I'm using [Cerberus](http://docs.python-cerberus.org/) to validate the data posted as JSON to a Flask-based ReST-API. I want at least one of the two fields `freight_id` and `tender_id` to be present.

These mappings would be considered as valid:

```
{"freight_id": 1, "price" : 12000}

{"tender_id": 1, "price": 12000}

{"freight_id": 1, "tender_id" : 2, "price": 12000}

```

While this one would not:

```
{"price": 12000}

```

How can I formulate a schema for such validation with Cerberus?

I almost read all documentation, but did not find any answer. The [`excludes`](http://docs.python-cerberus.org/en/stable/validation-rules.html#excludes)-rule doesn't fit my need.