// UserProfile
var UserSchema = new mongoose.Schema({
userId:{ type: mongoose.Schema.Types.ObjectId, ref: "User" },
picture : String,
telephone: {
  type: String,
  unique: false
},
adresse:String,
loc: {
  type: [Number], // [<longitude>, <latitude>]
  index: "2d" // create the geospatial index
},
metier:String,
pays:String,
ville:String,
code_postal:Number });

var UserSchema = new mongoose.Schema({
email: {
  type: String,
  unique: true,
  required: true,
  trim: true
},
password: {
  type: String,
  required: true,
},
passwordConf: {
  type: String,
  required: true,
},
picture : String,
telephone: {
  type: String,
  unique: false
},
adresse:String,
loc: {
  type: [Number], // [<longitude>, <latitude>]
  index: "2d" // create the geospatial index
},
metier:String,
pays:String,
ville:String },{timestamps: true});

User.aggregate([
  {
     $lookup:
     {
        from: "UserProfile",
        localField: "_id",
        foreignField: "userId",
        as: "profile"
     }
  }
], (err, user)=>{
  res.json({
     success: true,
     user : user
  });
})

{
"success": true,
"user": [
    {
        "_id": "5adcf607c187203e1c88d21e",
        "email": "userdemo@mail.com",
        "password": "demopass",
        "passwordConf": "demopass",
        "__v": 0,
        "profile": []
    },
....