在 mongodb / mongoose 中使用条件查找“来自”

Question

Is it possible to use conditional lookup source('from'),in mongodb or mongoose like as follows

 Collection.aggregate([
      {
        '$match': {
          'receivers.user': new ObjectId('..........')
        }
      }, {
        '$lookup': {
          'from': $switch: {
   type: [
      { case: 101, then: collection1 },
      { case: 102, then: collection2 },
   ]
}, 
          'localField': 'data.on_behalf', 
          'foreignField': '_id', 
          'as': 'data.on_behalf'
        }
      }, {
        '$unwind': {
          'path': '$data.on_behalf', 
          'preserveNullAndEmptyArrays': true
        }
      }
    ])

I want to switch my lookup collection according to the type value..

Answer 1

If you just have 2 cases, you can use conditional operator to specify the collection to lookup from:

 Collection.aggregate([
  ..., 
  {
    '$lookup': {
      'from': type == 101 ? 'collection1' : 'collection2', 
      'localField': 'data.on_behalf', 
      'foreignField': '_id', 
      'as': 'data.on_behalf'
    }
  },...

Or if you have more cases, you can create a switch statement outside to specify the collection then pass it to aggregate. Example:

let collection_to_lookup;
switch(type) {
  case 101: collection_to_lookup = "collection1"; break;
  case 102: collection_to_lookup = "collection2"; break;
  ...
  default: collection_to_lookup = "collection1"; break;
}

Collection.aggregate([
  ..., 
  {
    '$lookup': {
      'from': collection_to_lookup, 
      'localField': 'data.on_behalf', 
      'foreignField': '_id', 
      'as': 'data.on_behalf'
    }
  },...