Return a dict containing the value of the provided key and its uuid as
        value.

def key_occurrence(self, key, update=True):
        """
        Return a dict containing the value of the provided key and its uuid as
        value.
        """
        if update:
            self.update()
        result = {}
        for k, v in self.database.items():
            if key in v:
                result[str(v[key])] = k
        return result