How to return index of array item in Mongodb?

Question

The document is like below.

{
    "title": "Book1",
    "dailyactiviescores":[
        {
            "date": 2013-06-05,
            "score": 10,
        },
        {
            "date": 2013-06-06,
            "score": 21,
        },
    ]
}

The daily active score is intended to increase once the book is opened by a reader. The first solution comes to mind is use "$" to find whether target date has a score or not, and deal with it.

err = bookCollection.Update(
    {"title":"Book1", "dailyactivescore.date": 2013-06-06},
    {"$inc":{"dailyactivescore.$.score": 1}})
if err == ErrNotFound {
    bookCollection.Update({"title":"Book1"}, {"$push":...})
}

But I cannot help to think is there any way to return the index of an item inside array? If so, I could use one query to do the job rather than two. Like this.

index = bookCollection.Find(
    {"title":"Book1", "dailyactivescore.date": 2013-06-06}).Select({"$index"})
if index != -1 {
    incTarget = FormatString("dailyactivescore.%d.score", index)
    bookCollection.Update(..., {"$inc": {incTarget: 1}})
} else {
    //push here
}

Asya Kamsky · Accepted Answer

Incrementing a field that's not present isn't the issue as doing $inc:1 on it will just create it and set it to 1 post-increment. The issue is when you don't have an array item corresponding to the date you want to increment.

There are several possible solutions here (that don't involve multiple steps to increment).

One is to pre-create all the dates in the array elements with scores:0 like so:

{
    "title": "Book1",
    "dailyactiviescores":[
        {
            "date": 2013-06-01,
            "score": 0,
        },
        {
            "date": 2013-06-02,
            "score": 0,
        },
        {
            "date": 2013-06-03,
            "score": 0,
        },
        {
            "date": 2013-06-04,
            "score": 0,
        },
        {
            "date": 2013-06-05,
            "score": 0,
        },
        {
            "date": 2013-06-06,
            "score": 0
        },  { etc ... }
    ]
}

But how far into the future to go? So one option here is to "bucket" - for example, have an activities document "per month" and before the start of a month have a job that creates the new documents for next month. Slightly yucky. But it'll work.

Other options involve slight changes in schema.

You can use a collection with book, date, activity_scores. Then you can use a simple upsert to increment a score:

   db.books.update({title:"Book1", date:"2013-06-02", {$inc:{score:1}}, {upsert:true})

This will increment the score or insert new record with score:1 for this book and date and your collection will look like this:

{
    "title": "Book1",
    "date": 2013-06-01,
    "score": 10,
},
{
    "title": "Book1",
    "date": 2013-06-02,
    "score": 1,
}, ...

Depending on how much you simplified your example from your real use case, this might work well.

Another option is to stick with the array but switch to using the date string as a key that you increment:

Schema:

{
    "title": "Book1",
    "dailyactiviescores":{
            { "2013-06-01":10},
            { "2013-06-02":8}
    }
 }

Note it's now a subdocument and not an array and you can do:

db.books.update({title:"Book1"}, {"dailyactivityscores.2013-06-03":{$inc:1}})

and it will add a new date into the subdocument and increment it resulting in:

{
    "title": "Book1",
    "dailyactiviescores":{
            { "2013-06-01":10},
            { "2013-06-02":8},
            { "2013-06-03":1}
    }
 }

Note it's now harder to "add-up" the scores for the book so you can atomically also update a "subtotal" in the same update statement whether it's for all time or just for the month.

But here it's once again problematic to keep adding days to this subdocument - what happens when you're still around in a few years and these book documents grow hugely?

I suspect that unless you will only be keeping activity scores for the last N days (which you can do with capped array feature in 2.4) it will be simpler to have a separate collection for book-activity-score tracking where each book-day is a separate document than to embed the scores for each day into the book in a collection of books.

How to return index of array item in Mongodb?

Answers (2)

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