Question

I have two collections, Group and User. Originally, my Group document contained an array of User references. But I’ve changed my mapping so that Users now hold a reference to a Group. I am wondering how I can write a query to update all the existing User documents to reference the group that is currently referencing them, then remove any references on the groups.

Answer 1

I didn’t notice the language you are using at first – I had to do something very similar in Python so here is my example code:

# I changed my collection names to users/groups hope I mapped them right :)
users = db.users
groups = db.groups

for u in users.find():
    grp = groups.find( { "users" : u['userId'] } )
    u['groups'] = [ g['groupId'] for g in grp ]
    users.save(u)

Basically, for every user, I look in groups to find a list of groups which contain userId in the array users, and I make a list called groups in u and when I save it adds an array of groups to users document.

Should be not hard to do it in PHP

$users = $db->users;
$groups = $db->groups;

// find all users
$ucursor = $users->find();

// iterate through users, find groups which have the user,
// update user with array of groups
foreach ($ucursor as $u) {
$gcursor = $groups->find( … );
foreach ($gcursor as $g) {
…
}
}

But this seems like a one time operation, you could do it in mongo shell using something similar to:

var users = db.users.find();
while (users.hasNext()) {
   u = users.next();
   var gs = db.groups.find({"users":u.userId},{"groupId:1}).toArray();
   db.users.update({"groupId":groupId},{$set:{"groups":gs}});
}