Fixed #10202 -- Fixed another couple of slicing edge-cases with querysets.

Patch from Alex Gaynor and Ramiro Morales.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9924 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/db/models/sql/query.py

@@ -345,7 +345,7 @@ class BaseQuery(object):
         # in SQL (in variants that provide them) doesn't change the COUNT
         # output.
         number = max(0, number - self.low_mark)
-        if self.high_mark:
+        if self.high_mark is not None:
             number = min(number, self.high_mark - self.low_mark)
 
         return number
@@ -1760,7 +1760,7 @@ class BaseQuery(object):
 
         Typically, this means no limits or offsets have been put on the results.
         """
-        return not (self.low_mark or self.high_mark)
+        return not self.low_mark and self.high_mark is None
 
     def clear_select_fields(self):
         """

tests/regressiontests/queries/models.py

@@ -903,11 +903,17 @@ used in lookups.
 >>> Item.objects.filter(created__in=[time1, time2])
 [<Item: one>, <Item: two>]
 
-Bug #7698 -- People like to slice with '0' as the high-water mark.
+Bug #7698, #10202 -- People like to slice with '0' as the high-water mark.
 >>> Item.objects.all()[0:0]
 []
 >>> Item.objects.all()[0:0][:10]
 []
+>>> Item.objects.all()[:0].count()
+0
+>>> Item.objects.all()[:0].latest('created')
+Traceback (most recent call last):
+    ...
+AssertionError: Cannot change a query once a slice has been taken.
 
 Bug #7411 - saving to db must work even with partially read result set in
 another cursor.