Study.find(queryParams)
        .lean(true)
        .exec(function (err, studies) {...SOME LOGIC...}

var ParentSchema = {
      name           : { type: String, required: true },
      project        : { type: ObjectId, required: true },
      questions      : { type: [Question], default: [] },
    }
   mongoose.createModel('Parent', ParentSchema);

var QuestionSchema = {
    userId: ObjectId,
    thumbnail: String
});
mongoose.createModel('Question', QuestionSchema);