MongoDB: Should you still provide IDs linking to other collections to or just include collections?

Question

I'm pretty new to MongoDB and NoSQL in general. I have a collection Topics, where each topics can have many comments. Each comment will have metadata and whatnot making a Comments collection useful.

In MySQL I would use foreign keys to link to the Comments table, but in NoSQL should I just include the a Comments collection within the Topics collection or have it be in a separate collection and link by ids?

Thanks! Matt

Answer 1

It depends.

It depends on how many of each of these type of objects you expect to have. Can you fit them all into a single MongoDB document for a given Topic? Probably not.

It depends on the relationships - do you have one-to-many or many-to-many relationships? If it's one-to-many and the number of related entities is small you might chose to put embed them in an IList on a document. If it's many-to-many you might chose to use a more traditional relationship or you might chose to embed both sides as ILists.

You can still model relationships in MongoDB with separate collections BUT there are no joins in the database so you have to do that in code. Loading a Topic and then loading the Comments for it might be just fine from a performance perspective.

Other tips:

With MongoDB you can index INTO arrays on documents. So don't think of an Index as just being an index on a simple field on a document (like SQL). You can use, say, a Tag collection on a Topic and index into the tags. (See http://www.mongodb.org/display/DOCS/Indexes#Indexes-Arrays)

When you retrieve or write data you can do a partial read and a partial write of any document. (see http://www.mongodb.org/display/DOCS/Retrieving+a+Subset+of+Fields)

And, finally, when you can't see how to get what you want using collections and indexes, you might be able to achieve it using map reduce. For example, to find all the tags currently in use sorted by their frequency of use you would map each Topic emitting the tags used in it, and then you would reduce that set to get the result you want. You might then store the result of that map reduce permanently and only up date it when you need to.

It's a fairly significant mind-shift from relational thinking but it's worth it if you need the scalability and flexibility a NOSQL approach brings.

Answer 2

Also look at the Schema Design docs (http://www.mongodb.org/display/DOCS/Schema+Design). There are also some videos/slides of several 10Gen presentations on schema design linked on the Mongo site. See http://www.mongodb.org/pages/viewpage.action?pageId=17137769 for an overview.