如何在mongodb中使用aggregrate和$ lookup在第二个集合上应用条件

Question

I am totally stuck with one condition in MEAN Js. I have written my code below :

apiRouter.post('/findhospitalbyname', function(req, res) {

    Hospital.aggregate([
    {
        "$match":{
        title: { '$regex': '.*' + req.body.name + '.*', $options: 'i' }
        }
    },
    {   
        "$lookup": {  
            "localField": "_id",   
            "from": "favorites",         
            "foreignField": "hospital_id", 
       // "pipeline": [{ 
       //    "$match": {"user_id": mongoose.Types.ObjectId(req.body.user_id)}
       //  }],
            "as": "datafind"  
        } 
  } 
],   
function(err, hospital) {
            if (err) {
                return res.send({ 'user_data': err, 'error': 1 });
            }
            if (hospital.length != 0) {
                return res.send({ 'data': hospital, 'error':0});
            } else {
                return res.send({ 'data': '', 'error':2, 'msg': 'No data found' });
            }
        })   
});

This is my code. Here hospital is my first collection and favourite is my second collection. I want to add a condition on second collection. For first collection my code is working fine but it is not working for the second collection. There are some commented lines also which are showing my second collection where condition. Please reply if you have any solution.

Answer 1

I will try to use a unwind, match, group stages to get the result you need:

Hospital.aggregate([
{
    "$match":{
       title: { '$regex': '.*' + req.body.name + '.*', $options: 'i' }
    }
},
{   
    "$lookup": {  
        "from": "favorites",  
        "localField": "_id",        
        "foreignField": "hospital_id", 
        "as": "datafind"  
        } 
  },
  {
    "$unwind" : "$datafind"
  },
  { 
   "$match": {"$datafind.user_id": mongoose.Types.ObjectId(req.body.user_id)}
  },
  {
      $group: 
          {
              _id : "$_id" 
          }
  } 
],//the other code...

If you want to use pipeline first you need to know the version of your mongo database, pipeline only works on v3.6+ and the above code works on v3.2+. So yes mongo introduce and helpful conditional for lookup but remains on your database version.