db={"man":[{_id:1,name:"Jack",children:["Joy","Joe","James"]},{_id:2,name:"Molly",children:["Milly","Mou"]}],age:[{_id:1,name:"Joy",age:10},{_id:2,name:"Joe",age:12},{_id:3,name:"James",age:14},{_id:4,name:"Milly",age:9},{_id:5,name:"Mou",age:6}]}

db.man.aggregate([{$match:{name:"Jack"}},{$unwind:"$children"},{$lookup:{from:"age",foreignField:"name",localField:"children",as:"age"}},{$unwind:"$age"},{$project:{_id:0,name:"$children",age:"$age.age"}}])