diff --git a/django_extras/contrib/auth/models.py b/django_extras/contrib/auth/models.py
index 72487e9..23cfeeb 100644
--- a/django_extras/contrib/auth/models.py
+++ b/django_extras/contrib/auth/models.py
@@ -129,7 +129,8 @@ class Document(SingleOwnerMixin, models.Model):
 
     """
     owner = models.ForeignKey(USER_MODEL_NAME, verbose_name=_('owner'),
-                              related_name='%(app_label)s_%(class)s_owner')
+                              related_name='%(app_label)s_%(class)s_owner',
+                              on_delete=models.CASCADE, null=True, blank=False)
 
     objects = SingleOwnerMixinManager()
 
diff --git a/django_extras/core/types.py b/django_extras/core/types.py
index eb6073b..458c33e 100644
--- a/django_extras/core/types.py
+++ b/django_extras/core/types.py
@@ -57,7 +57,7 @@ def decimal_value(value):
     return value
 
 
-class Money(object):
+class Money(decimal.Decimal):
     """
     Represents a monetary quantity.
     """
diff --git a/django_extras/db/models/fields/__init__.py b/django_extras/db/models/fields/__init__.py
index dcf40c0..3d7e8e9 100644
--- a/django_extras/db/models/fields/__init__.py
+++ b/django_extras/db/models/fields/__init__.py
@@ -71,7 +71,7 @@ def get_db_prep_save(self, value, connection):
         value = self.to_python(value)
         if value is not None:
             value = value._amount
-        return connection.ops.value_to_db_decimal(value, self.max_digits, self.decimal_places)
+        return connection.ops.adapt_decimalfield_value(value, self.max_digits, self.decimal_places)
 
 
 class PercentField(models.FloatField):
diff --git a/django_extras/db/models/fields/jsonfield.py b/django_extras/db/models/fields/jsonfield.py
index e25ce91..6271163 100644
--- a/django_extras/db/models/fields/jsonfield.py
+++ b/django_extras/db/models/fields/jsonfield.py
@@ -35,7 +35,7 @@ def __repr__(self):
         return dumps(self)
 
 
-class JsonField(six.with_metaclass(models.SubfieldBase, models.TextField)):
+class JsonField(models.TextField):
     """Field that serializes/de-serializes a python list/dictionary to the
     database seamlessly."""