Multi key indexes

Important note: This article is in relation to online MongoDB course. For more information about the course and other posts describing its content please check my main page here: M101P: MongoDB for Developers course. 

Indexes

Insert some test data into db.

> db.students.insert( { name : "rado" , teachers: [0,1]} )
> db.students.insert( { name : "adam" , teachers: [0,1,3]} )
> db.students.find()
{ "_id" : ObjectId("52c186421fdaf7a8e42b43b8"), "name" : "rado", "teachers" : [  0,  1 ] }
{ "_id" : ObjectId("52c186531fdaf7a8e42b43b9"), "name" : "adam", "teachers" : [  0,  1,  3 ] }

That way you create an index on the array attribute.

> db.students.ensureIndex( { teachers:1 }  )
> db.system.indexes.find()
{ "v" : 1, "key" : { "teachers" : 1 }, "ns" : "school.students", "name" : "teachers_1" }

The usage of the index should be transparent when using the find method but you can always she the execution plan.

> db.students.find()
{ "_id" : ObjectId("52c186421fdaf7a8e42b43b8"), "name" : "rado", "teachers" : [  0,  1 ] }
{ "_id" : ObjectId("52c186531fdaf7a8e42b43b9"), "name" : "adam", "teachers" : [  0,  1,  3 ] }
{ "_id" : ObjectId("52c187621fdaf7a8e42b43bb"), "name" : "jon", "teachers" : [  2,  3 ] }
>
> db.students.find( { teachers : { $all : [0,1]} } )
{ "_id" : ObjectId("52c186421fdaf7a8e42b43b8"), "name" : "rado", "teachers" : [  0,  1 ] }
{ "_id" : ObjectId("52c186531fdaf7a8e42b43b9"), "name" : "adam", "teachers" : [  0,  1,  3 ] }


> db.students.find( { teachers : { $all : [0,1]} } ).explain()
{
        "cursor" : "BtreeCursor teachers_1",   ------ this proves that we use the index 
        "isMultiKey" : true,
        "n" : 2,
        "nscannedObjects" : 2,
        "nscanned" : 2,
        "nscannedObjectsAllPlans" : 2,
        "nscannedAllPlans" : 2,
        "scanAndOrder" : false,
        "indexOnly" : false,
        "nYields" : 0,
        "nChunkSkips" : 0,
        "millis" : 11,
        "indexBounds" : {
                "teachers" : [
                        [
                                0,
                                0
                        ]
                ]
        },
        "server" : "mongo2:27017"
}