`source_type_scope` should respect correct table alias

`join_scopes` in `PolymorphicReflection` is passed aliased `table`, so
it should be respected for `source_type_scope`.

Closes #13969.
Fixes #13920.
Fixes #15190.

activerecord/lib/active_record/reflection.rb

@@ -839,10 +839,6 @@ def join_scopes(table, predicate_builder) # :nodoc:
         source_reflection.join_scopes(table, predicate_builder) + super
       end
 
-      def source_type_scope
-        through_reflection.klass.where(foreign_type => options[:source_type])
-      end
-
       def has_scope?
         scope || options[:source_type] ||
           source_reflection.has_scope? ||
@@ -1011,15 +1007,15 @@ def initialize(reflection, previous_reflection)
 
       def join_scopes(table, predicate_builder) # :nodoc:
         scopes = @previous_reflection.join_scopes(table, predicate_builder) + super
-        scopes << @previous_reflection.source_type_scope
+        scopes << build_scope(table, predicate_builder).instance_exec(nil, &source_type_scope)
       end
 
       def constraints
-        @reflection.constraints + [source_type_info]
+        @reflection.constraints + [source_type_scope]
       end
 
       private
-        def source_type_info
+        def source_type_scope
           type = @previous_reflection.foreign_type
           source_type = @previous_reflection.options[:source_type]
           lambda { |object| where(type => source_type) }

activerecord/test/cases/associations/nested_through_associations_test.rb

@@ -24,6 +24,11 @@
 require "models/categorization"
 require "models/membership"
 require "models/essay"
+require "models/hotel"
+require "models/department"
+require "models/chef"
+require "models/cake_designer"
+require "models/drink_designer"
 
 class NestedThroughAssociationsTest < ActiveRecord::TestCase
   fixtures :authors, :author_addresses, :books, :posts, :subscriptions, :subscribers, :tags, :taggings,
@@ -574,6 +579,15 @@ def test_nested_has_many_through_should_not_be_autosaved
     assert !c.post_taggings.empty?
   end
 
+  def test_polymorphic_has_many_through_joined_different_table_twice
+    cake_designer = CakeDesigner.create!(chef: Chef.new)
+    drink_designer = DrinkDesigner.create!(chef: Chef.new)
+    department = Department.create!(chefs: [cake_designer.chef, drink_designer.chef])
+    hotel = Hotel.create!(departments: [department])
+
+    assert_equal hotel, Hotel.joins(:cake_designers, :drink_designers).take
+  end
+
   private
 
     def assert_includes_and_joins_equal(query, expected, association)