code: def assign_agent_to_resource(self, agent_id, resource_id):
          
          # Should check for existence of Agent? We may mever manage them.
          collection = JSONClientValidated('resource',
                                           collection='Resource',
                                           runtime=self._runtime)
          resource = collection.find_one({'_id': ObjectId(resource_id.get_identifier())})
  
          try:
              ResourceAgentSession(
                  self._catalog_id, self._proxy, self._runtime).get_resource_by_agent(agent_id)
          except errors.NotFound:
              pass
          else:
              raise errors.AlreadyExists()
          if 'agentIds' not in resource:
              resource['agentIds'] = [str(agent_id)]
          else:
              resource['agentIds'].append(str(agent_id))
          collection.save(resource)
docs: Adds an existing ``Agent`` to a ``Resource``.
  
          arg:    agent_id (osid.id.Id): the ``Id`` of the ``Agent``
          arg:    resource_id (osid.id.Id): the ``Id`` of the ``Resource``
          raise:  AlreadyExists - ``agent_id`` is already assigned to
                  ``resource_id``
          raise:  NotFound - ``agent_id`` or ``resource_id`` not found
          raise:  NullArgument - ``agent_id`` or ``resource_id`` is
                  ``null``
          raise:  OperationFailed - unable to complete request
          raise:  PermissionDenied - authorization failure
          *compliance: mandatory -- This method must be implemented.*