Question

Good day everyone.
Suppose we have a collection and a document which looks something like this:

    test_doc = {
    "ID" : "123",
    "a" : [
                    {
                        'x' : "/",
                        'y' : "2000",
                        'z' : "1000"
                    },
                    {
                        'x' : "/var",
                        'y' : "3500",
                        'z' : "3000"
                    }
           ]

      }

What i need is to retrieve a single property a.z .
In MongoDB i’m using the following query:

db.testcol.find({"ID":"123","a.x":"/"},{'a.z':1})

which returns this:

{ "_id" : ObjectId("skipped"), "a" : [ { "z" : "1000" }, { "z" : "3000" } ] }

As you can see it returns all the z properties, but i need only the first one or the second when condition is {"ID":"123","a.x":"/var"}
So, the question is: how do i get a single property in this situation? Is it just a matter of bad design or should i somehow process the returned document in code (python)? Any suggestions will be much appreciated.

Answer 1

In MongoDB 2.0 and older, this is not possible. What you want to do is return a specific element of the array – but that is not what your projection is actually doing, it will just return the whole array and then the z element of each one.

However, with 2.2 (rc2 as of writing this answer), things have gotten a bit better. You can now use $elemMatch as part of your projection (see SERVER-2238 for details) so that you only pull back the required array element. So, try something like this:

db.foo.find({"ID":"123",'a':{$elemMatch:{'x':"/"}}},{_id : 0, 'a.$': 1})
//returns
{ "a" : [ { "x" : "/", "y" : "2000", "z" : "1000" } ] }

Or, just use $elemMatch in the projection itself, which you may think is cleaner:

db.foo.find({"ID":"123"},{_id : 0, 'a':{$elemMatch:{'x':"/"}}})
//returns 
{ "a" : [ { "x" : "/", "y" : "2000", "z" : "1000" } ] }

So, now, at least the array returned is only the one containing only the entries you want and you can simply reference the relevant z element (elemMatch projections on a subdocument are not yet supported).

Last but not least, in 2.2 we have the aggregation framework, and one of the things it can do (with the $project operator, is to reshape your documents and change sub documents and array elements into top level arrays. To get your desired result, you would do something like this:

db.foo.aggregate( 
        {$match : {"ID":"123"}},  
        {$unwind : "$a"},  
        {$match : {"a.x":"/"}},  
        {$project : {_id : 0, z : "$a.z"}}
)

The result looks like this:

{ "result" : [ { "z" : "1000" } ], "ok" : 1 }