db={users:[{name:"John",children:["Mary"]},{name:"Mary",children:[]}],tokens:[{name:"someTokenName",owner:"John"}]}

db.users.aggregate([{$lookup:{from:"tokens",localField:"name",foreignField:"owner",as:"token"}},{$project:{name:1,children:1,token:{"$filter":{"input":"$token","cond":{$eq:["$$this.name","nonexists"]}}}}}])