db={"users":[{"_id":ObjectId("5a934e000102030405000005"),name:"User 1",org_id:"5a934e000102030405000007"},{"_id":ObjectId("5a934e000102030405000006"),name:"User 2",org_id:"5a934e000102030405000008"}],"organizations":[{"_id":ObjectId("5a934e000102030405000007"),"name":"Org 1"},{"_id":ObjectId("5a934e000102030405000008"),"name":"Org 2"}]}

db.users.aggregate([{$lookup:{from:"organizations",let:{objId:{$toObjectId:"$org_id"}},pipeline:[{$match:{$expr:{$eq:["$_id","$$objId"]}}},{$project:{_id:0,name:1}}],as:"org_name"}},{$set:{org_name:{$first:["$org_name.name"]}}}])