Make nested queries in MongoDB 3.0.7

Question

I have sub-events belonging to events belonging to an user, and I know the user's username - how do I get a list of the sub-events?

Is this going in the right direction? Or is it completely off?

db.subevents.find({_id: {$in: 
    db.events.find({_id: {$in:
        db.users.find({"username":"userx"},{_id:1})}},{_id:1})}})

Edit: Here is a sample of the data structure:

/* Event */
{
  "_id" : "XjhAqqNBkezKY3mdN",
  "name" : "My event",
  "userId" : "FiKsAAAgBb7cNoPH7"
}
/* Subevent */
{
  "_id" : "WkYAqBXNpJryp7rum",
  "name" : "The subevent",
  "eventId" : "hQXNzX3jbWppbAYFH"
}
/* User */
{
  "_id" : "RTHh5srhLMQp625zF",
  "username" : "userx"
}

you cannon use such syntax in mongo - but thh will be a nice feature — profesor79
– profesor79, Commented Jun 16, 2016 at 10:57
@KaSh there is lookup over lookup - so that is not implemented even in 3.2 :-) — profesor79
– profesor79, Commented Jun 16, 2016 at 11:21

Cos · Accepted Answer · 2016-06-17 07:34:10Z

1

Following profesor79's advice to use three different calls, I put together this solution to get all the sub-events belonging to the user:

var userIds = db.users.find({"username":"userx"}).map(function(user) { 
  return user._id; 
});
var eventIds= db.events.find({userId: {$in:userIds}}).map(function(event) { 
  return event._id; 
});
db.subevents.find({eventId:{$in:eventIds}});

answered Jun 17, 2016 at 7:34

Cos

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Comments

profesor79 · Accepted Answer · 2016-06-16 13:57:46Z

0

The only way is to make 3 ddifrent calls in mongo 3.0.7

GetUSerData
GetEventData
GetSubEventData

As you have relational schema here - this should be easy.

You could improve schema and remove subEvent collection by embedding subEvents documents into one events document

EDIT

/* Event */
{
    "_id" : "XjhAqqNBkezKY3mdN",
    "name" : "My event",
    "userId" : "FiKsAAAgBb7cNoPH7"
    "subewents" : [{
            "_id" : "WkYAqBXNpJryp7rum",
            "name" : "The subevent",
            "eventId" : "hQXNzX3jbWppbAYFH"
        }, {
            "_id" : "WkYAqBXNpJryp7rum2",
            "name" : "The subevent2",
            "eventId" : "hQXNzX3jbWppbAYFH"
        }, {
            "_id" : "WkYAqBXNpJryp7rum",
            "name" : "The subevent",
            "eventId" : "hQXNzX3jbWppbAYFH"
        }
    ]
}

/* User */
{
    "_id" : "RTHh5srhLMQp625zF",
    "username" : "userx"
}

edited Jun 16, 2016 at 13:57

answered Jun 16, 2016 at 11:35

profesor79

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3 Comments

Cos Over a year ago

So for example I should extract the eventIds into an array and then use that array to get the subEvent data? If it's not too much trouble, could you show us a little code sample?

profesor79 Over a year ago

@Cos - please see my edit with proposed structure - and could you tell me if this have a sense in your case

Cos Over a year ago

Thank you, however changing the schema is not an option for me at this point, I'd still be interested in the construction of the 3 different calls.

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