Add a first method to the QuerySet

[Wim Feijen]

Many times, get() is being used inside a try except Model.DoesNotExist clause. In order to save us lines of code and in order to prevent the Exception becoming common, a discussion followed which can be found here:

​https://groups.google.com/forum/?fromgroups=#!searchin/django-developers/first$20jacob/django-developers/3RwDxWKPZ_A/discussion

According to our BDFL:

After thinking a bit, I'm +1 on the idea, and I think
Queryset.first() is the right name (which implies we should probably
have a last() for completion).

This mirrors the name used by a few other ORM/collection APIs that I
think have really nice APIS:

• SQLalchemy (​http://docs.sqlalchemy.org/en/rel_0_7/orm/query.html#sqlalchemy.orm.query.Query.first)
• Storm (​http://people.canonical.com/~therve/storm/storm.store.ResultSet.html#first)
• Backbone (​http://backbonejs.org/#Collection-Underscore-Methods /

​http://documentcloud.github.com/underscore/#first)

If someone produces a patch I'll review it and commit it.

Jacob

[wim@…]

Patch for ticket 19326: added first. Unfortunately, my implementation fails

[anonymous]

Something like this should work:

try:
    return self.filter(*args, **kwargs)[0]
except IndexError:
    return None

I am not sure if the IndexError is correct error type...

If .last() is needed then that is going to be a bit more problematic as you will need to revert all of the ordering clauses of the query.

[wim@…]

My second take, using filter and IndexError as suggested

[bleskes]

Looks good to me.

[Anssi Kääriäinen]

Some polish done, available from ​https://github.com/akaariai/django/compare/ticket_19326

Biggest changes are addition of .last() method for symmetry, and automatic ordering by PK if there is no ordering defined otherwise. In addition some docs changes and move of the tests to new test module first_and_last.

I will still ask at django-developers if the API is the wanted one, otherwise this should be 100% ready for commit.

[Anssi Kääriäinen]

I'm setting this to DDN. If these methods were added, then the API would be confusing. .latest() vs .last() and .earliest() vs .first() will be confusing.

I still think it would be nice to have some way to get None instead of DoesNotExist if the None is expected, not exceptional value. However adding anything that is confusing is a bad idea.

Another API idea: .default(None).get(...), .default(someobj).latest() or .default(None).order_by('somecol')[0] could all make the ORM return None or any other given value instead of DoesNotExist. Does this sound sensible? Naming is of course tentative, for example .default_on_empty() could be better if somewhat long...

[Jacob]

I haven't seen a proposal addressing the first/last vs earliest/latest confusion that isn't even *more* confusing, so I'm marking this accepted. Let's just do first/last + earliest/latest and live with the slight redundancy.

[Selwin Ong]

I made a pull request implementing these features here: ​https://github.com/django/django/pull/1056 . Let me know if there's anything else that I need to change.

[Mike Fogel]

How about adding a test that first() and last() work as expected on empty querysets? Other than that the patch looks good to me.

[Anssi Kääriäinen <akaariai@…>]

In ea9a0857d4922fab1f9146f3a7828b67281edc89:

Fixed #19326 -- Added first() and last() methods to QuerySet