Replace or Skip Part of Array in MongoDB

Question

Assume I have a collection with documents that look like this:

{
    "_id": ";lkajsdflhkadfhaewifjasdkfjalksdfjs",
    "tree": [
        { "id": "a", "name": "One" },
        { "id": "b", "name": "Two" },
        { "id": "c", "name": "Three" },
        { "id": "d", "name": "Four" }
    ]
}

Now let's say I want to replace the a, b, and c entries in my tree array with e, f, and c entries. Is it possible to do that replace with an update query? If not, is there a way to select the document such that the tree array only contains the c and d entries (or just the d entry)? I want my document to look like this:

{
    "_id": ";lkajsdflhkadfhaewifjasdkfjalksdfjs",
    "tree": [
        { "id": "e", "name": "Five" },
        { "id": "f", "name": "Six" },
        { "id": "c", "name": "Three" },
        { "id": "d", "name": "Four" }
    ]
}

Order of the tree array matters. I'm aware of $splice, but I do not know ahead of time the index of the c entry. Also, the index may vary between documents. Can I do a query inside of $splice that lets me find the index?

Is your array really large normally? Most of the array operators are designed to work with a single index (or all). — WiredPrairie
– WiredPrairie, Commented Nov 22, 2013 at 16:51
The array shouldn't be too large, but it's not technically limited so I can't say it will always be small. — Nik
– Nik, Commented Nov 22, 2013 at 17:33
The easiest solution is just to load the entire doc and save it with whatever your platform/driver is. — WiredPrairie
– WiredPrairie, Commented Nov 22, 2013 at 17:56

Diver · Accepted Answer · 2013-11-22 16:55:00Z

1

How about doing a find().forEach?

db.test.find().forEach(function(doc){for (var i = 0; i < doc.tree.length; i++){
  switch(doc.tree[i].id){
    case "a": doc.tree[i] = { "id": "e", "name": "Five" };
    break;
    case "b": doc.tree[i] = { "id": "f", "name": "Six" };
    break;
  }} db.test.save(doc)});

Of course you can put in more specific logic to fit your rules but this will simply replace the a entries with e and b with f.

answered Nov 22, 2013 at 16:55

Diver

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1 Comment

Nik Over a year ago

I was using a,b,c,d,e,f as examples. It's unknown in practice what they will be.

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