Ticket #3982: lazy_attribute.diff

django/db/models/fields/__init__.py

@@ -357 +357 @@
         "Returns the value of this field in the given model instance."
         return getattr(obj, self.attname)
 
+    def lazy_attribute(self, cls, creator=None, init=None):
+        return AttributeProxy(self, cls, creator, init)
+
+class AttributeProxy(object):
+    def __init__(self, field, cls, creator=None, init=None):
+        self.field, self.cls = field, cls
+        self.creator = creator or self.cls
+        self.init = init or (lambda x: None)
+
+    def create(self, value):
+        # Creates the attribute and makes sure it's a valid type
+        value = self.creator(value)
+        assert isinstance(value, self.cls), \
+            "'%s.%s' did not return an object of type '%s'" % (
+                self.field.model.object_name,
+                self.field.attname,
+                self.cls.__name__,
+            )
+        return value
+        
+    def __get__(self, obj, type=None):
+        value = obj.__dict__[self.field.attname]
+        if not getattr(obj, self.field.get_cache_name()):
+            setattr(obj, self.field.attname, value)
+        return obj.__dict__[self.field.attname]
+
+    def __set__(self, obj, value):
+        if hasattr(obj, self.field.get_cache_name()):
+            setattr(obj, self.field.get_cache_name(), False)
+            if value is not None:
+                if not isinstance(value, self.cls):
+                    value = self.create(value)
+                if not getattr(obj, self.field.get_cache_name()):
+                    self.init(value)
+                    setattr(obj, self.field.get_cache_name(), True)
+        else:
+            # This cache indicates whether the attribute has been instantiated
+            setattr(obj, self.field.get_cache_name(), False)
+        obj.__dict__[self.field.attname] = value
+
 class AutoField(Field):
     empty_strings_allowed = False
     def __init__(self, *args, **kwargs):