Q:

EnsureIndex for likes in MongoDB

well, i am creating a network that allows users creating posts and like them. 
Asking on stackoverflow i've understood how to structure my database: 
A collection which includes a document for each post. 
A collection which includes a document for each like, in each of these documents there is a reference to post is referenced to. 
When i want to get ALL likes about a post i can query the like collection looking for the reference to that post.
And till here i am ok. But assuming i'll have millions documents in like collection, i wondered how could i query and search among them in not too long time. 
And i was advised of ensureIndex, in this case, i have to ensureindex of the field which contains reference to a post. 
But when do i have to create this index? is enough to create it once (for example when i set up my database) and it will be as default in mongodb or do i have to do it during application life-time? thank you 

A:

But assuming i'll have millions documents in like collection, i wondered how could i query and search among them in not too long time. 

I assume you would most likely want to do a count on the likes as an example?
You can't, instead you use optimizations to combat this. A count on millions of rows might get a bit slow.
A typical scenario are counters in SQL techs that you use to amend the parent row with a sum figure of its children.
Same applies to MongoDB.
You would aggregate important data to the top.
If you require to actually query the likes to show some who have liked it then you limit those likes. Google+ and other networks tend to limit the amount of likes they show to about 1,000.

And i was advised of ensureIndex,

Adding indexes to a database does help with actually searching for documents.

But when do i have to create this index? is enough to create it once

Yes, MongoDB will manage the index itself. You only need to ensure it once.

