add ActiveRecord::Base.has_attribute?

`has_attribute?` method to check wether a given attribute has been defined.

activerecord/lib/active_record/attribute_methods.rb

@@ -191,6 +191,18 @@ module ActiveRecord
           end
       end
 
+      # Returns true if the given attribute exists, otherwise false.
+      #
+      #   class Person < ActiveRecord::Base
+      #   end
+      #
+      #   Person.has_attribute?('name')   # => true
+      #   Person.has_attribute?(:age)     # => true
+      #   Person.has_attribute?(:nothing) # => false
+      def has_attribute?(attr_name)
+        attribute_types.key?(attr_name.to_s)
+      end
+
       # Returns the column object for the named attribute.
       # Returns a +ActiveRecord::ConnectionAdapters::NullColumn+ if the
       # named attribute does not exist.

activerecord/lib/active_record/inheritance.rb

@@ -51,7 +51,7 @@ module ActiveRecord
         end
 
         attrs = args.first
-        if attribute_names.include?(inheritance_column)
+        if has_attribute?(inheritance_column)
           subclass = subclass_from_attributes(attrs) || subclass_from_defaults
         end
 
@@ -163,7 +163,7 @@ module ActiveRecord
       end
 
       def using_single_table_inheritance?(record)
-        record[inheritance_column].present? && columns_hash.include?(inheritance_column)
+        record[inheritance_column].present? && has_attribute?(inheritance_column)
       end
 
       def find_sti_class(type_name)

activerecord/test/cases/base_test.rb

@@ -1345,6 +1345,19 @@ class BasicsTest < ActiveRecord::TestCase
                  Company.attribute_names
   end
 
+  def test_has_attribute
+    assert Company.has_attribute?('id')
+    assert Company.has_attribute?('type')
+    assert Company.has_attribute?('name')
+    assert_not Company.has_attribute?('lastname')
+    assert_not Company.has_attribute?('age')
+  end
+
+  def test_has_attribute_with_symbol
+    assert Company.has_attribute?(:id)
+    assert_not Company.has_attribute?(:age)
+  end
+
   def test_attribute_names_on_table_not_exists
     assert_equal [], NonExistentTable.attribute_names
   end