Querying data from Mongo collection using javascript

Question

This is the first time I am using mongo for a meteor project, which is to help collaborate a group of users to read the holy scriptures. The data structure for the collection looks as follows:

//collection is named as 'logs'
[
    {
        _id: someObjectId,
        startPage: 1,
        finishPage: 4,
        status: 'in progress',
        userId: someUserId1
    },
    {
        _id: someObjectId,
        startPage: 5,
        finishPage: 10,
        status: 'done',
        userId: someUserId2
    }
    //.... and so on and normally two users wont read the same page.
]

I am trying to figure out the following:

Total Pages "Done"
Total Pages "In Progress"
Next available Page
Missing Pages

Now due to my limited knowledge of mongo I'm stuck at this point. I have been looking at various solutions but not sure which one would be the right fit:

Aggregation (but I'm unclear as to how I would achieve the required data)
MapReduce (should this be used every time when a new log is added or MapReduce is only meant to run as a batch job)
Create a new separate collection that would hold all the tracking data and update it every time a log is either added or updated

e.g.

 [
    {
        page: 1,
        inProgress: 0, //users would normally not read the same page twice but they may
        done: 1
    },
    {
        page: 2,
        inProgress: 1, //users would normally not read the same page twice but they may
        done: 0
    }
]

I would be really grateful if somebody could provide some insight into this and preferred way of doing it. It may be obvious but I'm finding it a bit hard. Thanks.

Your question is unfortunately not very clear. Logs are normally written but never modified. You could also track accesses in the original collection that's being read but doing that would eventually cause your documents to get very large, perhaps exceeding the 16MB doc limit. What are you trying to do with the logs afterwards? Show who read what when and how many times? Presumably you want to create some reports. — Michel Floyd
– Michel Floyd, Commented Jan 19, 2016 at 8:02
Thanks @MichelFloyd. Apologies for that. I missed the main part out. I was trying to get the formatting right and the main bit got deleted (by fault of my own). I've updated the question now with what I am trying to figure out. You are also right that the logs are not meant to be modified and I should use a better name but in this case they can be modified. — Kamalpreet
– Kamalpreet, Commented Jan 19, 2016 at 10:37

Kevin Smith · Accepted Answer · 2016-01-19 14:07:25Z

2

I'd try to model your documents as per the usage you're trying to achieve, If you split up the logs in to just pages read then you can do a simple group by.

So say we change your models in to the following:

[{
    _id: new ObjectId(),
    pageNumber: 1,
    status: 'in progress',
    userId: 1
},
{
    _id: new ObjectId(),
    pageNumber: 2,
    status: 'in progress',
    userId: 1
},
{
    _id: new ObjectId(),
    pageNumber: 3,
    status: 'in progress',
    userId: 1
},
{
    _id: new ObjectId(),
    pageNumber: 4,
    status: 'in progress',
    userId: 1
},
{
    _id: new ObjectId(),
    pageNumber: 5,
    status: 'in progress',
    userId: 1
},
{
    _id: new ObjectId(),
    pageNumber: 6,
    status: 'done',
    userId: 2
},
{
    _id: new ObjectId(),
    pageNumber: 7,
    status: 'done',
    userId: 2
},
{
    _id: new ObjectId(),
    pageNumber: 8,
    status: 'done',
    userId: 2
},
{
    _id: new ObjectId(),
    pageNumber: 9,
    status: 'done',
    userId: 2
},
{
    _id: new ObjectId(),
    pageNumber: 10,
    status: 'done',
    userId: 2
},
{
    _id: new ObjectId(),
    pageNumber: 9,
    status: 'done',
    userId: 1
},
{
    _id: new ObjectId(),
    pageNumber: 10,
    status: 'done',
    userId: 1
}]

Then we can run the following aggregation query:

> db.logs.aggregate([ {$group: { _id: { "pageNumber" : "$pageNumber", "status" : "$status"}, count : {$sum : 1}}} ]).pretty()
{ "_id" : { "pageNumber" : 8, "status" : "done" }, "count" : 1 }
{ "_id" : { "pageNumber" : 7, "status" : "done" }, "count" : 1 }
{ "_id" : { "pageNumber" : 6, "status" : "done" }, "count" : 1 }
{ "_id" : { "pageNumber" : 4, "status" : "in progress" }, "count" : 1 }
{ "_id" : { "pageNumber" : 9, "status" : "done" }, "count" : 2 }
{ "_id" : { "pageNumber" : 3, "status" : "in progress" }, "count" : 1 }
{ "_id" : { "pageNumber" : 10, "status" : "done" }, "count" : 2 }
{ "_id" : { "pageNumber" : 2, "status" : "in progress" }, "count" : 1 }
{ "_id" : { "pageNumber" : 5, "status" : "in progress" }, "count" : 1 }
{ "_id" : { "pageNumber" : 1, "status" : "in progress" }, "count" : 1 }

answered Jan 19, 2016 at 14:07

Kevin Smith

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2 Comments

Kamalpreet Over a year ago

Thanks Kevin. That helps. However using this solution how can the missing pages be worked out?

Kevin Smith Over a year ago

you'd need like a book collection that you could store the book size in then you could match against that.

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