22 Column limit for procedures

问题

How can we overcome the 22 limit when calling procedures with Slick?

We currently have:

val q3 = sql"""call getStatements(${accountNumber})""".as[Transaction]

The problem is that we have to return more than 22 columns and Transaction case class cannot have more than 22 columns since when we do JSONFormat we get an error:

[error] E:\IdeaProjects\admin\Transaction.scala:59: No unapply or unapplySeq function found
[error]   implicit val jsonFormat = Json.format[Transaction]

Any suggestions?

回答1:

Alright - so if you can actually modify your Transaction case class than there is a better solution than HList (which to be honest may be a little cumbersome to operate with later on).

So here is the thing: let's imagine you have User table with following attributes:

id
name
surname
faculty
finalGrade
street
number
city
postCode

Above columns may not make sense but let's use them as example. The most straightforward way to deal with above is to create a case class:

case class User(
   id: Long,
   name: String,
   ...  // rest of the attributes here
   postCode: String)

which would be mapped from table on the application side.

Now what you can also do is to do this:

case class Address(street: String, number: String, city: String, postCode: String)

case class UniversityInfo(faculty: String, finalGrade: Double)

case class User(id: Long, name: String, surname: String, uniInfo: UniversityInfo, address: Address)

This composition will help you to avoid problem with too many columns (which is basically problem with too many attributes in your case class/tuple). Apart from that - I would argue that it is always (very often?) beneficial to do this if you have many columns - if for nothing else than simply for readability purposes.

How to do the mapping

class User(tag: Tag) extends Table(tag, "User") {

  // cricoss info
  def id = column[Long]("id")
  def name = column[String]("name")

  // ... all the other fields
  def postCode = column[String]("postCode")

  def * = (id, name, surname, uniInfoProjection, addressProjection) <>((User.apply _).tupled, User.unapply)

  def uniInfoProjection = (faculty, finalGrade) <>((UniversityInfo.apply _).tupled, UniversityInfo.unapply)

  def addressProjection = (street, number, city, city) <>((Address.apply _).tupled, Address.unapply)
}

The same can be done with custom SQL mapping.

implicit val getUserResult = GetResult(r => 
    User(r.nextLong, r.nextString, r.nextString, 
         UniversityInfo(r.nextString, r.nextDouble),
         Adress(r.nextString, r.nextString, r.nextString, r.nextString))
)

So to put things simply - try to segregate your fields into multiple nested case classes and your problem should go away (with added benefit of improved readability). If you do that approaching tuple/case class limit should virtually never be a problem (and you shouldn't even need to use HList).

来源：https://stackoverflow.com/questions/42399431/22-column-limit-for-procedures

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