Allow constructing Read from a list of underlying Read instances

Author: jatcwang

modules/core/src/main/scala/doobie/util/read.scala

@@ -4,6 +4,7 @@
 
 package doobie.util
 
+import cats.syntax.all.*
 import cats.Applicative
 import doobie.ResultSetIO
 import doobie.enumerated.Nullability
@@ -12,6 +13,7 @@ import doobie.free.resultset as IFRS
 
 import java.sql.ResultSet
 import scala.annotation.implicitNotFound
+import scala.collection.immutable.ArraySeq
 
 @implicitNotFound("""
 Cannot find or construct a Read instance for type:
@@ -119,7 +121,7 @@ object Read extends LowerPriority1Read {
     override lazy val length: Int = underlyingRead.length
   }
 
-  /** A Read instance consists of multiple underlying Read instances */
+  /** A Read instance consists of two underlying Read instances */
   class Composite[A, S0, S1](read0: Read[S0], read1: Read[S1], f: (S0, S1) => A) extends Read[A] {
     override def unsafeGet(rs: ResultSet, startIdx: Int): A = {
       val r0 = read0.unsafeGet(rs, startIdx)
@@ -145,6 +147,30 @@ object Read extends LowerPriority1Read {
     override lazy val length: Int = read0.length + read1.length
   }
 
+  /** A Composite made up of a list of underlying Read instances. This class but is intended to provide a simpler
+    * interface for other libraries that uses Doobie. This isn't used by Doobie itself for its derived instances.
+    *
+    * For large number of columns, this class may be more performant than chain of Read.Composite.
+    */
+  class CompositeOfInstances(readInstances: ArraySeq[Read[Any]]) extends Read[ArraySeq[Any]] {
+    override def unsafeGet(rs: ResultSet, startIdx: Int): ArraySeq[Any] = {
+      var columnIdx = startIdx
+      readInstances.map { r =>
+        val res = r.unsafeGet(rs, columnIdx)
+        columnIdx += r.length // This Read instance "consumed" x number of columns
+        res
+      }
+    }
+
+    override def gets: List[(Get[?], NullabilityKnown)] = readInstances.flatMap(_.gets).toList
+
+    override def toOpt: Read[Option[ArraySeq[Any]]] = {
+      readInstances.traverse(_.toOpt).map(arrayOfOptions => arrayOfOptions.sequence)
+    }
+
+    override def length: Int = readInstances.map(_.length).sum
+  }
+
 }
 
 trait LowerPriority1Read extends LowerPriority2Read {

modules/core/src/test/scala/doobie/util/ReadSuite.scala

@@ -9,12 +9,14 @@ import doobie.util.TestTypes.*
 import doobie.util.transactor.Transactor
 import doobie.testutils.VoidExtensions
 import doobie.syntax.all.*
+import doobie.util.Read.CompositeOfInstances
 import doobie.{ConnectionIO, Query}
 import doobie.util.analysis.{Analysis, ColumnMisalignment, ColumnTypeError, ColumnTypeWarning, NullabilityMisalignment}
 import doobie.util.fragment.Fragment
 import munit.Location
 
 import scala.annotation.nowarn
+import scala.collection.immutable.ArraySeq
 
 class ReadSuite extends munit.CatsEffectSuite with ReadSuitePlatform {
 
@@ -201,6 +203,54 @@ class ReadSuite extends munit.CatsEffectSuite with ReadSuitePlatform {
       q2.transact(xa).assertEquals(List(Some((1, 2, 3, 4))))
   }
 
+  test("Read.CompositeOfInstances reads columns correctly") {
+    import cats.syntax.functor.*
+    val rscc: Read[SimpleCaseClass] = Read.derived[SimpleCaseClass]
+
+    implicit val read: Read[ComplexCaseClass] =
+      new Read.CompositeOfInstances(ArraySeq(
+        rscc.widen[Any],
+        rscc.toOpt.widen[Any],
+        Read[Option[Int]].widen[Any],
+        Read[String].widen[Any]))
+        .map { arr =>
+          ComplexCaseClass(
+            arr(0).asInstanceOf[SimpleCaseClass],
+            arr(1).asInstanceOf[Option[SimpleCaseClass]],
+            arr(2).asInstanceOf[Option[Int]],
+            arr(3).asInstanceOf[String])
+        }
+
+    assertEquals(read.length, 8)
+
+    sql"SELECT 1, '2', '3', 4, '5', '6', 7, '8'"
+      .query[ComplexCaseClass].unique.transact(xa)
+      .assertEquals(
+        ComplexCaseClass(
+          SimpleCaseClass(Some(1), "2", Some("3")),
+          Some(SimpleCaseClass(Some(4), "5", Some("6"))),
+          Some(7),
+          "8"
+        )
+      )
+
+    // The 's' field in Option[SimpleCaseClass] is NULL, so whole case class value is None
+    sql"SELECT NULL, '2', '3', 4, NULL, '6', 7, '8'"
+      .query[ComplexCaseClass].unique.transact(xa)
+      .assertEquals(
+        ComplexCaseClass(
+          SimpleCaseClass(None, "2", Some("3")),
+          None,
+          Some(7),
+          "8"
+        )
+      )
+
+    sql"SELECT 1, NULL, '3', 4, '5', '6', 7, '8'"
+      .query[Option[ComplexCaseClass]].unique.transact(xa)
+      .assertEquals(None)
+  }
+
   test("Read should select correct columns when combined with `ap`") {
     import cats.syntax.all.*
     import doobie.implicits.*