Ensure value uniqueness.

        The rule's arguments are validated against this schema:
        {'type': 'boolean'}

def _validate_unique(self, unique, field, value):
        """Ensure value uniqueness.

        The rule's arguments are validated against this schema:
        {'type': 'boolean'}
        """
        if unique:
            root_key = self.schema_path[0]
            data = (doc[field] for doc in self.root_document[root_key])
            for key, count in collections.Counter(data).items():
                if count > 1:
                    msg = "'{}' is not unique".format(key)
                    self._error(field, msg)