When trying to ensure a unique index :

Using the shell :
`db.my_collection.ensureIndex(

{field1: 1, field2: 1}

,

{unique: true, safe: true}

)`

-> "errmsg" : "E11000 duplicate key error index: planotemp.my_collection.$field1_1_ field2_1 dup key: { : \"1319530\", : 360 }"

Using the driver :
db.collection('my_collection').ensureIndex(

{ field1: 1, field2: 2 }

,

{ unique: true, safe: true }

, function(err, res) {});

-> `err` is null