From c09829e03db611b46bb52e2054991222cf57bfbe Mon Sep 17 00:00:00 2001
From: Andrew Horner <andrew@tablexi.com>
Date: Thu, 18 Apr 2013 23:41:58 -0500
Subject: Preserve context for joins while merging relations

This is a backport of #10164, already merged into
master. The issue is described in lengthy detail
in issues #3002 and #5494.
---
 activerecord/CHANGELOG.md                          |  5 ++++
 .../active_record/associations/join_dependency.rb  |  2 +-
 .../join_dependency/join_association.rb            |  7 +++++-
 .../lib/active_record/relation/spawn_methods.rb    | 28 +++++++++++++++++++++-
 activerecord/test/cases/relations_test.rb          |  9 ++++++-
 5 files changed, 47 insertions(+), 4 deletions(-)

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index d27b11bddd..aa561bb8d3 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,5 +1,10 @@
 ## unreleased ##
 
+*   Maintain context for joins within ActiveRecord::Relation merges.
+    Backport #10164.
+
+    *Neeraj Singh + Andrew Horner*
+    
 *   Make sure the `EXPLAIN` command is never triggered by a `select_db` call.
 
     *Daniel Schierbeck*
diff --git a/activerecord/lib/active_record/associations/join_dependency.rb b/activerecord/lib/active_record/associations/join_dependency.rb
index cd366ac8b7..e3d8356f49 100644
--- a/activerecord/lib/active_record/associations/join_dependency.rb
+++ b/activerecord/lib/active_record/associations/join_dependency.rb
@@ -109,7 +109,7 @@ module ActiveRecord
         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 03963ab060..becf1a3f62 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 @@ module ActiveRecord
 
         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/spawn_methods.rb b/activerecord/lib/active_record/relation/spawn_methods.rb
index b8e384c2f3..90f2ac3cde 100644
--- a/activerecord/lib/active_record/relation/spawn_methods.rb
+++ b/activerecord/lib/active_record/relation/spawn_methods.rb
@@ -27,7 +27,7 @@ module ActiveRecord
         merge_relation_method(merged_relation, method, value) if value.present?
       end
 
-      merged_relation.joins_values += r.joins_values
+      merge_joins(merged_relation, r)
 
       merged_wheres = @where_values + r.where_values
 
@@ -146,6 +146,32 @@ module ActiveRecord
 
     private
 
+      def merge_joins(relation, other)
+        values = other.joins_values
+        return if values.blank?
+
+        if other.klass == relation.klass
+          relation.joins_values += values
+        else
+          joins_dependency, rest = values.partition do |join|
+            case join
+            when Hash, Symbol, Array
+              true
+            else
+              false
+            end
+          end
+
+          join_dependency = ActiveRecord::Associations::JoinDependency.new(
+            other.klass,
+            joins_dependency,
+            []
+          )
+
+          relation.joins_values += join_dependency.join_associations + rest
+        end
+      end
+
       def merge_relation_method(relation, method, value)
         relation.send(:"#{method}_values=", relation.send(:"#{method}_values") + value)
       end
diff --git a/activerecord/test/cases/relations_test.rb b/activerecord/test/cases/relations_test.rb
index 98fe6b7611..f14eee2eb8 100644
--- a/activerecord/test/cases/relations_test.rb
+++ b/activerecord/test/cases/relations_test.rb
@@ -4,6 +4,7 @@ require 'models/tagging'
 require 'models/post'
 require 'models/topic'
 require 'models/comment'
+require 'models/rating'
 require 'models/reply'
 require 'models/author'
 require 'models/comment'
@@ -19,7 +20,7 @@ require 'models/minivan'
 
 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
@@ -696,6 +697,12 @@ class RelationTest < ActiveRecord::TestCase
     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).count.length
+  end
+
   def test_count
     posts = Post.scoped
 
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