title: Django sum all related objects of a filtered QuerySet
body: <p>What I would like to do in psuedo code is:</p>
  
  <pre><code>def import_transaction_deposit_crypto(Importer):
      logger = get_nexchange_logger(__name__, True, True)
      existent_addresses = Address.objects.filter(
          currency__is_crypto=True,
          type=Address.DEPOSIT,
          currency__wallet__in=Importer.RELATED_NODES
  
      ).tx_to_set.count()
  </code></pre>
  
  <p>Example value of importer:</p>
  
  <pre><code>class LitecoinTxImporter:
      RELATED_NODES = 'ltc_rpc_1'
  </code></pre>
  
  <p><code>tx_to</code> is a related_field (reverse relation):</p>
  
  <pre><code>class Address(BtcBase, SoftDeletableModel):
      address_to = models.ForeignKey('core.Address',
                                     related_name='txs_to')
  </code></pre>
  
  <p>The idea is to count all 'already imported' transactions that belong to a <strong>specific</strong> RPC node (wallet), in order to feed it to the <code>from</code> parameter of the <code>listtransactions</code> RPC endpoint (generally speaking, for pagination purposes).</p>