Added Query.is_sliced property.

Previously, we used Query.can_filter() mainly to check if a query is
sliced what was confusing.

django/db/models/query.py

@@ -635,7 +635,7 @@ class QuerySet:
                 "arguments or 'get_latest_by' in the model's Meta."
             )
 
-        assert self.query.can_filter(), \
+        assert not self.query.is_sliced, \
             "Cannot change a query once a slice has been taken."
         obj = self._chain()
         obj.query.set_limits(high=1)
@@ -664,7 +664,7 @@ class QuerySet:
         Return a dictionary mapping each of the given IDs to the object with
         that ID. If `id_list` isn't provided, evaluate the entire QuerySet.
         """
-        assert self.query.can_filter(), \
+        assert not self.query.is_sliced, \
             "Cannot use 'limit' or 'offset' with in_bulk"
         if field_name != 'pk' and not self.model._meta.get_field(field_name).unique:
             raise ValueError("in_bulk()'s field_name must be a unique field but %r isn't." % field_name)
@@ -689,7 +689,7 @@ class QuerySet:
 
     def delete(self):
         """Delete the records in the current QuerySet."""
-        assert self.query.can_filter(), \
+        assert not self.query.is_sliced, \
             "Cannot use 'limit' or 'offset' with delete."
 
         if self._fields is not None:
@@ -731,7 +731,7 @@ class QuerySet:
         Update all elements in the current QuerySet, setting all the given
         fields to the appropriate values.
         """
-        assert self.query.can_filter(), \
+        assert not self.query.is_sliced, \
             "Cannot update a query once a slice has been taken."
         self._for_write = True
         query = self.query.chain(sql.UpdateQuery)
@@ -751,7 +751,7 @@ class QuerySet:
         code (it requires too much poking around at model internals to be
         useful at that level).
         """
-        assert self.query.can_filter(), \
+        assert not self.query.is_sliced, \
             "Cannot update a query once a slice has been taken."
         query = self.query.chain(sql.UpdateQuery)
         query.add_update_fields(values)
@@ -903,7 +903,7 @@ class QuerySet:
 
     def _filter_or_exclude(self, negate, *args, **kwargs):
         if args or kwargs:
-            assert self.query.can_filter(), \
+            assert not self.query.is_sliced, \
                 "Cannot filter a query once a slice has been taken."
 
         clone = self._chain()
@@ -1072,7 +1072,7 @@ class QuerySet:
 
     def order_by(self, *field_names):
         """Return a new QuerySet instance with the ordering changed."""
-        assert self.query.can_filter(), \
+        assert not self.query.is_sliced, \
             "Cannot reorder a query once a slice has been taken."
         obj = self._chain()
         obj.query.clear_ordering(force_empty=False)
@@ -1083,7 +1083,7 @@ class QuerySet:
         """
         Return a new QuerySet instance that will select only distinct results.
         """
-        assert self.query.can_filter(), \
+        assert not self.query.is_sliced, \
             "Cannot create distinct fields once a slice has been taken."
         obj = self._chain()
         obj.query.add_distinct_fields(*field_names)
@@ -1093,7 +1093,7 @@ class QuerySet:
               order_by=None, select_params=None):
         """Add extra SQL fragments to the query."""
         self._not_support_combined_queries('extra')
-        assert self.query.can_filter(), \
+        assert not self.query.is_sliced, \
             "Cannot change a query once a slice has been taken"
         clone = self._chain()
         clone.query.add_extra(select, select_params, where, params, tables, order_by)
@@ -1101,7 +1101,7 @@ class QuerySet:
 
     def reverse(self):
         """Reverse the ordering of the QuerySet."""
-        if not self.query.can_filter():
+        if self.query.is_sliced:
             raise TypeError('Cannot reverse a query once a slice has been taken.')
         clone = self._chain()
         clone.query.standard_ordering = not clone.query.standard_ordering

django/db/models/sql/query.py

@@ -413,7 +413,6 @@ class Query(BaseExpression):
         """
         if not self.annotation_select:
             return {}
-        has_limit = self.low_mark != 0 or self.high_mark is not None
         existing_annotations = [
             annotation for alias, annotation
             in self.annotations.items()
@@ -430,7 +429,7 @@ class Query(BaseExpression):
         # those operations must be done in a subquery so that the query
         # aggregates on the limit and/or distinct results instead of applying
         # the distinct and limit after the aggregation.
-        if (isinstance(self.group_by, tuple) or has_limit or existing_annotations or
+        if (isinstance(self.group_by, tuple) or self.is_sliced or existing_annotations or
                 self.distinct or self.combinator):
             from django.db.models.sql.subqueries import AggregateQuery
             outer_query = AggregateQuery(self.model)
@@ -438,7 +437,7 @@ class Query(BaseExpression):
             inner_query.select_for_update = False
             inner_query.select_related = False
             inner_query.set_annotation_mask(self.annotation_select)
-            if not has_limit and not self.distinct_fields:
+            if not self.is_sliced and not self.distinct_fields:
                 # Queries with distinct_fields need ordering and when a limit
                 # is applied we must take the slice from the ordered query.
                 # Otherwise no need for ordering.
@@ -548,7 +547,7 @@ class Query(BaseExpression):
         """
         assert self.model == rhs.model, \
             "Cannot combine queries on two different base models."
-        assert self.can_filter(), \
+        assert not self.is_sliced, \
             "Cannot combine queries once a slice has been taken."
         assert self.distinct == rhs.distinct, \
             "Cannot combine a unique query with a non-unique query."
@@ -1762,6 +1761,10 @@ class Query(BaseExpression):
         """Clear any existing limits."""
         self.low_mark, self.high_mark = 0, None
 
+    @property
+    def is_sliced(self):
+        return self.low_mark != 0 or self.high_mark is not None
+
     def has_limit_one(self):
         return self.high_mark is not None and (self.high_mark - self.low_mark) == 1
 
@@ -1771,7 +1774,7 @@ class Query(BaseExpression):
 
         Typically, this means no limits or offsets have been put on the results.
         """
-        return not self.low_mark and self.high_mark is None
+        return not self.is_sliced
 
     def clear_select_clause(self):
         """Remove all fields from SELECT clause."""