Deprecate `Class` handler in `PredicateBuilder`

Users should pass strings to queries instead of classes

3 files changed, 17 insertions, 4 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 298ee21e01..f0d9f1e538 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,8 @@
+*   Deprecate passing a class as a value in a query. Users should pass strings
+    instead.
+
+    *Melanie Gilman*
+
 *   `add_timestamps` and `remove_timestamps` now properly reversible with
     options.
 
diff --git a/activerecord/lib/active_record/relation/predicate_builder.rb b/activerecord/lib/active_record/relation/predicate_builder.rb
index eb21d01465..67e646bf18 100644
--- a/activerecord/lib/active_record/relation/predicate_builder.rb
+++ b/activerecord/lib/active_record/relation/predicate_builder.rb
@@ -35,7 +35,7 @@ module ActiveRecord
       # PriceEstimate.where(estimate_of: treasure)
       if klass && reflection = klass._reflect_on_association(column)
         if reflection.polymorphic? && base_class = polymorphic_base_class_from_value(value)
-          queries << self.class.build(table[reflection.foreign_type], base_class)
+          queries << self.class.build(table[reflection.foreign_type], base_class.name)
         end
 
         column = reflection.foreign_key
@@ -84,8 +84,7 @@ module ActiveRecord
     end
 
     register_handler(BasicObject, ->(attribute, value) { attribute.eq(value) })
-    # FIXME: I think we need to deprecate this behavior
-    register_handler(Class, ->(attribute, value) { attribute.eq(value.name) })
+    register_handler(Class, ->(attribute, value) { deprecate_class_handler; attribute.eq(value.name) })
     register_handler(Base, ->(attribute, value) { attribute.eq(value.id) })
     register_handler(Range, ->(attribute, value) { attribute.between(value) })
     register_handler(Relation, RelationHandler.new)
@@ -100,6 +99,13 @@ module ActiveRecord
     end
     private_class_method :handler_for
 
+    def self.deprecate_class_handler
+      ActiveSupport::Deprecation.warn(<<-MSG.squish)
+        Passing a class as a value in an Active Record query is deprecated and
+        will be removed. Pass a string instead.
+      MSG
+    end
+
     protected
 
     attr_reader :klass, :table
diff --git a/activerecord/test/cases/relations_test.rb b/activerecord/test/cases/relations_test.rb
index fc9637a167..3a0398d08d 100644
--- a/activerecord/test/cases/relations_test.rb
+++ b/activerecord/test/cases/relations_test.rb
@@ -712,7 +712,9 @@ class RelationTest < ActiveRecord::TestCase
 
   def test_find_by_classname
     Author.create!(:name => Mary.name)
-    assert_equal 1, Author.where(:name => Mary).size
+    assert_deprecated do
+      assert_equal 1, Author.where(:name => Mary).size
+    end
   end
 
   def test_find_by_id_with_list_of_ar