db.actions.aggregate([
  {
    '$unwind': '$actorId'
  },
  {
    $lookup: {
      from: 'actors',
      localField: 'actorId',
      foreignField: '_id',
      as: 'actor'

    }
  },
  {
    '$unwind': '$actor'
  }
]);

db.actors.aggregate(/* how can I get actions? */);