Grails projections ignoring sort order with MongoDB

Question

How do you order the results of projections in a Grails criteria when using MongoDB?

It seems that sorting is ignored by MongoDB. The code below correctly returns a list of sorted book titles when run with the Grails default in-memory HSQLDB database. Switching over to MongoDB causes the sorting to be ignored.

BookController.groovy

class BookController {

   def library = [
      [author: "Jan", title: "HTML5"],
      [author: "Lee", title: "CSS3"],
      [author: "Sue", title: "JavaScript"]
      ]

   def titles() {
      library.each { if (!Book.findByTitle(it.title)) new Book(it).save() }
      def ids = Book.createCriteria().list() {
         projections { id() }
         order "title"
         }
      def titles = ids.collect { Book.get(it).title }
      render titles as JSON
      }

}

Result with default DB (correct):

   ["CSS3","HTML5","JavaScript"]

Result with MongoDB (wrong):

   ["HTML5","CSS3","JavaScript"]

Note that the above book example is just some trivial code to illustrate the problem. The real goal is to generate a list of domain IDs sorted by a field of the domain so that the domain can be iterated over in the desired order.

The actual domain I'm dealing with is too large to fit in memory. In other words, this would crash the application: Book.list().title.sort()

Below is additional background information.

Book.groovy

class Book {    
   String title
   String author
   static mapWith = "mongo"
}

BuildConfig.groovy

...
compile ":mongodb:1.3.1"
...

DataSource.groovy

...
grails {
   mongo {
      host = "localhost"
      port = 27017
      databaseName = "book-store"
      }
   }

Answer 1

The projections support has been rewritten to use the MongoDb aggregation framework in 3.0 of the plugin. So the example where should work in 3.0 with or without ordering. See https://jira.grails.org/browse/GPMONGODB-305

Relevant commit https://github.com/grails/grails-data-mapping/commit/1d1155d8a9e29a25413effce081c21a36629137d