How to query relations in NoSQL?

5
votes

I have decided to move my database from MySQL to Mongo because majority of the time, my data is not structured. And it allowed me possibilities that was too complex in a traditional SQL.

There is one problem that I am currently facing and how to approach a traditional relational model of SQL in NoSQL. I have read many times that NoSQL is not designed to deal with relations. Do I need to add them as an array to the document with a relation?

Here is one situation that has made me stuck. In SQL I had a separate table for oauth access tokens that has user_id, client_id, access_token, expires as its attributes. It was 1-N relation between a user and an access_token. How would I do that in NoSQL? By adding an array field oauth_tokens? If I do that, how do I search for the token in the array? How do I query

search for a document where the _id is $user_id and there is an element
with $token in the access_tokens array?
1
mongodbnosql
You could use map-reduce concept in mongodb to form relationship with two different collections. - Kumar Kailash
@KumarKailash Likely that you could not. MongoDB works on one collection at a time, and it "deliberately" does this for a reason. This question needs to provide a specific point and example problem to solve or it is basically "too broad". So if the author wants a real answer, then it is suggested to actually describe that problem rather than ask for generalizations. - Neil Lunn
Before I asked this question, I had close to zero knowledge of "relations" in mongo; after googling and trying and testing I have a little bit of a knowledge. Sorry about that. I got what I needed for now but I am pretty sure I will be asking a similar question here in the nearest future and at that time I will be very specific. - Gasim

1 Answers

2
votes

You have at least 2 options here:

  1. You can store oauth_tokens in separate collection (just like you had in MySQL in other table), and add to oauth_token field like user_id containing _id value for this current user from users collection. Finding tokens for specified user is just searching in oauth_tokens collection for documents with given user_id. Keep in mind that this kind of relation isn't "supported" in any way by Mongo - database will not help you keeping values of field user_id correct.

Example:

db.tokens.insert({ client_id : "1", user_id : "20", access_token : "1234567890", expires : new Date(2014-12-31)})

query:

db.tokens.find({user_id:"20"})
  1. Just like you wrote: you can embedded tokens in user document and query for existing tokens. Check docs to see how you can query for embedded documents: link