Question

Is it possible to create a capped sub-collection. I’m trying to do something like:

user = {
  name: String,
  latest_messages: [String]
}

Where the latest_messages are capped to 10.

If not, what do you suggest as an alternative?

Update 1:

It appears as though keeping the array capped manually is the only solution. Here’s a way to do this:

joe = {name: 'Joe', latest_messages: ['', '', '', '', '', '', '', '', '', '']}
db.users.save(joe)

db.users.update({name: 'Joe'}, {$push: {'latest_messages': 'hello'}})
db.users.update({name: 'Joe'}, {$pop: {'latest_messages': -1}})

db.users.update({name: 'Joe'}, {$push: {'latest_messages': 'world'}})
db.users.update({name: 'Joe'}, {$pop: {'latest_messages': -1}})

Any suggestions on making this more efficient?

Update 2:

There’s an open Jira ticket, “SERVER-1050” that requests to add the ability to do the two (push & pop) as one atomic operation.

Answer 1

You can only specify the overall number of documents of a capped collection.
There is no way limit the size of an array unless through some checks in your app.