#5420: Allow database API users to specify the fields to exclude in a SELECT
statement
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Reporter: adrian | Owner: nobody
Status: new | Component: Database wrapper
Version: SVN | Keywords:
Stage: Unreviewed | Has_patch: 0
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This one will help people use their databases more efficiently.
The Django ORM should allow users to specify a list of field names to
*exclude* from a {{{QuerySet}}}. If a user attempts to access one of those
excluded fields on the resulting model instance, the field will be loaded
lazily via a separate query.
This is useful when you know you absolutely will not need to use a
particular field in your template, so there's no point in SELECTing that
data. This saves memory, and it saves on bandwidth between the database
server and the Web server.
Example:
{{{
#!python
class Person(models.Model):
name = models.CharField(maxlength=32)
age = models.IntegerField()
hometown = models.CharField(maxlength=32)
is_cool = models.BooleanField()
# My instinct is to call this hide(), but I'm sure there's a better name
for it.
>>> p = Person.objects.hide('hometown', 'is_cool').get(name='John Lennon')
>>> p.id
3
>>> p.name
u'John Lennon'
# Does a query to get "hometown", because it was hidden from the QuerySet.
# 'SELECT hometown FROM person WHERE id=3;'
>>> p.hometown
u'Liverpool'
# Does a query to get "is_cool", because it was hidden from the QuerySet.
# 'SELECT is_cool FROM person WHERE id=3;'
>>> p.is_cool
True
}}}
In the case of lazily loaded fields, the lazy loading *only* applies to
the particular field. E.g., when I accessed {{{p.hometown}}} in the above
example, it did *not* also lazily load the rest of the hidden fields
("is_cool").
We should also provide the inverse of {{{hide()}}} -- perhaps called
{{{expose()}}}? -- which would take a list of field names to *include*
rather than *exclude*. This would be an opt-in instead of an opt-out.
--
Ticket URL: <http://code.djangoproject.com/ticket/5420>
Django Code <http://code.djangoproject.com/>
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