Updating nested arrays in mongodb

Question

I have a document in mongodb with 2 level deep nested array of objects that I need to update, something like this:

{
    id: 1,
    items: [
        {
            id: 2,
            blocks: [
                {
                    id: 3
                    txt: 'hello'
                }
            ]
        }
    ] 
}

If there was only one level deep array I could use positional operator to update objects in it but for second level the only option I've came up is to use positional operator with nested object's index, like this:

db.objects.update({'items.id': 2}, {'$set': {'items.$.blocks.0.txt': 'hi'}})

This approach works but it seems dangerous to me since I'm building a web service and index number should come from client which can send say 100000 as index and this will force mongodb to create an array with 100000 indexes with null value.

Are there any other ways to update such nested objects where I can refer to object's ID instead of it's position or maybe ways to check if supplied index is out of bounds before using it in query?

Answer 1

Here's the big question, do you need to leverage Mongo's "addToSet" and "push" operations? If you really plan to modify just individual items in the array, then you should probably build these arrays as objects.

Here's how I would structure this:

{
    id: 1,
    items: 
        { 
          "2" : { "blocks" : { "3" : { txt : 'hello' } } },
          "5" : { "blocks" : { "1" : { txt : 'foo'}, "2" : { txt : 'bar'} } }
        }
}

This basically transforms everything in to JSON objects instead of arrays. You lose the ability to use $push and $addToSet but I think this makes everything easier. For example, your query would look like this:

db.objects.update({'items.2': {$exists:true} }, {'$set': {'items.2.blocks.0.txt': 'hi'}})

You'll also notice that I've dumped the "IDs". When you're nesting things like this you can generally replace "ID" with simply using that number as an index. The "ID" concept is now implied.

This feature has been added in 3.6 with expressive updates.

db.objects.update( {id: 1 }, { $set: { 'items.$[itm].blocks.$[blk].txt': "hi", } }, { multi: false, arrayFilters: [ { 'itm.id': 2 }, { 'blk.id': 3} ] } )