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magic-removal: Implemented 'laziness' for QuerySet slicing, to restor…

…e functionality that was previously possible with generic views via 'limit' and 'extra_lookup_args'

git-svn-id: http://code.djangoproject.com/svn/django/branches/magic-removal@2485 bcc190cf-cafb-0310-a4f2-bffc1f526a37
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commit ecd7f948dcc25222d9711254354d0f9292683566 1 parent f1c36bc
@spookylukey spookylukey authored
Showing with 93 additions and 11 deletions.
  1. +48 −11 django/db/models/query.py
  2. +45 −0 tests/modeltests/basic/models.py
View
59 django/db/models/query.py
@@ -95,17 +95,38 @@ def __iter__(self):
def __getitem__(self, k):
"Retrieve an item or slice from the set of results."
- # __getitem__ can't return QuerySet instances, because filter() and
- # order_by() on the result would break badly. This means we don't have
- # to worry about arithmetic with self._limit or self._offset -- they'll
- # both be None at this point.
if self._result_cache is None:
if isinstance(k, slice):
+ # Offset:
+ if self._offset is None:
+ offset = k.start
+ elif k.start is None:
+ offset = self._offset
+ else:
+ offset = self._offset + k.start
+ # Now adjust offset to the bounds of any existing limit:
+ if self._limit is not None and k.start is not None:
+ limit = self._limit - k.start
+ else:
+ limit = self._limit
+
+ # Limit:
if k.stop is not None and k.start is not None:
- limit = k.stop - k.start
+ if limit is None:
+ limit = k.stop - k.start
+ else:
+ limit = min((k.stop - k.start), limit)
else:
- limit = k.stop
- return list(self._clone(_offset=k.start, _limit=limit))[::k.step]
+ if limit is None:
+ limit = k.stop
+ else:
+ if k.stop is not None:
+ limit = min(k.stop, limit)
+
+ if k.step is None:
+ return self._clone(_offset=offset, _limit=limit)
+ else:
+ return list(self._clone(_offset=offset, _limit=limit))[::k.step]
else:
return self._clone(_offset=k, _limit=1).get()
else:
@@ -179,6 +200,8 @@ def latest(self, field_name=None):
"""
latest_by = field_name or self.model._meta.get_latest_by
assert bool(latest_by), "latest() requires either a field_name parameter or 'get_latest_by' in the model"
+ assert self._limit is None and self._offset is None, \
+ "Cannot change a query once a slice has been taken."
return self._clone(_limit=1, _order_by=('-'+latest_by,)).get()
def in_bulk(self, id_list):
@@ -186,6 +209,8 @@ def in_bulk(self, id_list):
Returns a dictionary mapping each of the given IDs to the object with
that ID.
"""
+ assert self._limit is None and self._offset is None, \
+ "Cannot use 'limit' or 'offset' with in_bulk"
assert isinstance(id_list, (tuple, list)), "in_bulk() must be provided with a list of IDs."
id_list = list(id_list)
assert id_list != [], "in_bulk() cannot be passed an empty ID list."
@@ -198,13 +223,14 @@ def delete(self):
"""
Deletes the records in the current QuerySet.
"""
+ assert self._limit is None and self._offset is None, \
+ "Cannot use 'limit' or 'offset' with delete."
+
del_query = self._clone()
# disable non-supported fields
del_query._select_related = False
del_query._order_by = []
- del_query._offset = None
- del_query._limit = None
# Collect all the objects to be deleted, and all the objects that are related to
# the objects that are to be deleted
@@ -251,6 +277,10 @@ def exclude(self, *args, **kwargs):
return self._filter_or_exclude(QNot, *args, **kwargs)
def _filter_or_exclude(self, qtype, *args, **kwargs):
+ if len(args) > 0 or len(kwargs) > 0:
+ assert self._limit is None and self._offset is None, \
+ "Cannot filter a query once a slice has been taken."
+
clone = self._clone()
if len(kwargs) > 0:
clone._filters = clone._filters & qtype(**kwargs)
@@ -264,6 +294,8 @@ def select_related(self, true_or_false=True):
def order_by(self, *field_names):
"Returns a new QuerySet instance with the ordering changed."
+ assert self._limit is None and self._offset is None, \
+ "Cannot reorder a query once a slice has been taken."
return self._clone(_order_by=field_names)
def distinct(self, true_or_false=True):
@@ -271,6 +303,8 @@ def distinct(self, true_or_false=True):
return self._clone(_distinct=true_or_false)
def extra(self, select=None, where=None, params=None, tables=None):
+ assert self._limit is None and self._offset is None, \
+ "Cannot change a query once a slice has been taken"
clone = self._clone()
if select: clone._select.extend(select)
if where: clone._where.extend(where)
@@ -301,8 +335,11 @@ def _clone(self, klass=None, **kwargs):
return c
def _combine(self, other):
- if self._distinct != other._distinct:
- raise ValueException, "Can't combine a unique query with a non-unique query"
+ assert self._limit is None and self._offset is None \
+ and other._limit is None and other._offset is None, \
+ "Cannot combine queries once a slice has been taken."
+ assert self._distinct == other._distinct, \
+ "Cannot combine a unique query with a non-unique query"
# use 'other's order by
# (so that A.filter(args1) & A.filter(args2) does the same as
# A.filter(args1).filter(args2)
View
45 tests/modeltests/basic/models.py
@@ -239,6 +239,51 @@ def __repr__(self):
>>> (s1 | s2 | s3)[::2]
[Area woman programs in Python, Third article]
+# Slices (without step) are lazy:
+>>> Article.objects.all()[0:5].filter()
+[Area woman programs in Python, Second article, Third article, Fourth article, Article 6]
+
+# Slicing again works:
+>>> Article.objects.all()[0:5][0:2]
+[Area woman programs in Python, Second article]
+>>> Article.objects.all()[0:5][:2]
+[Area woman programs in Python, Second article]
+>>> Article.objects.all()[0:5][4:]
+[Article 6]
+>>> Article.objects.all()[0:5][5:]
+[]
+
+# Some more tests!
+>>> Article.objects.all()[2:][0:2]
+[Third article, Fourth article]
+>>> Article.objects.all()[2:][:2]
+[Third article, Fourth article]
+>>> Article.objects.all()[2:][2:3]
+[Article 6]
+
+# Note that you can't use 'offset' without 'limit' (on some dbs), so this doesn't work:
+>>> Article.objects.all()[2:]
+Traceback (most recent call last):
+ ...
+AssertionError: 'offset' is not allowed without 'limit'
+
+# Also, once you have sliced you can't filter, re-order or combine
+>>> Article.objects.all()[0:5].filter(id=1)
+Traceback (most recent call last):
+ ...
+AssertionError: Cannot filter a query once a slice has been taken.
+
+>>> Article.objects.all()[0:5].order_by('id')
+Traceback (most recent call last):
+ ...
+AssertionError: Cannot reorder a query once a slice has been taken.
+
+>>> Article.objects.all()[0:1] & Article.objects.all()[4:5]
+Traceback (most recent call last):
+ ...
+AssertionError: Cannot combine queries once a slice has been taken.
+
+
# An Article instance doesn't have access to the "objects" attribute.
# That's only available on the class.
>>> a7.objects.all()
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