gistfile1.js

    ASchema = new mongoose.Schema({
      name: String,
      B: [BSchema]
    });
    
    var BSchema = new Schema({
      name: String
    });
    
    var CSchema = new Schema({
      name: String,
      B: {
        type: Schema.ObjectId,
        ref: 'BSchema'
      }
    });
    
    //This is OK, but not one time mongoose search:
    CSchema.find({
        name: 'C_xxx'
      }).exec(function(err, docC) {
    
      docC.forEach(function(o) {
        var BId = o.B;
        ASchema.findOne({
            'B._id': BId
          }, {
            'B.$': 1
          }
          ).exec(function(err, docA) {
          var Bname = docA.B[0].name;
          var Cname = docA.name;
        });
    
      });
    });
    
    //wrong in populate:
    CSchema.find({
        name: 'C_xxx'
      })
      .populate('ASchema.B')
      .exec(function(err, docC) {
      docC.forEach(function(o) {
        var Bname = o.B.name;
        //ERROR:o.B is a objectId,o.B.name is undefined
        //Aname can't find
      });
    });

/*
Data like this:

    ASchema = {
      name: "A_xxx",
      B: [{
          _id: 1,
          name: "B_xxx"
        }, {
          _id: 2,
          name: "B_xxx"
        }]
    }
    CSchema = [{
        name: "C_xxx",
        B: 1
      }, {
        name: "C_xxx",
        B: 2
      }]
    */