rating'
 require 'models/entrant'
 require 'models/developer'
 require 'models/reply'
@@ -19,7 +20,7 @@
 
 class RelationTest < ActiveRecord::TestCase
   fixtures :authors, :topics, :entrants, :developers, :companies, :developers_projects, :accounts, :categories, :categorizations, :posts, :comments,
-    :tags, :taggings, :cars, :minivans
+    :ratings, :tags, :taggings, :cars, :minivans
 
   def test_do_not_double_quote_string_id
     van = Minivan.last
@@ -731,6 +732,12 @@ def test_relation_merging_with_joins
     assert_equal 1, comments.count
   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 1, authors(:david).posts.merge(posts_with_special_comments_with_ratings).to_a.length
+  end
+
   def test_count
     posts = Post.scoped