Added the small changes necessary to make creating custom model fields easier.

Also includes some tests for this.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@6651 bcc190cf-cafb-0310-a4f2-bffc1f526a37

3 files changed, 56 insertions, 0 deletions

diff --git a/django/db/models/__init__.py b/django/db/models/__init__.py
index 4c712a0dc2..86763d99f9 100644
--- a/django/db/models/__init__.py
+++ b/django/db/models/__init__.py
@@ -7,6 +7,7 @@ from django.db.models.query import Q
 from django.db.models.manager import Manager
 from django.db.models.base import Model, AdminOptions
 from django.db.models.fields import *
+from django.db.models.fields.subclassing import SubfieldBase
 from django.db.models.fields.related import ForeignKey, OneToOneField, ManyToManyField, ManyToOneRel, ManyToManyRel, OneToOneRel, TABULAR, STACKED
 from django.db.models import signals
 from django.utils.functional import curry
diff --git a/django/db/models/fields/__init__.py b/django/db/models/fields/__init__.py
index 20a4f11f1d..b0dd55e3b9 100644
--- a/django/db/models/fields/__init__.py
+++ b/django/db/models/fields/__init__.py
@@ -147,6 +147,8 @@ class Field(object):
         # exactly which wacky database column type you want to use.
         data_types = get_creation_module().DATA_TYPES
         internal_type = self.get_internal_type()
+        if internal_type not in data_types:
+            return None
         return data_types[internal_type] % self.__dict__
 
     def validate_full(self, field_data, all_data):
diff --git a/django/db/models/fields/subclassing.py b/django/db/models/fields/subclassing.py
new file mode 100644
index 0000000000..1e4c8ca2e0
--- /dev/null
+++ b/django/db/models/fields/subclassing.py
@@ -0,0 +1,53 @@
+"""
+Convenience routines for creating non-trivial Field subclasses.
+
+Add SubfieldBase as the __metaclass__ for your Field subclass, implement
+to_python() and the other necessary methods and everything will work seamlessly.
+"""
+
+from django.utils.maxlength import LegacyMaxlength
+
+class SubfieldBase(LegacyMaxlength):
+    """
+    A metaclass for custom Field subclasses. This ensures the model's attribute
+    has the descriptor protocol attached to it.
+    """
+    def __new__(cls, base, name, attrs):
+        new_class = super(SubfieldBase, cls).__new__(cls, base, name, attrs)
+        new_class.contribute_to_class = make_contrib(
+                attrs.get('contribute_to_class'))
+        return new_class
+
+class Creator(object):
+    """
+    A placeholder class that provides a way to set the attribute on the model.
+    """
+    def __init__(self, field):
+        self.field = field
+
+    def __get__(self, obj, type=None):
+        if obj is None:
+            raise AttributeError('Can only be accessed via an instance.')
+        return self.value
+
+    def __set__(self, obj, value):
+        self.value = self.field.to_python(value)
+
+def make_contrib(func=None):
+    """
+    Returns a suitable contribute_to_class() method for the Field subclass.
+
+    If 'func' is passed in, it is the existing contribute_to_class() method on
+    the subclass and it is called before anything else. It is assumed in this
+    case that the existing contribute_to_class() calls all the necessary
+    superclass methods.
+    """
+    def contribute_to_class(self, cls, name):
+        if func:
+            func(self, cls, name)
+        else:
+            super(self.__class__, self).contribute_to_class(cls, name)
+        setattr(cls, self.name, Creator(self))
+
+    return contribute_to_class
+