Add LazyReferenceField

mongoengine/base/__init__.py

@@ -15,7 +15,7 @@ __all__ = (
     'UPDATE_OPERATORS', '_document_registry', 'get_document',
 
     # datastructures
-    'BaseDict', 'BaseList', 'EmbeddedDocumentList',
+    'BaseDict', 'BaseList', 'EmbeddedDocumentList', 'LazyReference',
 
     # document
     'BaseDocument',

mongoengine/base/datastructures.py

@@ -2,11 +2,12 @@ import itertools
 import weakref
 
 import six
+from bson import DBRef
 
 from mongoengine.common import _import_class
 from mongoengine.errors import DoesNotExist, MultipleObjectsReturned
 
-__all__ = ('BaseDict', 'BaseList', 'EmbeddedDocumentList')
+__all__ = ('BaseDict', 'BaseList', 'EmbeddedDocumentList', 'LazyReference')
 
 
 class BaseDict(dict):
@@ -445,3 +446,42 @@ class StrictDict(object):
 
             cls._classes[allowed_keys] = SpecificStrictDict
         return cls._classes[allowed_keys]
+
+
+class LazyReference(DBRef):
+    __slots__ = ('_cached_doc', 'passthrough', 'document_type')
+
+    def fetch(self, force=False):
+        if not self._cached_doc or force:
+            self._cached_doc = self.document_type.objects.get(pk=self.pk)
+            if not self._cached_doc:
+                raise DoesNotExist('Trying to dereference unknown document %s' % (self))
+        return self._cached_doc
+
+    @property
+    def pk(self):
+        return self.id
+
+    def __init__(self, document_type, pk, cached_doc=None, passthrough=False):
+        self.document_type = document_type
+        self._cached_doc = cached_doc
+        self.passthrough = passthrough
+        super(LazyReference, self).__init__(self.document_type._get_collection_name(), pk)
+
+    def __getitem__(self, name):
+        if not self.passthrough:
+            raise KeyError()
+        document = self.fetch()
+        return document[name]
+
+    def __getattr__(self, name):
+        if not object.__getattribute__(self, 'passthrough'):
+            raise AttributeError()
+        document = self.fetch()
+        try:
+            return document[name]
+        except KeyError:
+            raise AttributeError()
+
+    def __repr__(self):
+        return "<LazyReference(%s, %r)>" % (self.document_type, self.pk)