var mschema = new Schema({
    autoIncrementId: { type: Number,unique: true,index:true,sparse:true},
    elementName: {
        type: String,
        required: true
    },
    elementCanvasId: {
        type: String,
        required: true
    },
    parentId: {
        type: Schema.Types.ObjectId,
        required: false,
        ref: "Elements",
    },
    parentCanvasId: {
        type: String,
        required: false
    },
});

var modules = Mongoose.model('Elements', mschema);

db.employees.aggregate( [
   {
      $graphLookup: {
         from: "elements",
         startWith: "$parentCanvasId",
         connectFromField: "parentCanvasId",
         connectToField: "elementCanvasId",
         as: "elementHierarchy"
      }
   }
] )