MongoDB - Query embbeded documents

I've a collection named Events. Each Eventdocument have a collection of Participants as embbeded documents.

No开发者_开发知识库w is my question.. is there a way to query an Event and get all Participants thats ex. Age > 18?

When you query a collection in MongoDB, by default it returns the entire document which matches the query. You could slice it and retrieve a single subdocument if you want.

If all you want is the Participants who are older than 18, it would probably be best to do one of two things:

1. Store them in a subdocument inside of the event document called "Over18" or something. Insert them into that document (and possibly the other if you want) and then when you query the collection, you can instruct the database to only return the "Over18" subdocument. The downside to this is that you store your participants in two different subdocuments and you will have to figure out their age before inserting. This may or may not be feasible depending on your application. If you need to be able to check on arbitrary ages (i.e. sometimes its 18 but sometimes its 21 or 25, etc) then this will not work.

2. Query the collection and retreive the Participants subdocument and then filter it in your application code. Despite what some people may believe, this isnt terrible because you dont want your database to be doing too much work all the time. Offloading the computations to your application could actually benefit your database because it now can spend more time querying and less time filtering. It leads to better scalability in the long run.

Short answer: no. I tried to do the same a couple of months back, but mongoDB does not support it (at least in version <= 1.8). The same question has been asked in their Google Group for sure. You can either store the participants as a separate collection or get the whole documents and then filter them on the client. Far from ideal, I know. I'm still trying to figure out the best way around this limitation.

For future reference: This will be possible in MongoDB 2.2 using the new aggregation framework, by aggregating like this:

db.events.aggregate(
  { $unwind: '$participants' }, 
  { $match: {'age': {$gte: 18}}},
  { $project: {participants: 1}
)

This will return a list of n documents where n is the number of participants > 18 where each entry looks like this (note that the "participants" array field now holds a single entry instead):

{ 
  _id: objectIdOfTheEvent, 
  participants: { firstName: 'only one', lastName: 'participant'} 
}

It could probably even be flattened on the server to return a list of participants. See the officcial documentation for more information.