title: Pulling xcom from sub dag
body: <p>I am using a main dag (main_dag) that contains a number of subdags and each of those subdags has a number of tasks. I pushed an xcom from subdagA taskA, but I am pulling that xcom within subdagB taskB. Since the dag_id argument in xcom_pull() defaults to self.dag_id I have been unable to pull the necessary xcom. I was wondering how one would do this and/or if there is a better way to set this scenario up so I don't have to deal with this.</p>
  
  <p>example of what I am currently doing in subdagB:</p>
  
  <pre><code>def subdagB(parent_dag, child_dag, start_date, schedule_interval):
      subdagB = DAG('%s.%s' % (parent_dag, child_dag), start_date=start_date, schedule_interval=schedule_interval)
      start = DummyOperator(
          task_id='taskA',
          dag=subdagB)
      tag_db_template = '''echo {{ task_instance.xcom_pull(dag_id='dag.main_dag.subdagA', task_ids='taskA') }};'''
      t1 = BashOperator(
          task_id='taskB',
          bash_command=tag_db_template,
          xcom_push=True,
          dag=subdagB)
      end = DummyOperator(
          task_id='taskC',
          dag=subdagB)
      t0.set_upstream(start)
      t1.set_upstream(t0)
      end.set_upstream(t1)
      return subdagB
  </code></pre>
  
  <p>Thank you in advance for any help!</p>