diff --git a/actionpack/lib/action_controller/model_naming.rb b/actionpack/lib/action_controller/model_naming.rb
index 785221dc3d..2b33f67263 100644
--- a/actionpack/lib/action_controller/model_naming.rb
+++ b/actionpack/lib/action_controller/model_naming.rb
@@ -6,7 +6,7 @@ module ActionController
     end
 
     def model_name_from_record_or_class(record_or_class)
-      (record_or_class.is_a?(Class) ? record_or_class : convert_to_model(record_or_class).class).model_name
+      convert_to_model(record_or_class).model_name
     end
   end
 end
diff --git a/actionpack/lib/action_dispatch/routing/polymorphic_routes.rb b/actionpack/lib/action_dispatch/routing/polymorphic_routes.rb
index cd8b1ab066..427a5674bd 100644
--- a/actionpack/lib/action_dispatch/routing/polymorphic_routes.rb
+++ b/actionpack/lib/action_dispatch/routing/polymorphic_routes.rb
@@ -254,9 +254,9 @@ module ActionDispatch
           model = record.to_model
           name = if record.persisted?
                    args << model
-                   model.class.model_name.singular_route_key
+                   model.model_name.singular_route_key
                  else
-                   @key_strategy.call model.class.model_name
+                   @key_strategy.call model.model_name
                  end
 
           named_route = prefix + "#{name}_#{suffix}"
@@ -284,7 +284,7 @@ module ActionDispatch
               parent.model_name.singular_route_key
             else
               args << parent.to_model
-              parent.to_model.class.model_name.singular_route_key
+              parent.to_model.model_name.singular_route_key
             end
           }
 
@@ -297,9 +297,9 @@ module ActionDispatch
           else
             if record.persisted?
               args << record.to_model
-              record.to_model.class.model_name.singular_route_key
+              record.to_model.model_name.singular_route_key
             else
-              @key_strategy.call record.to_model.class.model_name
+              @key_strategy.call record.to_model.model_name
             end
           end
 
diff --git a/actionview/lib/action_view/helpers/form_helper.rb b/actionview/lib/action_view/helpers/form_helper.rb
index c6bc0c9e38..0582cb3e12 100644
--- a/actionview/lib/action_view/helpers/form_helper.rb
+++ b/actionview/lib/action_view/helpers/form_helper.rb
@@ -1863,8 +1863,8 @@ module ActionView
           object = convert_to_model(@object)
           key    = object ? (object.persisted? ? :update : :create) : :submit
 
-          model = if object.class.respond_to?(:model_name)
-            object.class.model_name.human
+          model = if object.respond_to?(:model_name)
+            object.model_name.human
           else
             @object_name.to_s.humanize
           end
diff --git a/actionview/lib/action_view/helpers/tags/label.rb b/actionview/lib/action_view/helpers/tags/label.rb
index a5bcaf8153..39b2f48c39 100644
--- a/actionview/lib/action_view/helpers/tags/label.rb
+++ b/actionview/lib/action_view/helpers/tags/label.rb
@@ -40,7 +40,7 @@ module ActionView
                         @object_name.gsub!(/\[(.*)_attributes\]\[\d+\]/, '.\1')
 
                         if object.respond_to?(:to_model)
-                          key = object.class.model_name.i18n_key
+                          key = object.model_name.i18n_key
                           i18n_default = ["#{key}.#{method_and_value}".to_sym, ""]
                         end
 
diff --git a/actionview/lib/action_view/model_naming.rb b/actionview/lib/action_view/model_naming.rb
index e09ebd60df..d42e436b17 100644
--- a/actionview/lib/action_view/model_naming.rb
+++ b/actionview/lib/action_view/model_naming.rb
@@ -6,7 +6,7 @@ module ActionView
     end
 
     def model_name_from_record_or_class(record_or_class)
-      (record_or_class.is_a?(Class) ? record_or_class : convert_to_model(record_or_class).class).model_name
+      convert_to_model(record_or_class).model_name
     end
   end
 end
diff --git a/actionview/test/activerecord/polymorphic_routes_test.rb b/actionview/test/activerecord/polymorphic_routes_test.rb
index e220dcb8cb..0d97ddb2f6 100644
--- a/actionview/test/activerecord/polymorphic_routes_test.rb
+++ b/actionview/test/activerecord/polymorphic_routes_test.rb
@@ -34,8 +34,8 @@ class ModelDelegator < ActiveRecord::Base
 end
 
 class ModelDelegate
-  def self.model_name
-    ActiveModel::Name.new(self)
+  def model_name
+    ActiveModel::Name.new(self.class)
   end
 
   def to_param
diff --git a/activemodel/lib/active_model/errors.rb b/activemodel/lib/active_model/errors.rb
index 1d025beeef..fc8034f9c7 100644
--- a/activemodel/lib/active_model/errors.rb
+++ b/activemodel/lib/active_model/errors.rb
@@ -434,7 +434,7 @@ module ActiveModel
 
       options = {
         default: defaults,
-        model: @base.class.model_name.human,
+        model: @base.model_name.human,
         attribute: @base.class.human_attribute_name(attribute),
         value: value
       }.merge!(options)
diff --git a/activemodel/lib/active_model/lint.rb b/activemodel/lib/active_model/lint.rb
index c6bc18b008..38087521a2 100644
--- a/activemodel/lib/active_model/lint.rb
+++ b/activemodel/lib/active_model/lint.rb
@@ -73,16 +73,19 @@ module ActiveModel
 
       # == \Naming
       #
-      # Model.model_name must return a string with some convenience methods:
-      # <tt>:human</tt>, <tt>:singular</tt> and <tt>:plural</tt>. Check
-      # ActiveModel::Naming for more information.
+      # Model.model_name and Model#model_name must return a string with some
+      # convenience methods: # <tt>:human</tt>, <tt>:singular</tt> and
+      # <tt>:plural</tt>. Check ActiveModel::Naming for more information.
       def test_model_naming
-        assert model.class.respond_to?(:model_name), "The model should respond to model_name"
+        assert model.class.respond_to?(:model_name), "The model class should respond to model_name"
         model_name = model.class.model_name
         assert model_name.respond_to?(:to_str)
         assert model_name.human.respond_to?(:to_str)
         assert model_name.singular.respond_to?(:to_str)
         assert model_name.plural.respond_to?(:to_str)
+
+        assert model.respond_to?(:model_name), "The model instance should respond to model_name"
+        assert_equal model.model_name, model.class.model_name
       end
 
       # == \Errors Testing
diff --git a/activemodel/lib/active_model/naming.rb b/activemodel/lib/active_model/naming.rb
index 241e88deeb..d9ec502f88 100644
--- a/activemodel/lib/active_model/naming.rb
+++ b/activemodel/lib/active_model/naming.rb
@@ -307,7 +307,7 @@ module ActiveModel
       if record_or_class.respond_to?(:model_name)
         record_or_class.model_name
       elsif record_or_class.respond_to?(:to_model)
-        record_or_class.to_model.class.model_name
+        record_or_class.to_model.model_name
       else
         record_or_class.class.model_name
       end
diff --git a/activemodel/lib/active_model/serializers/json.rb b/activemodel/lib/active_model/serializers/json.rb
index c58e73f6a7..77f2a64b11 100644
--- a/activemodel/lib/active_model/serializers/json.rb
+++ b/activemodel/lib/active_model/serializers/json.rb
@@ -93,7 +93,7 @@ module ActiveModel
         end
 
         if root
-          root = self.class.model_name.element if root == true
+          root = model_name.element if root == true
           { root => serializable_hash(options) }
         else
           serializable_hash(options)
diff --git a/activemodel/lib/active_model/serializers/xml.rb b/activemodel/lib/active_model/serializers/xml.rb
index 7f99536dbb..3ad3bf30ad 100644
--- a/activemodel/lib/active_model/serializers/xml.rb
+++ b/activemodel/lib/active_model/serializers/xml.rb
@@ -84,7 +84,7 @@ module ActiveModel
           @builder = options[:builder]
           @builder.instruct! unless options[:skip_instruct]
 
-          root = (options[:root] || @serializable.class.model_name.element).to_s
+          root = (options[:root] || @serializable.model_name.element).to_s
           root = ActiveSupport::XmlMini.rename_key(root, options)
 
           args = [root]
diff --git a/activemodel/lib/active_model/validations.rb b/activemodel/lib/active_model/validations.rb
index f67a3be5c1..7ee033ba5f 100644
--- a/activemodel/lib/active_model/validations.rb
+++ b/activemodel/lib/active_model/validations.rb
@@ -39,6 +39,7 @@ module ActiveModel
     extend ActiveSupport::Concern
 
     included do
+      extend ActiveModel::Naming
       extend ActiveModel::Callbacks
       extend ActiveModel::Translation
 
diff --git a/activerecord/lib/active_record/integration.rb b/activerecord/lib/active_record/integration.rb
index 31e2518540..15b2f65dcb 100644
--- a/activerecord/lib/active_record/integration.rb
+++ b/activerecord/lib/active_record/integration.rb
@@ -55,16 +55,16 @@ module ActiveRecord
     def cache_key(*timestamp_names)
       case
       when new_record?
-        "#{self.class.model_name.cache_key}/new"
+        "#{model_name.cache_key}/new"
       when timestamp_names.any?
         timestamp = max_updated_column_timestamp(timestamp_names)
         timestamp = timestamp.utc.to_s(cache_timestamp_format)
-        "#{self.class.model_name.cache_key}/#{id}-#{timestamp}"
+        "#{model_name.cache_key}/#{id}-#{timestamp}"
       when timestamp = max_updated_column_timestamp
         timestamp = timestamp.utc.to_s(cache_timestamp_format)
-        "#{self.class.model_name.cache_key}/#{id}-#{timestamp}"
+        "#{model_name.cache_key}/#{id}-#{timestamp}"
       else
-        "#{self.class.model_name.cache_key}/#{id}"
+        "#{model_name.cache_key}/#{id}"
       end
     end