diff --git a/activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb b/activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb
index 9ae452e7a1..6f17b755db 100644
--- a/activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb
+++ b/activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb
@@ -6,7 +6,7 @@ module ActiveRecord
     class BelongsToPolymorphicAssociation < BelongsToAssociation #:nodoc:
       def klass
         type = owner[reflection.foreign_type]
-        type.presence && type.constantize
+        type.presence && owner.class.polymorphic_class_for(type)
       end
 
       def target_changed?
diff --git a/activerecord/lib/active_record/inheritance.rb b/activerecord/lib/active_record/inheritance.rb
index 5ca48fa18c..bb67f7f1b0 100644
--- a/activerecord/lib/active_record/inheritance.rb
+++ b/activerecord/lib/active_record/inheritance.rb
@@ -162,14 +162,40 @@ module ActiveRecord
         defined?(@abstract_class) && @abstract_class == true
       end
 
+      # Returns the value to be stored in the inheritance column for STI.
       def sti_name
         store_full_sti_class ? name : name.demodulize
       end
 
+      # Returns the class for the provided +type_name+.
+      #
+      # It is used to find the class correspondent to the value stored in the inheritance column.
+      def sti_class_for(type_name)
+        if store_full_sti_class
+          ActiveSupport::Dependencies.constantize(type_name)
+        else
+          compute_type(type_name)
+        end
+      rescue NameError
+        raise SubclassNotFound,
+          "The single-table inheritance mechanism failed to locate the subclass: '#{type_name}'. " \
+          "This error is raised because the column '#{inheritance_column}' is reserved for storing the class in case of inheritance. " \
+          "Please rename this column if you didn't intend it to be used for storing the inheritance class " \
+          "or overwrite #{name}.inheritance_column to use another column for that information."
+      end
+
+      # Returns the value to be stored in the polymorphic type column for Polymorphic Associations.
       def polymorphic_name
         base_class.name
       end
 
+      # Returns the class for the provided +name+.
+      #
+      # It is used to find the class correspondent to the value stored in the polymorphic type column.
+      def polymorphic_class_for(name)
+        name.constantize
+      end
+
       def inherited(subclass)
         subclass.instance_variable_set(:@_type_candidates_cache, Concurrent::Map.new)
         super
@@ -224,22 +250,12 @@ module ActiveRecord
 
         def find_sti_class(type_name)
           type_name = base_class.type_for_attribute(inheritance_column).cast(type_name)
-          subclass = begin
-            if store_full_sti_class
-              ActiveSupport::Dependencies.constantize(type_name)
-            else
-              compute_type(type_name)
-            end
-          rescue NameError
-            raise SubclassNotFound,
-              "The single-table inheritance mechanism failed to locate the subclass: '#{type_name}'. " \
-              "This error is raised because the column '#{inheritance_column}' is reserved for storing the class in case of inheritance. " \
-              "Please rename this column if you didn't intend it to be used for storing the inheritance class " \
-              "or overwrite #{name}.inheritance_column to use another column for that information."
-          end
+          subclass = sti_class_for(type_name)
+
           unless subclass == self || descendants.include?(subclass)
             raise SubclassNotFound, "Invalid single-table inheritance type: #{subclass.name} is not a subclass of #{name}"
           end
+
           subclass
         end