dropIndex( ) , is used to remove the index from a collection.
By default, mongoDB system creates an index on _id when first document is inserted into collection.
As per my understanding of mongoDB, a user can remove indexes which are created by user. I meant to say, index on _id can't be removed.
When I tried to remove index on _id, system throws following error.
"nIndexesWas" : 1, "ok" : 0, "errmsg" : "cannot drop _id index" }