How to retrieve data from related documents by ID? - mongodb

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?

Answer 1

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 } }
});

Answer 2

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.