A couple of short commands to add unique constraints on sequential index fields in a mongoDB document

create_unique_sequential_index_mongo_db.js

// Mongo shell

// Let's say you need to have a sequential id; here, it is '_i'
// the other entry is a reference to another 'post' containing its natural MongoDB ObjectId
db.your_collection_name.insert({"_i":new NumberLong(1), "post_id":"4cf68ff393eae6291e00003a"});

// let's make sure we add an index that is defined as unique
db.your_collection_name.ensureIndex({"_i":1}, {unique:true}); 

// TODO: still need to provide the methodology used to auto-increment the unique field '_i'