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