What's a good way to combine pagination and non query grouping in Slick 3.0?

Question

For simplification let's say I have three tables:

val postTable = TableQuery[Posts]
val postTagTable = TableQuery[PostTags]
val tagTable = TableQuery[Tags]

One post can have multiple tags and postTagTable just contains the relation.

Now I could query the posts and tags like this:

val query = for {
    post <- postTable
    postTag <- postTagTable if post.id === postTag.postId
    tag <- tagTable if postTag.tagId === tag.id
} yield (post, tag)

val postTags = db.run(query.result).map {
    case result: Seq[(Post,Tag)] => 
        result.groupBy(_._1).map {
            case (post, postTagSeq) => (post, postTagSeq.map(_._2))
        }
}

Which would give me a Future[Seq[(Post, Seq(Tag))]].

So far so good.

But what if I want to add pagination for the posts? Since one Post can have multiple Tags with the above query, I don't know how many rows to take from the query, in order to get, let's say, 10 Posts.

Does anyone know a good way of getting the same result with a specific number of Posts in a single query?

I am actually not even sure how I would approach this in native SQL without nested queries, so if someone has a suggestion in that direction I would also be glad to hear it.

Thanks!

EDIT

Just so you know, what kind of query I am currently doing:

val pageQuery = postTable drop(page * pageSize) take(pageSize)

val query = for {
    pagePost <- pageQuery
    post <- postTable if pagePost.id === post.id
    postTag <- postTagTable if post.id === postTag.postId
    tag <- tagTable if postTag.tagId === tag.id
} yield (post, tag)

val postTags = db.run(query.result).map {
    case result: Seq[(Post,Tag)] => 
        result.groupBy(_._1).map {
            case (post, postTagSeq) => (post, postTagSeq.map(_._2))
        }
}

But this obviously results in a nested query. And this is what I would like to avoid.

EDIT 2

Another 2-query solution that would be possible:

val pageQuery = postTable drop(page * pageSize) map(_.id) take(pageSize)

db.run(pageQuery.result) flatMap {
    case ids: Seq[Int] => 
        val query = for {
            post <- postTable if post.id inSetBind ids
            postTag <- postTagTable if post.id === postTag.postId
            tag <- tagTable if postTag.tagId === tag.id
        } yield (post, tag)

        val postTags = db.run(query.result).map {
                case result: Seq[(Post,Tag)] => 
                     result.groupBy(_._1).map {
                         case (post, postTagSeq) => (post, postTagSeq.map(_._2))
                     }
        }
 }

But this would take two trips to the database and uses the in operator, so it's probably not as good as the join query.

Any suggestions?

Answer 1

You can do like this:

  def findPagination(from: Int, to: Int): Future[Seq[(Post, Seq[Tag])]] = {
    val query:DBIO[Seq[(Album,Seq[Genre])]] = postRepository.findAll(from, to).flatMap{posts=>
      DBIO.sequence(
        posts.map{ post=>
          tagRepository.findByPostId(post.id).map(tags=>(post,tags))
        }
      )
    }
    db.run(query)
  }

Inside PostRepository

def findAll(from: Int, limit: Int): DBIO[Seq[Post]] = postTable.drop(from).take(limit).result

Inside TagRepository

  def findByPostId(id: Int): DBIO[Seq[Tag]] = {
    val query = for {
      tag <- tagTable
      pstTag <- postTagTable if pstTag.postId === id && tag.id === pstTag.tagId
    } yield tag
    query.result
  }

EDIT

I thing you can't do it without subselect in a single query. Your current solution is the best one. Also you can optimize your query by removing unnecessary "join"

val query = for {
    pagePost <- pageQuery
    postTag <- postTagTable if pagePost.id === postTag.postId
    tag <- tagTable if postTag.tagId === tag.id
} yield (pagePost, tag)

And you'll get approximately next SQL (Slick 3.0.1):

SELECT x2.`postname`,
       x2.`id`,
       x3.`tagname`,
       x3.`id`
FROM
  (SELECT x4.`postname` AS `postname`, x4.`id` AS `id`
   FROM `POST` x4 LIMIT 10, 1) x2,
     `POST_TAG` x5,
     `TAG` x3
WHERE (x2.`id` = x5.`postId`)
  AND (x5.`tagId` = x3.`id`)

Maybe in your case, also it's more efficient to pre-compile this query http://slick.typesafe.com/doc/3.0.0/queries.html#compiled-queries