How to use 'django unique_together' in Python

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def unique_together_constraint(model, instance):
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    if not instance._meta.unique_together:
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        return
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    error_fields = []
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    for unique_fields in instance._meta.unique_together:
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        check = {}
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        for field_name in unique_fields:
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            if not get_field(instance, field_name).primary_key:
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                check[field_name] = getattr(instance, field_name)
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        if all(e is None for e in check.values()):
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            continue
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        if model._default_manager.filter(**check).exists():
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            error_fields.extend([
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                get_field(instance, field_name)
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                for field_name in unique_fields
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            ])
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    if error_fields:
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        raise InvalidConstraint(error_fields)

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def test_pointing_to_m2m_field(self):
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    class Model(models.Model):
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        m2m = models.ManyToManyField('self')
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        class Meta:
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            index_together = [['m2m']]
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    self.assertEqual(Model.check(), [
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        Error(
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            "'index_together' refers to a ManyToManyField 'm2m', but "
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            "ManyToManyFields are not permitted in 'index_together'.",
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            obj=Model,
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            id='models.E013',
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        ),
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    ])