Check that inserting a child is within the limits.

def _check_child_limits(self, child_pid):
        """Check that inserting a child is within the limits."""
        if self.max_children is not None and \
                self.children.count() >= self.max_children:
            raise PIDRelationConsistencyError(
                "Max number of children is set to {}.".
                format(self.max_children))
        if self.max_parents is not None and \
                PIDRelation.query.filter_by(
                    child=child_pid,
                    relation_type=self.relation_type.id)\
                .count() >= self.max_parents:
            raise PIDRelationConsistencyError(
                "This pid already has the maximum number of parents.")