db={"users":[{_id:2,name:"Foo",phoneverified:true},{_id:3,name:"Bar",phoneverified:true}],"usersmeta":[{"user":2,meta:"city",value:"london"},{"user":3,meta:"town",value:"london"}]}

db.users.aggregate([{$lookup:{from:"usersmeta",localField:"_id",foreignField:"user",as:"metas"}},{$match:{phoneverified:true,metas:{$elemMatch:{meta:"city",value:"london"}}}}])