Meta constructor for struct types

[tpolecat]

I think the path from here to there is:

• On Meta we need toObject and fromObject that perform a cast at the lowest level and then apply any accumulated invariant mappings. Propagate this to Atom.
• On Composite we then implement toObjectArray and fromObjectArray inductively, and from there trivially we can implement Meta.fromComposite.

[tpolecat]

Examples given in #94

[tpolecat]

Bah, ok this doesn't work with nulls because we can't distinguish a null Int from 0. So the derivation needs to be distinct from Composite. For now maybe punt and just have a raw struct mapping that you have to nxmap.

[tpolecat]

Well, actually, nuts to this. The Postgres driver doesn't support structs so you have to go via JSON or something. Pushing to backlog.

[wedens]

Just for the future reference and in case someone is looking for at least half-arsed solution:

  def pgStruct[A >: Null <: AnyRef: TypeTag](
    typeName: String,
    encode: A => String,
    decode: String => A
  )(implicit A: ClassTag[A]): AdvancedMeta[A] = {
    val toPGobject: A => PGobject = a => {
      val o = new PGobject
      o.setType(typeName)
      o.setValue(encode(a))
      o
    }
    Meta.advanced[A](
      NonEmptyList.of(JdbcType.Struct),
      NonEmptyList.of(typeName),
      (rs, n) => rs.getObject(n) match {
        case null => null
        case a: PGobject => decode(a.getValue)
        case a => throw InvalidObjectMapping(A.runtimeClass, a.getClass)
      },
      (ps, n, a) => ps.setObject(n, toPGobject(a)),
      (rs, n, a) => rs.updateObject(n, toPGobject(a))
    )
  }

encode and decode assumes string in a format (value1,value2,...) (I can't find any specification on this format)

[tpolecat]

Yeah the PGObject format seems to be undocumented. I assume it's the same string encoding that's used by the wire protocol, which is the literal encoding you use in SQL statements. I have looked at the driver's implementation and I can't figure it out. It's very complicated and side-effecty. It would be a good project to try to make sense of it.

[Malax]

I think this is the documentation for syntax of the string: https://www.postgresql.org/docs/current/static/rowtypes.html#ROWTYPES-IO-SYNTAX. I hacked something that can escape/unescape this, maybe it's worth iterating on it to work out the edge cases.

Worth noting that Postgres changes the escaping on its own (tested with 10). If you escape it using backslashes, it'll come back quoted.

object CompositeTextRepresentation {
  def build(s: List[String]): String = {
    s.map(_.replace(",", "\\,").replace("\"", "\\\"")).mkString("(", ",", ")")
  }

  def parse(s: String): List[String] = {
    val inner = s.substring(1, s.length - 1)

    var inQuotes = false
    var inEscapedQuote = false

    var builder = StringBuilder.newBuilder
    var results = List.empty[String]

    for (c <- inner) {
      if (c != '"' && c != ',') {
        builder.append(c)
      }

      if (c == '"') {
        if (!inQuotes) {
          inQuotes = true
        } else if (inQuotes && !inEscapedQuote) {
          inEscapedQuote = true
        } else {
          builder.append(c)
          inEscapedQuote = false
        }
      }

      if (c == ',') {
        if (inQuotes && !inEscapedQuote) {
          builder.append(c)
        } else {
          inQuotes = false
          inEscapedQuote = false
          results = builder.toString() :: results
          builder = StringBuilder.newBuilder
        }
      }
    }

    (builder.toString() :: results).reverse
  }
}

This still requires a way to get from individual types to a single string tho.

[wedens]

This still requires a way to get from individual types to a single string tho.

Yeah, it asks for a typeclass. Similarly to this PR #625