From 3faf3c84be55f709d828ea7837db988c56b0239e Mon Sep 17 00:00:00 2001
From: Jonathan Prates <jonathan.prates@corp.terra.com.br>
Date: Tue, 27 May 2014 16:33:38 -0300
Subject: [PATCH] Avoid to open all documents from cursors in an if stmt

Using a cursos in an if statement:

cursor = Collection.objects

	if cursor:
		(...)

Will open all documents, because there are not an __nonzero__ method.
This change check only one document (if present) and returns True or False.
---
 mongoengine/queryset/base.py |  9 +++++++++
 tests/queryset/queryset.py   | 23 +++++++++++++++++++++++
 2 files changed, 32 insertions(+)

diff --git a/mongoengine/queryset/base.py b/mongoengine/queryset/base.py
index c2ad027e..823bc164 100644
--- a/mongoengine/queryset/base.py
+++ b/mongoengine/queryset/base.py
@@ -154,6 +154,15 @@ class BaseQuerySet(object):
     def __iter__(self):
         raise NotImplementedError
 
+    def __nonzero__(self):
+        """ Avoid to open all records in an if stmt """
+
+        for value in self:
+            self.rewind()
+            return True
+
+        return False        
+
     # Core functions
 
     def all(self):
diff --git a/tests/queryset/queryset.py b/tests/queryset/queryset.py
index 7ff2965d..f274e0ee 100644
--- a/tests/queryset/queryset.py
+++ b/tests/queryset/queryset.py
@@ -3814,6 +3814,29 @@ class QuerySetTest(unittest.TestCase):
         self.assertEqual(Example.objects(size=instance_size).count(), 1)
         self.assertEqual(Example.objects(size__in=[instance_size]).count(), 1)
 
+    def test_cursor_in_an_if_stmt(self):
+
+        class Test(Document):
+            test_field = StringField()
+
+        Test.drop_collection()
+        queryset = Test.objects
+
+        if queryset:
+            raise AssertionError('Empty cursor returns True')
+
+        test = Test()
+        test.test_field = 'test'
+        test.save()
+
+        queryset = Test.objects
+        if not test:
+            raise AssertionError('There is data, but cursor returned False')
+
+        queryset.next()
+        if queryset:
+            raise AssertionError('There is no data left in cursor')
+
 
 if __name__ == '__main__':
     unittest.main()