Combining querysets while maintaining desired ordering?

[Richard Brockie]

Hi,

I am combining querysets of the same model in the following manner:

class ExampleModel(models.Model):
    many_field1 = models.ManyToManyField('Model', related_name='name1')
    many_field2 = models.ManyToManyField('Model', related_name='name2')

    def combined_many_fields(self):
        qs1 = self.many_field1.all().order_by('some', 'fields')
        qs2 = self.many_field2.all().order_by('some', 'fields')

        return qs1 | qs2

The result of the above code is that the order_by() methods are applied after the querysets are combined. This means that the two querysets can intermingle. I want them to be sorted before combination, not after, and retain being a queryset.

There are ways to force the evaluation of the individual querysets first and return a combination in the preferred order.

    def combined_many_fields(self):

        from itertools import chain

       return chain(qs1, qs2)

However, this is no longer a queryset, and therefore cannot be operated on with subsequent queryset methods. Is there a way to apply the ordering first and retain the queryset nature of the combination?

Am I trying to do something that is outside the scope of a queryset?

[Simon Charette]

Hello Richard,

Since Django 1.11 you should be able to use the QuerySet.union() method[0] for that.

queryset = self.many_field1.order_by('some', 'fields')

combined = queryset.union(self.many_field2.order_by('some', 'fields'))

Best,

Simon

[0] https://docs.djangoproject.com/en/1.11/ref/models/querysets/#union

[Richard Brockie]

Hi Simon,

Thanks for the clarification of the operation of the .union() method - the operative phrase in the docs appears to be "combine the results of two or more QuerySets" - perfect!

My project is currently using 1.8 LTS - I can defer this until I roll forward to 1.11 LTS (hopefully sometime soon).

Thanks again,

R.

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    R.

Richard Brockie

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