From d8d98b6143198b3c6cbe6f3aa4b5ad5bb099b1a3 Mon Sep 17 00:00:00 2001
From: Ollie Ford <me@ojford.com>
Date: Sat, 3 Dec 2016 21:10:05 +0000
Subject: [PATCH] Support Falsey `primary_key`s (#1354)

---
 mongoengine/document.py    |  6 +++---
 tests/document/instance.py | 15 +++++++++++++++
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/mongoengine/document.py b/mongoengine/document.py
index 607b1bd9..91dcafc4 100644
--- a/mongoengine/document.py
+++ b/mongoengine/document.py
@@ -472,7 +472,7 @@ class Document(BaseDocument):
         Raises :class:`OperationError` if called on an object that has not yet
         been saved.
         """
-        if not self.pk:
+        if self.pk is None:
             if kwargs.get('upsert', False):
                 query = self.to_mongo()
                 if "_cls" in query:
@@ -604,7 +604,7 @@ class Document(BaseDocument):
         elif "max_depth" in kwargs:
             max_depth = kwargs["max_depth"]
 
-        if not self.pk:
+        if self.pk is None:
             raise self.DoesNotExist("Document does not exist")
         obj = self._qs.read_preference(ReadPreference.PRIMARY).filter(
             **self._object_key).only(*fields).limit(
@@ -655,7 +655,7 @@ class Document(BaseDocument):
     def to_dbref(self):
         """Returns an instance of :class:`~bson.dbref.DBRef` useful in
         `__raw__` queries."""
-        if not self.pk:
+        if self.pk is None:
             msg = "Only saved documents can have a valid dbref"
             raise OperationError(msg)
         return DBRef(self.__class__._get_collection_name(), self.pk)
diff --git a/tests/document/instance.py b/tests/document/instance.py
index cb2c1746..1342f981 100644
--- a/tests/document/instance.py
+++ b/tests/document/instance.py
@@ -3202,5 +3202,20 @@ class InstanceTest(unittest.TestCase):
             self.assertEqual(b._instance, a)
         self.assertEqual(idx, 2)
 
+    def test_falsey_pk(self):
+        """Ensure that we can create and update a document with Falsey PK.
+        """
+        class Person(Document):
+            age = IntField(primary_key=True)
+            height = FloatField()
+
+        person = Person()
+        person.age = 0
+        person.height = 1.89
+        person.save()
+
+        person.update(set__height=2.0)
+
+
 if __name__ == '__main__':
     unittest.main()