Fix STI scopes using benolee's suggestion. Fixes #11939

activerecord/lib/active_record/relation/spawn_methods.rb

@@ -151,7 +151,7 @@ module ActiveRecord
         values = other.joins_values
         return if values.blank?
 
-        if other.klass == relation.klass
+        if other.klass >= relation.klass
           relation.joins_values += values
         else
           joins_dependency, rest = values.partition do |join|

activerecord/test/cases/inheritance_test.rb

@@ -236,6 +236,11 @@ class InheritanceTest < ActiveRecord::TestCase
     assert_nothing_raised { s = SpecialSubscriber.new("name" => "And breaaaaathe!"); s.id = 'roger'; s.save }
   end
 
+  def test_scope_inherited_properly
+    assert_nothing_raised { Company.of_first_firm }
+    assert_nothing_raised { Client.of_first_firm }
+  end
+
   private
     def switch_to_alt_inheritance_column
       # we don't want misleading test results, so get rid of the values in the type column

activerecord/test/models/company.rb

@@ -12,6 +12,11 @@ class Company < AbstractCompany
   has_many :contracts
   has_many :developers, :through => :contracts
 
+  scope :of_first_firm, lambda {
+    joins(:account => :firm).
+    where('firms.id' => 1)
+  }
+
   def arbitrary_method
     "I am Jack's profound disappointment"
   end