def get_quota_usage_by_tenant(self, tenant_id, resource):
        """Get a specific quota usage by tenant."""

        quotas = QuotaUsage.find_all(tenant_id=tenant_id, resource=resource).all()
        if len(quotas) == 0:
            return QuotaUsage.create(tenant_id=tenant_id, in_use=0, reserved=0, resource=resource)
        return quotas[0]

    def get_all_quota_usages_by_tenant(self, tenant_id, resources):
        """
        Retrieve the quota usagess for the given tenant.

        :param tenant_id: The ID of the tenant to return quotas for.
        :param resources: A list of the registered resources to get.
        """

        all_usages = QuotaUsage.find_all(tenant_id=tenant_id).all()
        result_usages = {usage.resource: usage for usage in all_usages if usage.resource in resources}
        if len(result_usages) != len(resources):
            for resource in resources:
                # Not in the DB, return default value
                if resource not in result_usages:
                    usage = QuotaUsage.create(tenant_id=tenant_id, in_use=0, reserved=0, resource=resource)
                    result_usages[resource] = usage

        return result_usages

    def rollback(self, reservations):
        """Roll back reservations.

        :param reservations: A list of the reservation UUIDs, as
                             returned by the reserve() method.
        """

        for reservation in reservations:
            usage = QuotaUsage.find_by(id=reservation.usage_id)
            usage.reserved -= reservation.delta
            reservation.status = Reservation.Statuses.ROLLEDBACK
            usage.save()
            reservation.save()

    def commit(self, reservations):
        """Commit reservations.

        :param reservations: A list of the reservation UUIDs, as
                             returned by the reserve() method.
        """

        for reservation in reservations:
            usage = QuotaUsage.find_by(id=reservation.usage_id)
            usage.in_use += reservation.delta
            usage.reserved -= reservation.delta
            reservation.status = Reservation.Statuses.COMMITTED
            usage.save()
            reservation.save()