How to retrieve data from related documents by ID?

Question

I'm trying to understand how to set up basic relations in mongoDB. I've read a bit about it in the documentation but it's a little terse.

This should be pretty simple: I'm trying to log a list of impressions and the users who are responsible for the impressions. Here's some examples of log documents:

{type: '1', userId:'xxx-12345'}
{type: '1', userId:'xxx-12345'}
{type: '1', userId:'xxx-12345'}
{type: '2', userId:'zzz-84638'}
{type: '2', userId:'xxx-12345'}

Here's an example of a user document:

{userId: 'xxx-12345', location: 'US'}

Is there a way to count the total number of documents which "belong" to a userId of xxx-12345, where type is 1?

In the above case, I'd want to see a result like { '1':3, '2':1 }.

Also, is the above an acceptable way of creating the relationships?

You would like to count the impressions per type per userId? Or you would like to count the impressions per type for a given userid? — attish, Oct 05 '13 at 09:29

score 1 · Accepted Answer · answered Oct 05 '13 at 09:37

For your 1st question Is there a way to count the total number of documents which "belong" to a userId of xxx-12345, where type is 1?, below is the solution:

db.impressions.aggregate({
                           $match: {
                                userId: 'xxx-12345',
                                type: 1
                           }
                         },
                         {
                              $group: { _id: null, count: { $sum: 1 } }
                         });

To get the solution in format you specified (In the above case, I'd want to see a result like { '1':3, '2':1 }.), use below code:

db.impressions.aggregate({
                       $match: {
                            userId: 'xxx-12345',
                       }
                     },
                     {
                          $group: { _id: '$type', totalImpressions: { $sum: 1 } }
                     });

Agis · Answer 2 · 2013-10-05T10:02:12.447

You can use the Aggregation Pipeline introduced in version 2.2:

db.a.aggregate([
  { $match: { userId: 'xxx-12345' } },
  { $group: { _id: "$type", total: { $sum: 1 } } }
])

This will output:

{
        "result" : [
                {
                        "_id" : "2",
                        "total" : 1
                },
                {
                        "_id" : "1",
                        "total" : 3
                }
        ],
        "ok" : 1
}

where "_id" is the type and "total" is the count that type appears in user "xxx-12345".

However, if you want to get only the total number of documents which belong to "xxx-12345" where the type is "1" you can do it like this:

db.a.aggregate([
  { $match: { userId: 'xxx-12345', type: "1" } },
  { $group: { _id: null, count: { $sum: 1} } }
])

which will output the following:

{ "result" : [ { "_id" : null, "count" : 3 } ], "ok" : 1 }

where "count" is what you're looking for.

How to retrieve data from related documents by ID?

2 Answers2