Updated .only() behaviour - now like exclude it is chainable (#202)

mongoengine/queryset/field_list.py

@@ -7,11 +7,20 @@ class QueryFieldList(object):
     ONLY = 1
     EXCLUDE = 0
 
-    def __init__(self, fields=[], value=ONLY, always_include=[]):
+    def __init__(self, fields=None, value=ONLY, always_include=None, _only_called=False):
+        """The QueryFieldList builder
+
+        :param fields: A list of fields used in `.only()` or `.exclude()`
+        :param value: How to handle the fields; either `ONLY` or `EXCLUDE`
+        :param always_include: Any fields to always_include eg `_cls`
+        :param _only_called: Has `.only()` been called?  If so its a set of fields
+           otherwise it performs a union.
+        """
         self.value = value
-        self.fields = set(fields)
-        self.always_include = set(always_include)
+        self.fields = set(fields or [])
+        self.always_include = set(always_include or [])
         self._id = None
+        self._only_called = _only_called
         self.slice = {}
 
     def __add__(self, f):
@@ -26,7 +35,10 @@ class QueryFieldList(object):
             self.slice = {}
         elif self.value is self.ONLY and f.value is self.ONLY:
             self._clean_slice()
-            self.fields = self.fields.intersection(f.fields)
+            if self._only_called:
+                self.fields = self.fields.union(f.fields)
+            else:
+                self.fields = f.fields
         elif self.value is self.EXCLUDE and f.value is self.EXCLUDE:
             self.fields = self.fields.union(f.fields)
             self._clean_slice()
@@ -46,6 +58,9 @@ class QueryFieldList(object):
                 self.fields = self.fields.union(self.always_include)
             else:
                 self.fields -= self.always_include
+
+        if getattr(f, '_only_called', False):
+            self._only_called = True
         return self
 
     def __nonzero__(self):