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Adding a .first() method to Django's QuerySet

· Web Dev Zone

In my last Django project, we had a set of helper functions that we used a lot. The most used was helpers.first, which takes a query set and returns the first element, or None if the query set was empty.

Instead of writing this:

try:
    object = MyModel.objects.get(key=value)
except model.DoesNotExist:
    object = None

You can write this:

def first(query):
    try:
        return query.all()[0]
    except:
        return None

object = helpers.first(MyModel.objects.filter(key=value))

Note, that this is not identical. The get method will ensure that there is exactly one row in the database that matches the query. The helper.first() method will silently eat all but the first matching row. As long as you're aware of that, you might choose to use the second form in some cases, primarily for style reasons.

But the syntax on the helper is a little verbose, plus you're constantly including helpers.py. Here is a version that makes this available as a method on the end of your query set chain. All you have to do is have your models inherit from this AbstractModel.

class FirstQuerySet(models.query.QuerySet):

    def first(self):
        try:
            return self[0]
        except:
            return None


class ManagerWithFirstQuery(models.Manager):

    def get_query_set(self):
        return FirstQuerySet(self.model)


class AbstractModel(models.Model):
    objects = ManagerWithFirstQuery()

    class Meta:
        abstract = True


class MyModel(AbstractModel):
    ...

Now, you can do the following.

object = MyModel.objects.filter(key=value).first()

 

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Published at DZone with permission of Chase Seibert, DZone MVB. See the original article here.

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