<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _model_exists_step(self, model, should_exist):
 """ Test for the existence of a model matching the given data. """


    model = get_model(model)
    data = guess_types(self.hashes)

    queryset = model.objects

    try:
        existence_check = _TEST_MODEL[model]
    except KeyError:
        existence_check = test_existence

    failed = 0
    try:
        for hash_ in data:
            match = existence_check(queryset, hash_)

            if should_exist:
                assert match, \
                    "%s does not exist: %s" % (model.__name__, hash_)
            else:
                assert not match, \
                    "%s exists: %s" % (model.__name__, hash_)

    except AssertionError as exc:
        print(exc)
        failed += 1

    if failed:
        print("Rows in DB are:")
        for existing_model in queryset.all():
            _dump_model(existing_model,
                        attrs=[k[1:]
                               for k in data[0].keys()
                               if k.startswith('@')])

        if should_exist:
            raise AssertionError("%i rows missing" % failed)
        else:
            raise AssertionError("%i rows found" % failed)