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post.scala

def categoriesByProducts(productsIds: Seq[ProductId]): Future[Seq[(Seq[ProductId], Category)]] =
    db.run(
      Taxonometry
        .filter(_.productId inSet productsIds)
        .join(Categories).on(_.categoryId === _.id)
        .result)
      .map { result =>
        result.groupBy(_._2.id).toVector.map {
          case (_, categories) ⇒ categories.map(_._1.productId) → categories.head._2
        }
      }