diff --git a/actionpack/lib/action_controller/model_naming.rb b/actionpack/lib/action_controller/model_naming.rb
index 785221dc3dca644866d1f40c7d3b09f250f70f3b..2b33f6726315e7703ac6e4d49651ed9cc101000c 100644
--- a/actionpack/lib/action_controller/model_naming.rb
+++ b/actionpack/lib/action_controller/model_naming.rb
@@ -6,7 +6,7 @@ def convert_to_model(object)
     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 bd3696cda16d07636b1b427e7f1e6d12450817c5..746e4cd2458728cc733735c3e10bf69c1797526d 100644
--- a/actionpack/lib/action_dispatch/routing/polymorphic_routes.rb
+++ b/actionpack/lib/action_dispatch/routing/polymorphic_routes.rb
@@ -249,9 +249,9 @@ def handle_model(record)
           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}"
@@ -279,7 +279,7 @@ def handle_list(list)
               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
           }
 
@@ -292,9 +292,9 @@ def handle_list(list)
           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 789a413c8d2501882733623ae93fcebeb6e9de97..bd8e3b19735aae87e415e0cec67dc0444d29841b 100644
--- a/actionview/lib/action_view/helpers/form_helper.rb
+++ b/actionview/lib/action_view/helpers/form_helper.rb
@@ -1815,8 +1815,8 @@ def submit_default_value
           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 a5bcaf81537249c6d92ed4ca345e57621610a1bb..39b2f48c390b1000ce9e94581a428b6ae37ba900 100644
--- a/actionview/lib/action_view/helpers/tags/label.rb
+++ b/actionview/lib/action_view/helpers/tags/label.rb
@@ -40,7 +40,7 @@ def render(&block)
                         @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 e09ebd60df95bc77cc3a9e9020078d257ea674e3..d42e436b1706b415a7907decffab7c8d180af8e2 100644
--- a/actionview/lib/action_view/model_naming.rb
+++ b/actionview/lib/action_view/model_naming.rb
@@ -6,7 +6,7 @@ def convert_to_model(object)
     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 fef27ef492af7fec012ce883b8f50ae3d1090ab9..b03fe66e2363f36d0edc79ca3d9339b5c3c3b7f3 100644
--- a/actionview/test/activerecord/polymorphic_routes_test.rb
+++ b/actionview/test/activerecord/polymorphic_routes_test.rb
@@ -34,8 +34,8 @@ def to_model
 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 917d3b9142f4ddeb6fffc963775a288daf733ded..95029fd5383f209ce082c97ca3a8b2dc7cea7185 100644
--- a/activemodel/lib/active_model/errors.rb
+++ b/activemodel/lib/active_model/errors.rb
@@ -434,7 +434,7 @@ def generate_message(attribute, type = :invalid, options = {})
 
       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 c6bc18b00864607f0f6f0301314d083d3281a22f..38087521a2f5f54022a23853d5bfff2014b4743f 100644
--- a/activemodel/lib/active_model/lint.rb
+++ b/activemodel/lib/active_model/lint.rb
@@ -73,16 +73,19 @@ def test_persisted?
 
       # == \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 8cf1a191f4b5aa4b58c37b57d7745063af1b8b5e..ef3449b7cb1b55d56b59a052fcee11428fa0e33d 100644
--- a/activemodel/lib/active_model/naming.rb
+++ b/activemodel/lib/active_model/naming.rb
@@ -308,7 +308,7 @@ def self.model_name_from_record_or_class(record_or_class) #:nodoc:
       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 c58e73f6a78e980699587d19826fbc7ca77d3eb4..77f2a64b11cc16f4cc8bcf804a82f5a09043f167 100644
--- a/activemodel/lib/active_model/serializers/json.rb
+++ b/activemodel/lib/active_model/serializers/json.rb
@@ -93,7 +93,7 @@ def as_json(options = nil)
         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 7f99536dbbc56cc545da1fe24666784654220917..3ad3bf30ad2eb2a55ec6e0e6c86a14551f2e720f 100644
--- a/activemodel/lib/active_model/serializers/xml.rb
+++ b/activemodel/lib/active_model/serializers/xml.rb
@@ -84,7 +84,7 @@ def serialize
           @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 cf97f45dba94ab1127891dc55ebbefaa17dfc3fb..e5468633051c323cea463768c40a70e3aae4526d 100644
--- a/activemodel/lib/active_model/validations.rb
+++ b/activemodel/lib/active_model/validations.rb
@@ -39,6 +39,7 @@ module Validations
     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 31e2518540d6af3550a459af2e3310905fa84570..15b2f65dcb2f49b5ee46f1dbe08a4f72372f7f0f 100644
--- a/activerecord/lib/active_record/integration.rb
+++ b/activerecord/lib/active_record/integration.rb
@@ -55,16 +55,16 @@ def to_param
     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