Added support for multiple slices

Also made slicing chainable. (#170) (#190) (#191)

docs/changelog.rst

@@ -21,6 +21,7 @@ Changes in 0.8
 - Remove _types and just use _cls for inheritance (#148)
 - Only allow QNode instances to be passed as query objects (#199)
 - Dynamic fields are now validated on save (#153) (#154)
+- Added support for multiple slices and made slicing chainable. (#170) (#190) (#191)
 
 Changes in 0.7.9
 ================

mongoengine/queryset/field_list.py

@@ -12,20 +12,31 @@ class QueryFieldList(object):
         self.fields = set(fields)
         self.always_include = set(always_include)
         self._id = None
+        self.slice = {}
 
     def __add__(self, f):
+        if isinstance(f.value, dict):
+            for field in f.fields:
+                self.slice[field] = f.value
             if not self.fields:
                 self.fields = f.fields
+        elif not self.fields:
+            self.fields = f.fields
             self.value = f.value
+            self.slice = {}
         elif self.value is self.ONLY and f.value is self.ONLY:
+            self._clean_slice()
             self.fields = self.fields.intersection(f.fields)
         elif self.value is self.EXCLUDE and f.value is self.EXCLUDE:
             self.fields = self.fields.union(f.fields)
+            self._clean_slice()
         elif self.value is self.ONLY and f.value is self.EXCLUDE:
             self.fields -= f.fields
+            self._clean_slice()
         elif self.value is self.EXCLUDE and f.value is self.ONLY:
             self.value = self.ONLY
             self.fields = f.fields - self.fields
+            self._clean_slice()
 
         if '_id' in f.fields:
             self._id = f.value
@@ -42,10 +53,18 @@ class QueryFieldList(object):
 
     def as_dict(self):
         field_list = dict((field, self.value) for field in self.fields)
+        if self.slice:
+            field_list.update(self.slice)
         if self._id is not None:
             field_list['_id'] = self._id
         return field_list
 
     def reset(self):
         self.fields = set([])
+        self.slice = {}
         self.value = self.ONLY
+
+    def _clean_slice(self):
+        if self.slice:
+            for field in set(self.slice.keys()) - self.fields:
+                del self.slice[field]

tests/queryset/field_list.py

@@ -115,6 +115,35 @@ class OnlyExcludeAllTest(unittest.TestCase):
         qs = qs.only(*only)
         self.assertEqual(qs._loaded_fields.as_dict(), {'b': 1, 'c': 1})
 
+    def test_slicing(self):
+
+        class MyDoc(Document):
+            a = ListField()
+            b = ListField()
+            c = ListField()
+            d = ListField()
+            e = ListField()
+            f = ListField()
+
+        include = ['a', 'b', 'c', 'd', 'e']
+        exclude = ['d', 'e']
+        only = ['b', 'c']
+
+        qs = MyDoc.objects.fields(**dict(((i, 1) for i in include)))
+        qs = qs.exclude(*exclude)
+        qs = qs.only(*only)
+        qs = qs.fields(slice__b=5)
+        self.assertEqual(qs._loaded_fields.as_dict(),
+            {'b': {'$slice': 5}, 'c': 1})
+
+        qs = qs.fields(slice__c=[5, 1])
+        self.assertEqual(qs._loaded_fields.as_dict(),
+            {'b': {'$slice': 5}, 'c': {'$slice': [5, 1]}})
+
+        qs = qs.exclude('c')
+        self.assertEqual(qs._loaded_fields.as_dict(),
+            {'b': {'$slice': 5}})
+
     def test_only(self):
         """Ensure that QuerySet.only only returns the requested fields.
         """