在MongoDB中使用WHERE和JOIN

Question

I have 2 collections, I want to select the records from a collection according to another collection,

let me explain my mean by example:

Collection1:

id, user_id, completed
1 , 1     , true
2 , 1     , false
3 , 2     , true
4 , 3     , true
5 , 4     , false

Collection2:

id, username
1 , test
2 , test 2
3 , test 3
4 , test 4

I want to select users from collection2 which the completed column in collection1 are false (the result should be (1,test),(2,test 4) )

because I'm a newbie in MongoDB I don't know what should I do, I can do it in MySQL and PostgreSQL with join but here what should I do?

I read the aggregation-pipeline in MongoDB documentation but I cannot figure out.