java - MongoDB $lookup query returning empty field -

there 2 collections in database, projects , tasks. each project can have number of tasks associated it. trying create mongodb query in java return task , embed project linked it, code have completed far returns empty array (i have named something appears [] below).

unfortunately have not found many examples on how use $lookup in java. changes need perform on code something field comes project? should using $lookup or aggregation operator?

the java method using mongo-java-driver version 3.5.0:

public static string readtask() {     mongoclient mongoclient = new mongoclient();     mongodatabase database = mongoclient.getdatabase("exampledatabase");      bson lookup = new document("$lookup",             new document("from", "project")                     .append("localfield", "project._id")                     .append("foreignfield", "_id")                     .append("as", "something"));      list<bson> filters = new arraylist<>();     filters.add(lookup);      aggregateiterable<document> = database.getcollection("task").aggregate(filters);     system.out.println("first document: " + it.first().tostring());      return it.first().tostring(); } 

this method returns following:

{     _id = 599a62cac29d9a2684c64012,     constructionroomnumber = 15,     type = example,     summary = summary text,     description = description text,     status = open,     project = {         "$ref": "project",         "$id": "5996582a0983347784fb2ff4"     },     = [] } 

expected result:

{     _id: objectid('599a62cac29d9a2684c64012')     constructionroomnumber: "15"     type: "example"     summary: "summary text"     description: "description text"     status: "open"     project: {         _id: objectid('5996582a0983347784fb2ff4')         projectcode: "v1000"         projectname: "example project"         projectlocation: "1 somewhere street, city"         clientname: "whatever client"         isactive: true     } } 

here example of data stored in mongodb looks like:

example project:

_id: objectid('5996582a0983347784fb2ff4') projectcode: "v1000" projectname: "example project" projectlocation: "1 somewhere street, city" clientname: "whatever client" isactive: true 

example task:

_id: objectid('599a62cac29d9a2684c64012') constructionroomnumber: "15" type: "example" summary: "summary text" description: "description text" status: "open" project: dbref(project, 5996582a0983347784fb2ff4, undefined) 

wiki