> db.user.drop()
true
> db.user.ensureIndex({a: 1, b: 1}, {sparse: true, unique: true})
{
        "createdCollectionAutomatically" : true,
        "numIndexesBefore" : 1,
        "numIndexesAfter" : 2,
        "ok" : 1
}
> db.user.insert({a: 1, b: 1})
WriteResult({ "nInserted" : 1 })
> db.user.insert({a: 1, b: 2})
WriteResult({ "nInserted" : 1 })
> db.user.insert({a: 1, b: 3})
WriteResult({ "nInserted" : 1 })
> db.user.insert({a: 2, b: 1})
WriteResult({ "nInserted" : 1 })
> db.user.insert({a: 2, b: 2})
WriteResult({ "nInserted" : 1 })
> db.user.insert({a: 2, b: 3})
WriteResult({ "nInserted" : 1 })
> db.user.insert({a: 3, b: 1})
WriteResult({ "nInserted" : 1 })
> db.user.insert({a: 3, b: 2})
WriteResult({ "nInserted" : 1 })
> db.user.insert({a: 3})
WriteResult({ "nInserted" : 1 })
> db.user.insert({a: 4, b: 1})
WriteResult({ "nInserted" : 1 })
> db.user.insert({a: 4, b: 2})
WriteResult({ "nInserted" : 1 })
> db.user.insert({a: 4})
WriteResult({ "nInserted" : 1 })
> db.user.insert({a: 4})
WriteResult({
        "nInserted" : 0,
        "writeError" : {
                "code" : 11000,
                "errmsg" : "E11000 duplicate key error collection: project1.user index: a_1_b_1 dup key: { : 4.0, : null }"
        }
})
> db.user.getInd
db.user.getIndexKeys(   db.user.getIndexSpecs(  db.user.getIndexes(     db.user.getIndices(
> db.user.getIndexes()
[
        {
                "v" : 1,
                "key" : {
                        "_id" : 1
                },
                "name" : "_id_",
                "ns" : "project1.user"
        },
        {
                "v" : 1,
                "unique" : true,
                "key" : {
                        "a" : 1,
                        "b" : 1
                },
                "name" : "a_1_b_1",
                "ns" : "project1.user",
                "sparse" : true
        }
]
>