Updated to handle the converntional api style for slicing a field

Added testcase to demonstrate embedded slicing as well.

Refs #167

mongoengine/queryset.py

@@ -548,7 +548,7 @@ class QuerySet(object):
         return '.'.join(parts)
 
     @classmethod
-    def _transform_query(cls, _doc_cls=None, **query):
+    def _transform_query(cls, _doc_cls=None, _field_operation=False, **query):
         """Transform a query from Django-style format to Mongo format.
         """
         operators = ['ne', 'gt', 'gte', 'lt', 'lte', 'in', 'nin', 'mod',
@@ -915,13 +915,26 @@ class QuerySet(object):
         You can use the $slice operator to retrieve a subrange of elements in
         an array ::
 
-            post = BlogPost.objects(...).fields(comments={"$slice": 5}) // first 5 comments
+            post = BlogPost.objects(...).fields(slice__comments=5) // first 5 comments
 
         :param kwargs: A dictionary identifying what to include
 
         .. versionadded:: 0.5
         """
-        fields = sorted(kwargs.iteritems(), key=operator.itemgetter(1))
+
+        # Check for an operator and transform to mongo-style if there is
+        operators = ["slice"]
+        cleaned_fields = []
+        for key, value in kwargs.items():
+            parts = key.split('__')
+            op = None
+            if parts[0] in operators:
+                op = parts.pop(0)
+                value = {'$' + op: value}
+            key = '.'.join(parts)
+            cleaned_fields.append((key, value))
+
+        fields = sorted(cleaned_fields, key=operator.itemgetter(1))
         for value, group in itertools.groupby(fields, lambda x: x[1]):
             fields = [field for field, value in group]
             fields = self._fields_to_dbfields(fields)

tests/queryset.py

@@ -597,10 +597,9 @@ class QuerySetTest(unittest.TestCase):
 
         Email.drop_collection()
 
-    def test_custom_fields(self):
+    def test_slicing_fields(self):
         """Ensure that query slicing an array works.
         """
-
         class Numbers(Document):
             n = ListField(IntField())
 
@@ -610,25 +609,69 @@ class QuerySetTest(unittest.TestCase):
         numbers.save()
 
         # first three
-        numbers = Numbers.objects.fields(n={"$slice": 3}).get()
+        numbers = Numbers.objects.fields(slice__n=3).get()
         self.assertEquals(numbers.n, [0, 1, 2])
 
         # last three
-        numbers = Numbers.objects.fields(n={"$slice": -3}).get()
+        numbers = Numbers.objects.fields(slice__n=-3).get()
         self.assertEquals(numbers.n, [-3, -2, -1])
 
         # skip 2, limit 3
-        numbers = Numbers.objects.fields(n={"$slice": [2, 3]}).get()
+        numbers = Numbers.objects.fields(slice__n=[2, 3]).get()
         self.assertEquals(numbers.n, [2, 3, 4])
 
         # skip to fifth from last, limit 4
-        numbers = Numbers.objects.fields(n={"$slice": [-5, 4]}).get()
+        numbers = Numbers.objects.fields(slice__n=[-5, 4]).get()
         self.assertEquals(numbers.n, [-5, -4, -3, -2])
 
         # skip to fifth from last, limit 10
+        numbers = Numbers.objects.fields(slice__n=[-5, 10]).get()
+        self.assertEquals(numbers.n, [-5, -4, -3, -2, -1])
+
+        # skip to fifth from last, limit 10 dict method
         numbers = Numbers.objects.fields(n={"$slice": [-5, 10]}).get()
         self.assertEquals(numbers.n, [-5, -4, -3, -2, -1])
 
+    def test_slicing_nested_fields(self):
+        """Ensure that query slicing an embedded array works.
+        """
+
+        class EmbeddedNumber(EmbeddedDocument):
+            n = ListField(IntField())
+
+        class Numbers(Document):
+            embedded = EmbeddedDocumentField(EmbeddedNumber)
+
+        Numbers.drop_collection()
+
+        numbers = Numbers()
+        numbers.embedded = EmbeddedNumber(n=[0,1,2,3,4,5,-5,-4,-3,-2,-1])
+        numbers.save()
+
+        # first three
+        numbers = Numbers.objects.fields(slice__embedded__n=3).get()
+        self.assertEquals(numbers.embedded.n, [0, 1, 2])
+
+        # last three
+        numbers = Numbers.objects.fields(slice__embedded__n=-3).get()
+        self.assertEquals(numbers.embedded.n, [-3, -2, -1])
+
+        # skip 2, limit 3
+        numbers = Numbers.objects.fields(slice__embedded__n=[2, 3]).get()
+        self.assertEquals(numbers.embedded.n, [2, 3, 4])
+
+        # skip to fifth from last, limit 4
+        numbers = Numbers.objects.fields(slice__embedded__n=[-5, 4]).get()
+        self.assertEquals(numbers.embedded.n, [-5, -4, -3, -2])
+
+        # skip to fifth from last, limit 10
+        numbers = Numbers.objects.fields(slice__embedded__n=[-5, 10]).get()
+        self.assertEquals(numbers.embedded.n, [-5, -4, -3, -2, -1])
+
+        # skip to fifth from last, limit 10 dict method
+        numbers = Numbers.objects.fields(embedded__n={"$slice": [-5, 10]}).get()
+        self.assertEquals(numbers.embedded.n, [-5, -4, -3, -2, -1])
+
     def test_find_embedded(self):
         """Ensure that an embedded document is properly returned from a query.
         """