foo.scala

val posts = model.Post.joins(model.Post.tagsRef).findById(1) // thorws TooManyRowsException

insert.sql

insert into posts(
  id,
  title,
  body,
  created_at
) values (
  1,
  'test',
  'text\body',
  now()
);

insert into tags(
  id,
  name,
  created_at
) select
  1,
  'tag1',
  now()
union all select
  2,
  'tag2',
  now();

insert into posts_tags(
  id,
  tag_id,
  post_id,
  created_at
)select
  1,
  1,
  1,
  now()
union all select
  1,
  2,
  1,
  now();

Post.scala

package model

import skinny.orm._, feature._
import scalikejdbc._, SQLInterpolation._
import org.joda.time._

case class Post(
  id: Long,
  title: String,
  body: String,
  tags: Seq[Tag] = Nil,
  createdAt: DateTime,
  updatedAt: Option[DateTime] = None
)

object Post extends SkinnyCRUDMapper[Post] with TimestampsFeature[Post] {
  override val tableName = "posts"
  override val defaultAlias = createAlias("p")

  val tagsRef = hasManyThrough[Tag](
    through = PostTag,
    many = Tag,
    merge = (p, t) => p.copy(tags = t)) // .byDefault

  override def extract(rs: WrappedResultSet, rn: ResultName[Post]): Post = new Post(
    id = rs.long(rn.id),
    title = rs.string(rn.title),
    body = rs.string(rn.body),
    createdAt = rs.dateTime(rn.createdAt),
    updatedAt = rs.dateTimeOpt(rn.updatedAt)
  )
}

PostTag.scala

package model

import skinny.orm._, feature._
import scalikejdbc._, SQLInterpolation._
import org.joda.time._

case class PostTag(
  id:Long,
  tagId: Int,
  postId: Int,
  createdAt: DateTime
)

object PostTag extends SkinnyJoinTable[PostTag] {
  override val tableName = "posts_tags"
  override val defaultAlias = createAlias("pt")

  override def extract(rs: WrappedResultSet, rn: ResultName[PostTag]): PostTag = new PostTag(
    id = rs.long(rn.id),
    tagId = rs.int(rn.tagId),
    postId = rs.int(rn.postId),
    createdAt = rs.dateTime(rn.createdAt)
  )
}

Tag.scala

package model

import skinny.orm._, feature._
import scalikejdbc._, SQLInterpolation._
import org.joda.time._

case class Tag(
  id: Long,
  name: String,
  createdAt: DateTime,
  updatedAt: Option[DateTime] = None
)

object Tag extends SkinnyCRUDMapper[Tag] with TimestampsFeature[Tag] {
  override val tableName = "tags"
  override val defaultAlias = createAlias("t")

  override def extract(rs: WrappedResultSet, rn: ResultName[Tag]): Tag = new Tag(
    id = rs.long(rn.id),
    name = rs.string(rn.name),
    createdAt = rs.dateTime(rn.createdAt),
    updatedAt = rs.dateTimeOpt(rn.updatedAt)
  )
}