diff options
| -rw-r--r-- | django/db/models/sql/datastructures.py | 7 | ||||
| -rw-r--r-- | django/db/models/sql/query.py | 15 | ||||
| -rw-r--r-- | docs/db-api.txt | 13 | ||||
| -rw-r--r-- | tests/regressiontests/queries/models.py | 7 |
4 files changed, 22 insertions, 20 deletions
diff --git a/django/db/models/sql/datastructures.py b/django/db/models/sql/datastructures.py index bc21fb3b68..cb54a564f8 100644 --- a/django/db/models/sql/datastructures.py +++ b/django/db/models/sql/datastructures.py @@ -9,7 +9,12 @@ class EmptyResultSet(Exception): class FullResultSet(Exception): pass -class JoinError(Exception): +class MultiJoin(Exception): + """ + Used by join construction code to indicate the point at which a + multi-valued join was attempted (if the caller wants to treat that + exceptionally). + """ def __init__(self, level): self.level = level diff --git a/django/db/models/sql/query.py b/django/db/models/sql/query.py index 771ee55e12..c153fe1e55 100644 --- a/django/db/models/sql/query.py +++ b/django/db/models/sql/query.py @@ -18,7 +18,7 @@ from django.db.models.sql.where import WhereNode, EverythingNode, AND, OR from django.db.models.sql.datastructures import Count from django.db.models.fields import FieldDoesNotExist from django.core.exceptions import FieldError -from datastructures import EmptyResultSet, Empty, JoinError +from datastructures import EmptyResultSet, Empty, MultiJoin from constants import * try: @@ -523,11 +523,8 @@ class Query(object): pieces = name.split(LOOKUP_SEP) if not alias: alias = self.get_initial_alias() - try: - field, target, opts, joins, last = self.setup_joins(pieces, opts, - alias, False, False) - except JoinError: - raise FieldError("Cannot order by many-valued field: '%s'" % name) + field, target, opts, joins, last = self.setup_joins(pieces, opts, + alias, False) alias = joins[-1] col = target.column @@ -848,7 +845,7 @@ class Query(object): try: field, target, opts, join_list, last = self.setup_joins(parts, opts, alias, (connector == AND), allow_many) - except JoinError, e: + except MultiJoin, e: self.split_exclude(filter_expr, LOOKUP_SEP.join(parts[:e.level])) return final = len(join_list) @@ -1007,7 +1004,7 @@ class Query(object): if not allow_many and (m2m or not direct): for alias in joins: self.unref_alias(alias) - raise JoinError(pos + 1) + raise MultiJoin(pos + 1) if model: # The field lives on a base class of the current model. alias_list = [] @@ -1175,7 +1172,7 @@ class Query(object): name.split(LOOKUP_SEP), opts, alias, False, allow_m2m, True) self.select.append((joins[-1], target.column)) - except JoinError: + except MultiJoin: raise FieldError("Invalid field name: '%s'" % name) def add_ordering(self, *ordering): diff --git a/docs/db-api.txt b/docs/db-api.txt index af5ba8ce49..e15b0b2176 100644 --- a/docs/db-api.txt +++ b/docs/db-api.txt @@ -510,12 +510,13 @@ primary key if there is no ``Meta.ordering`` specified. For example:: ...since the ``Blog`` model has no default ordering specified. -You can only order by model fields that have a single value attached to them -for each instance of the model. For example, non-relations, ``ForeignKey`` and -``OneToOneField`` fields. Explicitly, you can't order by a ``ManyToManyField`` -or a reverse ``ForeignKey`` relation. There's no naturally correct ordering -for many-valued fields and a lot of the alternatives are not psosible to -express in SQL very efficiently. +It is permissible to specify a multi-valued field to order the results by (for +example, a ``ManyToMany`` field). Normally this won't be a sensible thing to +do and it's really an advanced usage feature. However, if you know that your +queryset's filtering or available data implies that there will only be one +ordering piece of data for each of the main items you are selecting, the +ordering may well be exactly what you want to do. Use ordering on multi-valued +fields with care and make sure the results are what you expect. **New in Django development version:** If you don't want any ordering to be applied to a query, not even the default ordering, call ``order_by()`` with no diff --git a/tests/regressiontests/queries/models.py b/tests/regressiontests/queries/models.py index a9783569a7..a9517628b4 100644 --- a/tests/regressiontests/queries/models.py +++ b/tests/regressiontests/queries/models.py @@ -424,11 +424,10 @@ FieldError: Infinite loop caused by ordering. [<Ranking: 1: a3>, <Ranking: 2: a2>, <Ranking: 3: a1>] # Ordering by a many-valued attribute (e.g. a many-to-many or reverse -# ForeignKey) doesn't make sense (there's no natural ordering). +# ForeignKey) is legal, but the results might not make sense. That isn't +# Django's problem. Garbage in, garbage out. >>> Item.objects.all().order_by('tags') -Traceback (most recent call last): -... -FieldError: Cannot order by many-valued field: 'tags' +[...] # If we replace the default ordering, Django adjusts the required tables # automatically. Item normally requires a join with Note to do the default |