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29050ef999
In extreme cases, some fields are expensive to load from the database (e.g. GIS fields requiring conversion, or large text fields). This commit adds defer() and only() methods to querysets that allow the caller to specify which fields should not be loaded unless they are accessed. git-svn-id: http://code.djangoproject.com/svn/django/trunk@10090 bcc190cf-cafb-0310-a4f2-bffc1f526a37
90 lines
2.7 KiB
Python
90 lines
2.7 KiB
Python
"""
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Tests for defer() and only().
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"""
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from django.db import models
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from django.db.models.query_utils import DeferredAttribute
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class Secondary(models.Model):
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first = models.CharField(max_length=50)
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second = models.CharField(max_length=50)
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class Primary(models.Model):
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name = models.CharField(max_length=50)
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value = models.CharField(max_length=50)
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related = models.ForeignKey(Secondary)
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def count_delayed_fields(obj, debug=False):
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"""
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Returns the number of delayed attributes on the given model instance.
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"""
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count = 0
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for field in obj._meta.fields:
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if isinstance(obj.__class__.__dict__.get(field.attname),
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DeferredAttribute):
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if debug:
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print field.name, field.attname
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count += 1
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return count
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__test__ = {"API_TEST": """
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To all outward appearances, instances with deferred fields look the same as
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normal instances when we examine attribut values. Therefore we test for the
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number of deferred fields on returned instances (by poking at the internals),
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as a way to observe what is going on.
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>>> s1 = Secondary.objects.create(first="x1", second="y1")
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>>> p1 = Primary.objects.create(name="p1", value="xx", related=s1)
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>>> qs = Primary.objects.all()
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>>> count_delayed_fields(qs.defer('name')[0])
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1
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>>> count_delayed_fields(qs.only('name')[0])
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2
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>>> count_delayed_fields(qs.defer('related__first')[0])
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0
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>>> obj = qs.select_related().only('related__first')[0]
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>>> count_delayed_fields(obj)
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2
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>>> obj.related_id == s1.pk
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True
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>>> count_delayed_fields(qs.defer('name').extra(select={'a': 1})[0])
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1
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>>> count_delayed_fields(qs.extra(select={'a': 1}).defer('name')[0])
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1
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>>> count_delayed_fields(qs.defer('name').defer('value')[0])
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2
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>>> count_delayed_fields(qs.only('name').only('value')[0])
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2
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>>> count_delayed_fields(qs.only('name').defer('value')[0])
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2
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>>> count_delayed_fields(qs.only('name', 'value').defer('value')[0])
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2
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>>> count_delayed_fields(qs.defer('name').only('value')[0])
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2
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>>> obj = qs.only()[0]
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>>> count_delayed_fields(qs.defer(None)[0])
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0
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>>> count_delayed_fields(qs.only('name').defer(None)[0])
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0
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User values() won't defer anything (you get the full list of dictionaries
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back), but it still works.
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>>> qs.defer('name').values()[0] == {'id': p1.id, 'name': u'p1', 'value': 'xx', 'related_id': s1.id}
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True
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>>> qs.only('name').values()[0] == {'id': p1.id, 'name': u'p1', 'value': 'xx', 'related_id': s1.id}
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True
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Using defer() and only() with get() is also valid.
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>>> count_delayed_fields(qs.defer('name').get(pk=p1.pk))
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1
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>>> count_delayed_fields(qs.only('name').get(pk=p1.pk))
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2
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# KNOWN NOT TO WORK: >>> count_delayed_fields(qs.only('name').select_related('related')[0])
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# KNOWN NOT TO WORK >>> count_delayed_fields(qs.defer('related').select_related('related')[0])
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"""}
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