Membuat Index Unique
Single Field
`db.collection.createIndex( <key and index type specification>, { unique: true } )`
Contoh `db.members.createIndex( { “user_id”: 1 }, { unique: true } )`
Drop Index Unique
`db.groups.dropIndex(“nama_key”)`
Contoh `db.members.createIndex( “user_id”)`
