Using as a graph database for finding "friends" of "friends" in MongoDb

Question

I have been investigating a graph database and I have found neo4j and although this seems ideal I have also come across Mongodb.

Mongodb is not an official graph database but I wondered if it could be used for my scenario.

I am writing an application where users can have friends and those friends can have friends etc, the typical social part of a social network.

I was wondering in my situation whether Mongodb might suffice. How easy would it be to implement or do I really need to focus on REAL graph databases?

I do notice foursquare are using Mongodb so I presume it supports their infrastructure.

But how easy would it be to find all friends of my friends that also have friends in common, for example?

Answer 1

Although it wouldn't be impossible, MongoDB would not be a good fit for this scenario.

The reason is that MongoDB does not do JOINs. When you need a query which spans multiple documents, you need a separate query for each document.

In your example, each user document would have an array with the _id's of their friends. To find "all friends of the friends of UserA who are also friends of UserB" would mean that you would:

1. find userA and get his friends-array
2. find all users in that array and get their friend-arrays
3. find all users in these arrays who have UserB in their friends-array

These are three queries you have to perform. Between each of these queries, the result set has to be sent to the application, the application has to formulate a new query and send it back to the database. The result-set returned from the 2nd query can be quite large, which means that the 3rd query could take a while.

tl;dr: Use the right tool for the job. When your data is graph-based and you want to do graph-based queries on it, use a graph database.