13 files changed, 82 insertions, 16 deletions
diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 90b5cb3b8f..0b7fbe5a48 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,16 @@
+*   `scope_chain` should not be mutated for other reflections.
+
+    Currently `scope_chain` uses same array for building different
+    `scope_chain` for different associations. During processing
+    these arrays are sometimes mutated and because of in-place
+    mutation the changed `scope_chain` impacts other reflections.
+
+    Fix is to dup the value before adding to the `scope_chain`.
+
+    Fixes #3882.
+
+    *Neeraj Singh*
+
 *   Prevent the inversed association from being reloaded on save.
 
     Fixes #9499.
diff --git a/activerecord/lib/active_record/associations/preloader/through_association.rb b/activerecord/lib/active_record/associations/preloader/through_association.rb
index 3166df57eb..2a8530af62 100644
--- a/activerecord/lib/active_record/associations/preloader/through_association.rb
+++ b/activerecord/lib/active_record/associations/preloader/through_association.rb
@@ -29,29 +29,30 @@ module ActiveRecord
                                          source_reflection.name,
                                          reflection_scope)
 
+          @preloaded_records = preloaders.flat_map(&:preloaded_records)
+
           middle_to_pl = preloaders.each_with_object({}) do |pl,h|
             pl.owners.each { |middle|
               h[middle] = pl
             }
           end
 
+          record_offset = {}
+          @preloaded_records.each_with_index do |record,i|
+            record_offset[record] = i
+          end
+
           through_records.each_with_object({}) { |(lhs,center),records_by_owner|
             pl_to_middle = center.group_by { |record| middle_to_pl[record] }
 
             records_by_owner[lhs] = pl_to_middle.flat_map do |pl, middles|
               rhs_records = middles.flat_map { |r|
-                r.send(source_reflection.name)
+                association = r.association source_reflection.name
+
+                association.reader
               }.compact
 
-              loaded_records = pl.preloaded_records
-              i = 0
-              record_index = loaded_records.each_with_object({}) { |r,indexes|
-                indexes[r] = i
-                i += 1
-              }
-              records = rhs_records.sort_by { |rhs| record_index[rhs] }
-              @preloaded_records.concat rhs_records
-              records
+              rhs_records.sort_by { |rhs| record_offset[rhs] }
             end
           }
         end
diff --git a/activerecord/lib/active_record/reflection.rb b/activerecord/lib/active_record/reflection.rb
index e88c5d17cb..bce7766501 100644
--- a/activerecord/lib/active_record/reflection.rb
+++ b/activerecord/lib/active_record/reflection.rb
@@ -574,7 +574,7 @@ module ActiveRecord
           # Add to it the scope from this reflection (if any)
           scope_chain.first << scope if scope
 
-          through_scope_chain = through_reflection.scope_chain
+          through_scope_chain = through_reflection.scope_chain.map(&:dup)
 
           if options[:source_type]
             through_scope_chain.first <<
diff --git a/activerecord/test/cases/reflection_test.rb b/activerecord/test/cases/reflection_test.rb
index 0e5c7df2cc..d7ad5ed29f 100644
--- a/activerecord/test/cases/reflection_test.rb
+++ b/activerecord/test/cases/reflection_test.rb
@@ -18,6 +18,11 @@ require 'models/subscription'
 require 'models/tag'
 require 'models/sponsor'
 require 'models/edge'
+require 'models/hotel'
+require 'models/chef'
+require 'models/department'
+require 'models/cake_designer'
+require 'models/drink_designer'
 
 class ReflectionTest < ActiveRecord::TestCase
   include ActiveRecord::Reflection
@@ -227,6 +232,17 @@ class ReflectionTest < ActiveRecord::TestCase
     assert_equal expected, actual
   end
 
+  def test_scope_chain_does_not_interfere_with_hmt_with_polymorphic_case
+    @hotel = Hotel.create!
+    @department = @hotel.departments.create!
+    @department.chefs.create!(employable: CakeDesigner.create!)
+    @department.chefs.create!(employable: DrinkDesigner.create!)
+
+    assert_equal 1, @hotel.cake_designers.size
+    assert_equal 1, @hotel.drink_designers.size
+    assert_equal 2, @hotel.chefs.size
+  end
+
   def test_nested?
     assert !Author.reflect_on_association(:comments).nested?
     assert Author.reflect_on_association(:tags).nested?
diff --git a/activerecord/test/fixtures/sponsors.yml b/activerecord/test/fixtures/sponsors.yml
index bfc6b238b1..2da541c539 100644
--- a/activerecord/test/fixtures/sponsors.yml
+++ b/activerecord/test/fixtures/sponsors.yml
@@ -8,5 +8,5 @@ boring_club_sponsor_for_groucho:
   sponsorable_type: Member
 crazy_club_sponsor_for_groucho:
   sponsor_club: crazy_club
-  sponsorable_id: 2
+  sponsorable_id: 3
   sponsorable_type: Member
diff --git a/activerecord/test/models/cake_designer.rb b/activerecord/test/models/cake_designer.rb
new file mode 100644
index 0000000000..9c57ef573a
--- /dev/null
+++ b/activerecord/test/models/cake_designer.rb
@@ -0,0 +1,3 @@
+class CakeDesigner < ActiveRecord::Base
+  has_one :chef, as: :employable
+end
diff --git a/activerecord/test/models/chef.rb b/activerecord/test/models/chef.rb
new file mode 100644
index 0000000000..67a4e54f06
--- /dev/null
+++ b/activerecord/test/models/chef.rb
@@ -0,0 +1,3 @@
+class Chef < ActiveRecord::Base
+  belongs_to :employable, polymorphic: true
+end
diff --git a/activerecord/test/models/department.rb b/activerecord/test/models/department.rb
new file mode 100644
index 0000000000..08004a0ed3
--- /dev/null
+++ b/activerecord/test/models/department.rb
@@ -0,0 +1,4 @@
+class Department < ActiveRecord::Base
+  has_many :chefs
+  belongs_to :hotel
+end
diff --git a/activerecord/test/models/drink_designer.rb b/activerecord/test/models/drink_designer.rb
new file mode 100644
index 0000000000..2db968ef11
--- /dev/null
+++ b/activerecord/test/models/drink_designer.rb
@@ -0,0 +1,3 @@
+class DrinkDesigner < ActiveRecord::Base
+  has_one :chef, as: :employable
+end
diff --git a/activerecord/test/models/hotel.rb b/activerecord/test/models/hotel.rb
new file mode 100644
index 0000000000..b352cd22f3
--- /dev/null
+++ b/activerecord/test/models/hotel.rb
@@ -0,0 +1,6 @@
+class Hotel < ActiveRecord::Base
+  has_many :departments
+  has_many :chefs, through: :departments
+  has_many :cake_designers, source_type: 'CakeDesigner', source: :employable, through: :chefs
+  has_many :drink_designers, source_type: 'DrinkDesigner', source: :employable, through: :chefs
+end
diff --git a/activerecord/test/schema/schema.rb b/activerecord/test/schema/schema.rb
index 75711673a7..88a686d436 100644
--- a/activerecord/test/schema/schema.rb
+++ b/activerecord/test/schema/schema.rb
@@ -787,6 +787,22 @@ ActiveRecord::Schema.define do
     t.string 'from'
   end
 
+  create_table :hotels, force: true do |t|
+  end
+  create_table :departments, force: true do |t|
+    t.integer :hotel_id
+  end
+  create_table :cake_designers, force: true do |t|
+  end
+  create_table :drink_designers, force: true do |t|
+  end
+  create_table :chefs, force: true do |t|
+    t.integer :employable_id
+    t.string :employable_type
+    t.integer :department_id
+  end
+
+
   except 'SQLite' do
     # fk_test_has_fk should be before fk_test_has_pk
     create_table :fk_test_has_fk, :force => true do |t|
diff --git a/activesupport/lib/active_support/core_ext/class/attribute.rb b/activesupport/lib/active_support/core_ext/class/attribute.rb
index 83038f9da5..f2a221c396 100644
--- a/activesupport/lib/active_support/core_ext/class/attribute.rb
+++ b/activesupport/lib/active_support/core_ext/class/attribute.rb
@@ -118,7 +118,10 @@ class Class
   end
 
   private
-    def singleton_class?
-      ancestors.first != self
+
+    unless respond_to?(:singleton_class?)
+      def singleton_class?
+        ancestors.first != self
+      end
     end
 end
diff --git a/guides/source/active_record_validations.md b/guides/source/active_record_validations.md
index 797b996357..0df52a655f 100644
--- a/guides/source/active_record_validations.md
+++ b/guides/source/active_record_validations.md
@@ -438,8 +438,6 @@ provide a personalized message or use `presence: true` instead. When
 `:in` or `:within` have a lower limit of 1, you should either provide a
 personalized message or call `presence` prior to `length`.
 
-The `size` helper is an alias for `length`.
-
 ### `numericality`
 
 This helper validates that your attributes have only numeric values. By