ordered queries

Once you have your query filter, you can specify an ordering of the results using an orderBy clause. In the last section, we retrieved all blog posts for a given blog from the past week, but it would be nice to get them back in chronological order as well. This will return the posts in ascending chronological order:

import longevity.persistence.PState
import scala.concurrent.Future

val blog: Blog = getBlogFromSomewhere()

val recentPosts: Future[Vector[PState[BlogPost]]] = repo.queryToVector {
  import com.github.nscala_time.time.Imports._
  import BlogPost.queryDsl._
  import BlogPost.props._

  blogUri eqs blog.blogUri and postDate gt DateTime.now - 1.week orderBy postDate
}

Unfortunately, the DSL will not tolerate a line-break before the orderBy operator. If you need a line-break, you might try variations such as this:

{ blogUri eqs blog.blogUri and postDate gt DateTime.now - 1.week
} orderBy postDate

If we want them in descending order, we just change postDate to postDate.desc:

import longevity.persistence.PState
import scala.concurrent.Future

val blog: Blog = getBlogFromSomewhere()

val recentPosts: Future[Vector[PState[BlogPost]]] = repo.queryToVector {
  import com.github.nscala_time.time.Imports._
  import BlogPost.queryDsl._
  import BlogPost.props._

  blogUri eqs blog.blogUri and postDate gt DateTime.now - 1.week orderBy postDate.desc
}

If you prefer to to leave out the dot before asc or desc, you will need to import scala.language.postfixOps:

import longevity.persistence.PState
import scala.concurrent.Future

val blog: Blog = getBlogFromSomewhere()

val recentPosts: Future[Vector[PState[BlogPost]]] = repo.queryToVector {
  import com.github.nscala_time.time.Imports._
  import BlogPost.queryDsl._
  import BlogPost.props._
  import scala.language.postfixOps

  blogUri eqs blog.blogUri and postDate gt DateTime.now - 1.week orderBy postDate desc
}

If you want to specify multiple properties in your orderBy clause, you will have to surround them in parentheses. Here’s an example where we retrieve all the blog posts in the last week from every blog. We order them first by blogUri, and then by postDate

postDate gt DateTime.now - 1.week orderBy (blogUri, postDate.desc)

The orderBy clauses are fully processed by your back end, and consequently, will affect the performance of your queries. Only in limited circumstances will the back end will be able to collect your query results in the right order in place. This means that ordered queries that return a large number of results may be costly.

For MongoDB, ordered results can be assembled in place only when the orderBy clause matches the primary key property. For Cassandra, any accepted orderBy clause will assemble the ordered results in place. However, the orderBy clauses accepted by Cassandra are limited, as explained here.