Ticket #5012: negative_sliced_querysets.patch

django/db/backends/mysql/base.py

@@ -177 +177 @@
-def get_datetime_cast_sql():
-    return None
-
-
+
+
+
-    sql = "LIMIT "
-
+
+
+
+
+
+
-        sql += "%s," % offset
-    return sql + str(limit)
-

django/db/backends/mysql_old/base.py

@@ -192 +192 @@
-def get_datetime_cast_sql():
-    return None
-
-
+
+
+
-    sql = "LIMIT "
-
+
+
+
+
+
+
-        sql += "%s," % offset
-    return sql + str(limit)
-

django/db/backends/postgresql/base.py

@@ -158 +158 @@
-def get_datetime_cast_sql():
-    return None
-
-
-
-
+
+
+
+
+
+
-        sql += " OFFSET %s" % offset
-    return sql
-

django/db/backends/postgresql_psycopg2/base.py

@@ -112 +112 @@
-def get_datetime_cast_sql():
-    return None
-
-
-
-
+
+
+
+
+
-        sql += " OFFSET %s" % offset
-    return sql
-

django/db/backends/sqlite3/base.py

@@ -142 +142 @@
-def get_datetime_cast_sql():
-    return None
-
-
-
+
+
+
+
+
-    if offset and offset != 0:
-        sql += " OFFSET %s" % offset
-    return sql

django/db/models/query.py

@@ -15 +15 @@
-except NameError:
-    from sets import Set as set   # Python 2.3 fallback
-
+try:
+    reversed
+except NameError:
+    from django.utils.itercompat import reversed   # Python 2.3 fallback 
+
-# The string constant used to separate query parts
-LOOKUP_SEPARATOR = '__'
-
@@ -82 +87 @@
-    else:
-        return backend.quote_name(word)
-
+def negative_slice(k):
+    return (k.start and k.start < 0) or (k.stop and k.stop < 0)
+
+def invert_slice(k):
+    """
+    Reverse the slice, inverting the negation and swapping the start and stop
+    slice positions.
+    """
+    new_start = k.stop and -k.stop or None
+    if k.stop == 0:
+        # Edge case of [-x:0] should remain empty.
+        new_stop = 0
+    else:
+        new_stop = k.start and -k.start or None
+    return slice(new_start, new_stop)
+
+def invert_item(k):
+    "Reverse the __getitem__ integer item position."
+    return -k - 1
+
-class _QuerySet(object):
-    "Represents a lazy database lookup for a set of objects"
-    def __init__(self, model=None):
@@ -97 +122 @@
-        self._tables = []            # List of extra tables to use.
-        self._offset = None          # OFFSET clause.
-        self._limit = None           # LIMIT clause.
+        self._reversed = None        # Reverse the db results (used for negative slicing)
-        self._result_cache = None
-
-    ########################
@@ -116 +142 @@
-        "Retrieve an item or slice from the set of results."
-        if not isinstance(k, (slice, int)):
-            raise TypeError
-
-
-
+
+
+
+
+
+
+
-        if self._result_cache is None:
+            extra_clone_args = {}
-            if isinstance(k, slice):
+                if self._reversed:
+                    # Because we will be dealing with a reversed QuerySet, the
+                    # slice should be inverted.
+                    if not negative_slice(k):
+                        # Can't negative slice an already reversed QuerySet, so
+                        # just slice it as a list. Note, we haven't done the
+                        # inversion yet, hence reverse logic in the condition.
+                        return self._get_data()[k]
+                    k = invert_slice(k) 
+                elif negative_slice(k):
+                    extra_clone_args['_reversed'] = True
+                    k = invert_slice(k)
-                # Offset:
-                if self._offset is None:
-                    offset = k.start
@@ -137 +180 @@
-                # Limit:
-                if k.stop is not None and k.start is not None:
-                    if limit is None:
-k.stop - k.start
+max(k.stop - k.start, 0)
-                    else:
-                        limit = min((k.stop - k.start), limit)
-                else:
@@ -147 +190 @@
-                        if k.stop is not None:
-                            limit = min(k.stop, limit)
-
-
-
+
+
+
-                else:
-
+
+
-            else:
+                if self._reversed:
+                    # Because we will be dealing with a reversed QuerySet, the
+                    # item position should be inverted.
+                    if k >= 0:
+                        # Can't negate already offset reverse slice via the
+                        # database way, just have to get it as a list. Note, we
+                        # haven't done the inversion yet, hence reverse logic in
+                        # the condition.
+                        return self._get_data()[k]
+                    # Reversed queryset lookups need to take the current offset
+                    # into consideration.
+                    k += self._offset
+                    k = invert_item(k)
+                elif k < 0:
+                    k = invert_item(k)
+                    extra_clone_args['_reversed'] = True
-                try:
-
+
+
-                except self.model.DoesNotExist, e:
-                    raise IndexError, e.args
-        else:
@@ -174 +236 @@
-    ####################################
-
-    def iterator(self):
+        if self._reversed:
+            # Being a reverse slice, the order will be reversed from what the
+            # user expects.
+            return reversed(list(self._iterator()))
+        return self._iterator()
+    def _iterator(self):
-        "Performs the SELECT database lookup of this QuerySet."
-        try:
-            select, sql, params = self._get_sql_clause()
-        except EmptyResultSet:
-            raise StopIteration
+        if self._limit == 0:
+            raise StopIteration
-
-        # self._select is a dictionary, and dictionaries' key order is
-        # undefined, so we convert it to a list of tuples.
@@ -452 +522 @@
-        c._tables = self._tables[:]
-        c._offset = self._offset
-        c._limit = self._limit
+        c._reversed = self._reversed
-        c.__dict__.update(kwargs)
-        return c
-
@@ -525 +596 @@
-
-        # ORDER BY clause
-        order_by = []
-
-
-
-
-
+
-            if f == '?': # Special case.
-                order_by.append(backend.get_random_function_sql())
-            else:
@@ -554 +621 @@
-            sql.append("ORDER BY " + ", ".join(order_by))
-
-        # LIMIT and OFFSET clauses
-
+ or self._offset is not None
-            sql.append("%s " % backend.get_limit_offset_sql(self._limit, self._offset))
-        else:
-            assert self._offset is None, "'offset' is not allowed without 'limit'"
-
-        return select, " ".join(sql), params
-
+    def _get_ordering(self):
+        if self._order_by is not None:
+            ordering = self._order_by
+        else:
+            ordering = self.model._meta.ordering
+        if not self._reversed:
+            return ordering
+        # Negative indexing requires inverting the ordering.
+        assert ordering, 'Negative indexing requires explicit ordering'
+        reverse_ordering = lambda f: (f.startswith('-') and f[1:]
+                                      or '-%s' % f)
+        return [reverse_ordering(f) for f in ordering]
+
-# Use the backend's QuerySet class if it defines one, otherwise use _QuerySet.
-if hasattr(backend, 'get_query_set_class'):
-    QuerySet = backend.get_query_set_class(_QuerySet)
@@ -573 +651 @@
-        # select_related isn't supported in values().
-        self._select_related = False
-
-
+_
-        try:
-            select, sql, params = self._get_sql_clause()
-        except EmptyResultSet:
@@ -621 +699 @@
-        return c
-
-class DateQuerySet(QuerySet):
-
+_
-        from django.db.backends.util import typecast_timestamp
-        from django.db.models.fields import DateTimeField
-        self._order_by = () # Clear this because it'll mess things up otherwise.

docs/db-api.txt

@@ -434 +434 @@
-Note, however, that the first of these will raise ``IndexError`` while the
-second will raise ``DoesNotExist`` if no objects match the given criteria.
-
+**New in Django development version**
+
+You can also use negative slicing and negative indexes, provided the
+``QuerySet``s has an explicit order (either via ``.order_by()`` or the model's
+meta ``ordering`` option). For example::
+
+    entries_by_date_order = Entry.objects.order_by('pub_date')
+    oldest_entry = entries_by_date_order[0]
+    latest_entry = entries_by_date_order[-1]
+    latest_three_entries = entries_by_date_order[:-3]
+
-QuerySet methods that return new QuerySets
-------------------------------------------
-

tests/modeltests/basic/models.py

@@ -246 +246 @@
->>> s3 = Article.objects.filter(id__exact=3)
->>> (s1 | s2 | s3)[::2]
-[<Article: Area woman programs in Python>, <Article: Third article>]
+>>> Article.objects.all()[2:]
+[<Article: Third article>, <Article: Article 6>, <Article: Default headline>, <Article: Fourth article>, <Article: Article 7>, <Article: Updated article 8>]
-
+
-# Slices (without step) are lazy:
->>> Article.objects.all()[0:5].filter()
-[<Article: Area woman programs in Python>, <Article: Second article>, <Article: Third article>, <Article: Article 6>, <Article: Default headline>]
@@ -268 +271 @@
-[<Article: Third article>, <Article: Article 6>]
->>> Article.objects.all()[2:][2:3]
-[<Article: Default headline>]
+>>> Article.objects.all()[-5:-2]
+[<Article: Article 6>, <Article: Default headline>, <Article: Fourth article>]
+>>> Article.objects.all()[-5:-2][1]
+<Article: Default headline>
+>>> Article.objects.all()[-5:-2][-1]
+<Article: Fourth article>
+>>> Article.objects.all()[-5:-2][1:]
+[<Article: Default headline>, <Article: Fourth article>]
+>>> Article.objects.all()[-5:-2][-2:]
+[<Article: Default headline>, <Article: Fourth article>]
+>>> Article.objects.all()[-5:-2][:2]
+[<Article: Article 6>, <Article: Default headline>]
+>>> Article.objects.all()[-5:-2][:-2]
+[<Article: Article 6>]
+>>> Article.objects.all()[-5:-2][:-10]
+[]
+>>> Article.objects.all()[-5:-2][-10:]
+[<Article: Article 6>, <Article: Default headline>, <Article: Fourth article>]
-
-
-
-
-
-
-
-
+
->>> Article.objects.all()[0:5].filter(id=1)
-Traceback (most recent call last):
-    ...
@@ -291 +306 @@
-    ...
-AssertionError: Cannot combine queries once a slice has been taken.
-
-not supported, due to database constraints
-hint: inverting your ordering might do what you need
+possible, provided if the QuerySet has explicit ordering
+done by internally inverting the ordering
->>> Article.objects.all()[-1]
+<Article: Updated article 8>
+>>> Article.objects.all()[0:-5]
+[<Article: Area woman programs in Python>, <Article: Second article>, <Article: Third article>]
+>>> Article.objects.all()[-2:]
+[<Article: Article 7>, <Article: Updated article 8>]
+>>> Article.objects.all()[:-7]
+[<Article: Area woman programs in Python>]
+>>> Article.objects.all()[-2:-1]
+[<Article: Article 7>]
+>>> Article.objects.all()[-2:0]
+[]
+>>> Article.objects.all()[-2:-3]
+[]
+
+# Can't mix negative and positive positions when slicing.
+>>> Article.objects.all()[1:-1]
-Traceback (most recent call last):
-    ...
-Negative indexing
-0:-5
+Slicing with both positive and negative indexes
+-5:2
-Traceback (most recent call last):
-    ...
-Negative indexing
+Slicing with both positive and negative indexes
-
-# An Article instance doesn't have access to the "objects" attribute.
-# That's only available on the class.