django filter on the basis of text length

Question

I would like to filter my model on the basis of the length of the text Something like

MyModel.objects.filter(len(text) > 10)

where text

Answer 1

A nice solution for Django >= 1.9 is possible by registering the builtin function Length as a Transform for CharField lookup.

Register the transformation in the project once. (The best place is probably models.py.)

from django.db.models import CharField
from django.db.models.functions import Length

CharField.register_lookup(Length, 'length')

Use:

result = MyModel.objects.filter(text__length__gt=10)

See exactly the same example in docs for Length as a transform.

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It works correctly for all backends, compiled by LENGTH() for most backends and by CHAR_LENGTH() for MySQL. It is then registered for all subclasses of CharField automatically, e.g. for EmailField. The TextField must be registered individually. It is safe to register the name "length", because a transform name could never shade or be shaded by an equally named field name or related field name.

The only disadvantage could be readability puzzle: Where did the "length" come from? (The lookup is global, but the same can be luckily safely registered repeatedly in more modules, if useful for readability, without any possible overhead at query runtime.)

Other similarly valuable solution is the hobs's above that is shorter if a registration counts and if a similar query is not used repeatedly.