MongoDB - upsert involving lists

Question

I'm a MongoDB newbie and wanted to ask how to write an update command involving upsert and list.

Basically I want to accomplish something like this:

{"_id" : ObjectId("4c28f62cbf8544c60506f11d"),
"some_other_data":"goes here",
"trips": [
    {"name": "2010-05-10",
     "loc": [{"lat":21.321231, "lng": 16.8783234, "updated_at": "Mon May 10 2010 15:24:35"}, 
        {"lat":21.321231, "lng": 16.8783234, "updated_at": "Mon May 10 2010 15:24:24"}]
    },
    {"name": "2010-05-08",
     "loc": [{"lat":21.324239, "lng": 16.8735234, "updated_at": "Mon May 8 2010 11:18:05"},
        {"lat":21.311234, "lng": 16.8743271, "updated_at": "Mon May 8 2010 11:17:55"}, 
        {"lat":21.321238, "lng": 16.8782219, "updated_at": "Mon May 8 2010 11:17:45"}]
    }
]}

Note that:

• You supply a trip name and the current location
• If the trip does not exist already, it needs to be created
• trips.name should be unique so that if it exists, you append to the location array

This is the query I wrote combining the positional operator with $push.

    db.mycollection.update({"application_id": "MyTestApp", 
                            "trips.name": "2010-05-10"},
                           {$push: {'trips.$.loc': {"lat":11, "lng":11} }}, 
                           true);

But this results in data like this:

> db.mycollection.find({"application_id":"MyTestApp"})          
{ "_id" : ObjectId("4c29044ebf8544c60506f11f"), 
"application_id" : "MyTestApp", 
"trips" : { "$" : { "loc" : [ { "lat" : 11, "lng" : 11 } ] }, 
"name" : "2010-05-10" } 
}

You can see that

• "trips" is not an array
• it took "$" literally and created a key with that (doh!)

So far I've been pretty happy with MongoDB, but there's definitely a steep learning curve with writing complicated queries.

Any feedback will be appreciated.

Thanks in advance, Amie

Answer 1

You can't mix the positional operator ("$") and an upsert; the "$" will be treated as a field name during the insert. You can't do this for new documents, only existing one.

I suggested a structure more like this:

{"_id" : ObjectId("4c28f62cbf8544c60506f11d"),
"some_other_data":"goes here",
"trips": { 
    "2010-05-10":
       [{"lat":21.321231, "lng": 16.8783234, "updated_at": "Mon May 10 2010 15:24:35"}, 
        {"lat":21.321231, "lng": 16.8783234, "updated_at": "Mon May 10 2010 15:24:24"}],
    "2010-05-08": 
       [{"lat":21.324239, "lng": 16.8735234, "updated_at": "Mon May 8 2010 11:18:05"},
        {"lat":21.311234, "lng": 16.8743271, "updated_at": "Mon May 8 2010 11:17:55"}, 
        {"lat":21.321238, "lng": 16.8782219, "updated_at": "Mon May 8 2010 11:17:45"}]
    }
}

Then you can issue an update like this:

db.mycollection.update({application_id: "MyTestApp", "trips.2010-05-10":{$exists:true}},
                       {$push: {"trips.2010-05-10": {lat:11, lng:11} }}, 
                       true);

results in this being inserted.

> db.mycollection.find()
{ "_id" : ObjectId("4c2931d17b210000000045f0"), 
    "application_id" : "MyTestApp", 
    "trips" : { "2010-05-10" : [ { "lat" : 11, "lng" : 11 } ] } }

and running it again give you this:

> db.mycollection.find()
{ "_id" : ObjectId("4c2932db7b210000000045f2"), 
    "application_id" : "MyTestApp", 
    "trips" : { "2010-05-10" : 
        [ { "lat" : 11, "lng" : 11 }, 
          { "lat" : 11, "lng" : 11 } ] } }

Answer 2

EDITED TO INCLUDE CORRECT SOLUTION

This is exactly the problem I hit learning Mongo - you're looking for the $addToSet operator (see docs here) that's used with the update command, in conjunction with the $ positional operator you were using.

$addToSet

{ $addToSet : { field : value } }

Adds value to the array only if its not in the array already.

The query thus becomes (db.stack is the collection I used for testing purposes), sample run to follow:

db.stack.update({ "trips.name":"2010-05-10" }, 
                { $addToSet: { "trips.$.loc":{"lat":11, "lng":12} } }
               );

TEST RUN (with some abbreviations for space of the elements that aren't important):

#### YOUR ITEM IN THE DB
> db.stack.find({"trips.name":"2010-05-10"})
{ "_id" : ObjectId("4c28f62cbf8544c60506f11d"), "some_other_data" : "goes here", 
    "trips" : [
    { "name" : "2010-05-10",
        "loc" : [ {
                "lat" : 21.321231,
                "lng" : 16.8783234,
                "updated_at" : "Mon May 10 2010 15:24:35"
            }, { "lat" : 21.321231,
                "lng" : 16.8783234,
                "updated_at" : "Mon May 10 2010 15:24:24"
            } ] },
    { "name" : "2010-05-08",
        "loc" : [ ... ]
    } ] }
#### SUCCESSFULLY ADDS ITEM TO PROPER ARRAY
> db.stack.update({"trips.name":"2010-05-10"}, {$addToSet: {"trips.$.loc":{"lat":11, "lng":11}}});
> db.stack.findOne()
{ "_id" : ObjectId("4c28f62cbf8544c60506f11d"), "some_other_data" : "goes here",
    "trips" : [
        { "loc" : [
                { "lat" : 21.321231,
                    "lng" : 16.8783234,
                    "updated_at" : "Mon May 10 2010 15:24:35"
                }, { "lat" : 21.321231,
                    "lng" : 16.8783234,
                    "updated_at" : "Mon May 10 2010 15:24:24"
                }, { "lat" : 11,
                    "lng" : 11
                }
            ], "name" : "2010-05-10"
        },
        { "name" : "2010-05-08",
            "loc" : [ ...  ]
        } ] }
#### ON REPEAT RUN DOESN'T ADD NEW ELEMENT
> db.stack.update({"trips.name":"2010-05-10"}, {$addToSet: {"trips.$.loc":{"lat":11, "lng":11}}});
> db.stack.findOne()
{ "_id" : ObjectId("4c28f62cbf8544c60506f11d"), "some_other_data" : "goes here",
    "trips" : [ {
            "loc" : [
                { "lat" : 21.321231,
                    "lng" : 16.8783234,
                    "updated_at" : "Mon May 10 2010 15:24:35"
                }, { "lat" : 21.321231,
                    "lng" : 16.8783234,
                    "updated_at" : "Mon May 10 2010 15:24:24"
                }, { "lat" : 11,
                    "lng" : 11
                }
            ], "name" : "2010-05-10"
        },
        { "name" : "2010-05-08",
            "loc" : [ ...  ]
        } ] }
#### BUT WILL CORRECTLY ADD ANOTHER ELEMENT TO THE SAME ARRAY IF IT'S NOT PRESENT
> db.stack.update({"trips.name":"2010-05-10"}, {$addToSet: {"trips.$.loc":{"lat":11, "lng":12}}});
> db.stack.findOne()
{ "_id" : ObjectId("4c28f62cbf8544c60506f11d"), "some_other_data" : "goes here",
    "trips" : [
        { "loc" : [
                { "lat" : 21.321231,
                    "lng" : 16.8783234,
                    "updated_at" : "Mon May 10 2010 15:24:35"
                }, { "lat" : 21.321231,
                    "lng" : 16.8783234,
                    "updated_at" : "Mon May 10 2010 15:24:24"
                }, { "lat" : 11,
                    "lng" : 11
                }, { "lat" : 11,
                    "lng" : 12
                }
            ], "name" : "2010-05-10"
        },
        { "name" : "2010-05-08",
            "loc" : [ ... ]
    } ] }