Filtering embedded documents in MongoDB

Question

I am having trouble grasping how to filter embedded documents in MongoDB, and am starting to think I should be using a relational association, but that feels wrong in the document-store context.

Sticking with a typical blog/comment system, I have a collection of blogs, and each blog has many comments. The comments are stored as embedded documents inside the blog document.

It is very simple to filter my blogs collection, but in order to filter my comments embedded in each blog, I am having to load them all into memory (retrieve all into a Ruby array), and loop through each comment, returning ones that match a specific criteria.

My efforts to filter embedded documents using dot notation is failing, and bringing back all sub documents.

Is there a better way of getting MongoDB to filter these for me, or should I resign myself to relational associations? (Pulling back all embedded documents and manually filtering is going to be too intensive in the long run)

Answer 1

There's currently no way to filter on embedded docs in the way you're describing. Using the dot notation allows you to match on an embedded doc, but the entire document, parent and all, will still be returned. It's also possible to select which fields will be returned, but that doesn't really help your case, either.

We have a "virtual collections" case, which would implement the desired functionality; feel free to vote on it:

http://jira.mongodb.org/browse/SERVER-142

In the meantime, you should probably treat comments as their own collection. In general, if you need to work with a given data set on its own, make it a collection. If it's better conceived of as part of some other set, it's better to embed.