Custom `length` lookup for Django ORM

models.py

class LengthLookup(models.Transform):
    lookup_name = 'length'

    def as_sql(self, compiler, connection):
        lhs, params = compiler.compile(self.lhs)
        return 'CHAR_LENGTH(%s)' % lhs, params

    # SQLite doesn't implement CHAR_LENGTH, but its implementation of LENGTH conforms
    # with standard SQL CHAR_LENGTH (i.e. number of chars, not number of bytes)
    def as_sqlite(self, compiler, connection):
        lhs, params = compiler.compile(self.lhs)
        return 'LENGTH(%s)' % lhs, params

    @property
    def output_field(self):
        return models.IntegerField()

models.CharField.register_lookup(LengthLookup)