title: Apache Airflow does not pickle DAGs
body: <p>I would like to recover DAG objects so that I can better inspect certain dependencies after DAG runs (e.g. what data is consumed by specific operators). I am using <code>postgres:9.6</code> as metadata database backend.</p>
  
  <p>This seems to be supported via the <code>donot_pickle</code> configuration variable, which by default indicates all DAGs must be pickled:</p>
  
  <pre><code>[core]
  # Whether to disable pickling dags
  donot_pickle = False
  </code></pre>
  
  <p>I have some test DAGs (3) available but their corresponding <code>pickle_id</code> is empty:</p>
  
  <pre><code>&gt; select pickle_id from dag;
  pickle_id
  ---------
  
  
  (3 rows)
  </code></pre>
  
  <p>Pickles table is also empty:</p>
  
  <pre><code>&gt; select count(*) from dag_pickle;
   count
  ------
      0
  (1 row)
  </code></pre>
  
  <p>What might be going wrong here? I was not able to find any reference in the docs.</p>