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Juliano Alves
Juliano Alves

Posted on • Updated on • Originally published at juliano-alves.com

Contributing to Quill, a Pairing Session

#scala #quill #opensource #howto

As one of Quill's maintainer and an open source enthusiast, I am always thinking of ways to spread the word and have more people contributing to the project. Pair programming is a great technique to share knowledge and introduce newcomers to a project, however, I can't pair with someone in the US, India or Brazil willing to contribute, can I?

Of course I can! Welcome to our your first remote pairing blog session! In this post I will have you as my co-pilot while implementing a new feature to Quill.

Think about it as a proper pairing session, where I am talking directly to you, speaking my mind. Sometimes it can feel like I am jumping around the code way too easily, but this is natural in a pairing session where one person has more experience that the other. And don't be ashamed, express your opinions and suggestions in the comments.

Getting started

The feature we will implement is Add SQL Server Support for returning via OUTPUT, meaning it's mostly about quill-sql and quill-jdbc. Let's walk through this.

Being a SQL Server feature, SQLServerDialectSpec can be a good starting point. However, there aren't any tests regarding returning. Still thinking about Sql Server specifically, it's worth checking sqlserver/JdbcContextSpec as well. There we can find something:

"Insert with returning with single column table" in {
  val inserted = testContext.run {
    qr4.insert(lift(TestEntity4(0))).returningGenerated(_.i)
  }
  testContext.run(qr4.filter(_.i == lift(inserted))).head.i mustBe inserted
}

"Insert with returning with multiple columns and query embedded" in {
  val inserted = testContext.run {
    qr4Emb.insert(lift(TestEntity4Emb(EmbSingle(0)))).returningGenerated(_.emb.i)
  }
  testContext.run(qr4Emb.filter(_.emb.i == lift(inserted))).head.emb.i mustBe inserted
}
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Let's try to change these tests to use .returning:

[error] .../quill/quill-jdbc/src/test/scala/io/getquill/context/jdbc/sqlserver/JdbcContextSpec.scala:57:49: The 'returning' clause is not supported by the io.getquill.SQLServerDialect idiom. Use 'returningGenerated' instead.
[error]       qr4.insert(lift(TestEntity4(0))).returning(_.i)
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Maybe we can search for the error message. That takes us to Parsing:

idiomReturnCapability match {
  case ReturningMultipleFieldSupported | ReturningClauseSupported =>
  case ReturningSingleFieldSupported =>
    c.fail(s"The 'returning' clause is not supported by the ${currentIdiom.getOrElse("specified")} idiom. Use 'returningGenerated' instead.")
  case ReturningNotSupported =>
    c.fail(s"The 'returning' or 'returningGenerated' clauses are not supported by the ${currentIdiom.getOrElse("specified")} idiom.")
}
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That definitely doesn't help now, but let's keep it in mind, eventually we will get back to it.

Let's look for something more specific. Maybe SQLServerDialect can tell us something. The trait's signature is:

trait SQLServerDialect
  extends SqlIdiom
  with QuestionMarkBindVariables
  with ConcatSupport
  with CanReturnField
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We are getting close, I can feel it! Following CanReturnField we will see that it extends ReturningCapability, which has the definitions for all existing returning behaviours.

ReturningCapability is returned by idiomReturningCapability from trait Capabilities and its descendants:

trait Capabilities {
  def idiomReturningCapability: ReturningCapability
}

trait CanReturnClause extends Capabilities {
  override def idiomReturningCapability: ReturningClauseSupported = ReturningClauseSupported
}

trait CanReturnField extends Capabilities {
  override def idiomReturningCapability: ReturningSingleFieldSupported = ReturningSingleFieldSupported
}
...
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Following idiomReturningCapability we will finally find the place where the SQL is generated, in SqlIdiom:

case r @ ReturningAction(Insert(table: Entity, Nil), alias, prop) =>
  idiomReturningCapability match {
    // If there are queries inside of the returning clause we are forced to alias the inserted table (see #1509). Only do this as
    // a last resort since it is not even supported in all Postgres versions (i.e. only after 9.5)
    case ReturningClauseSupported if (CollectAst.byType[Entity](prop).nonEmpty) =>
      SqlIdiom.withActionAlias(this, r)
    case ReturningClauseSupported =>
      stmt"INSERT INTO ${table.token} ${defaultAutoGeneratedToken(prop.token)} RETURNING ${returnListTokenizer.token(ExpandReturning(r)(this, strategy).map(_._1))}"
    case other =>
      stmt"INSERT INTO ${table.token} ${defaultAutoGeneratedToken(prop.token)}"
  }

case r @ ReturningAction(action, alias, prop) =>
  idiomReturningCapability match {
    // If there are queries inside of the returning clause we are forced to alias the inserted table (see #1509). Only do this as
    // a last resort since it is not even supported in all Postgres versions (i.e. only after 9.5)
    case ReturningClauseSupported if (CollectAst.byType[Entity](prop).nonEmpty) =>
      SqlIdiom.withActionAlias(this, r)
    case ReturningClauseSupported =>
      stmt"${action.token} RETURNING ${returnListTokenizer.token(ExpandReturning(r)(this, strategy).map(_._1))}"
    case other =>
      stmt"${action.token}"
  }
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It starts to get complex. Before we carry on, we should summarize what we've learned so far:

  • SQLServerDialect extends CanReturnField, in order to support returningGenerated only;
  • CanReturnField ensures this behaviour, defining that idiomReturningCapability returns a ReturningSingleFieldSupported;
  • ReturningSingleFieldSupported is a ReturningCapability, mother of all the return behaviours;
  • Parsing verifies the type of the returning clause and fails the compilation if necessary;
  • SqlIdiom decides how to generate the return statement after checking the current idiomReturningCapability

All this acquired knowledge is vital to implement the new feature. Like the Github issue says, we already have a dialect that does what we need, PostgresDialect:

trait PostgresDialect
  extends SqlIdiom
  with QuestionMarkBindVariables
  with ConcatSupport
  with OnConflictSupport
  with CanReturnClause
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As we know, CanReturnField.idiomReturnCapability returns a ReturningClauseSupported:

/**
 * An actual `RETURNING` clause is supported in the SQL dialect of the specified database e.g. Postgres.
 * this typically means that columns returned from Insert/Update/etc... clauses can have other database
 * operations done on them such as arithmetic `RETURNING id + 1`, UDFs `RETURNING udf(id)` or others.
 * In JDBC, the following is done:
 * `connection.prepareStatement(sql, Statement.RETURN_GENERATED_KEYS))`.
 */
sealed trait ReturningClauseSupported extends ReturningCapability
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CanReturnClause is extended by MirrorSqlDialectWithReturnClause as well:

trait MirrorSqlDialectWithReturnClause
  extends SqlIdiom
  with QuestionMarkBindVariables
  with ConcatSupport
  with CanReturnClause
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Which is used for tests in SqlActionMacroSpec, for returning and returningGenerated:

"returning clause - single" in testContext.withDialect(MirrorSqlDialectWithReturnClause) { ctx =>
  import ctx._
  val q = quote {
    qr1.insert(lift(TestEntity("s", 0, 1L, None))).returning(_.l)
  }

  val mirror = ctx.run(q)
  mirror.string mustEqual "INSERT INTO TestEntity (s,i,l,o) VALUES (?, ?, ?, ?) RETURNING l"
  mirror.returningBehavior mustEqual ReturnRecord
}
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Now we know enough to set up a plan of action.

Development plan

  • We need a new ReturningCapability, exposed by a new Capabilities via idiomReturningCapability;
  • SqlIdiom has to accomodate the new ReturningCapability generating the OUTPUT clause;
  • Parsing needs to allow the new code to compile;
  • SQLServerDialect will extend the new Capabilities

And we can deal with the unexpected surprises along the way. The game is on!

OutputClauseSupported and CanOutputClause

Following the stablished convention, the names make sense:

sealed trait OutputClauseSupported extends ReturningCapability

object OutputClauseSupported extends OutputClauseSupported

trait CanOutputClause extends Capabilities {
  override def idiomReturningCapability: OutputClauseSupported = OutputClauseSupported
}
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Now a new MirrorSqlDialect extending CanOutputClause:

trait MirrorSqlDialectWithOutputClause
  extends SqlIdiom
  with QuestionMarkBindVariables
  with ConcatSupport
  with CanOutputClause

object MirrorSqlDialectWithOutputClause extends MirrorSqlDialectWithOutputClause {
  override def prepareForProbing(string: String) = string
}
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So far so good... or maybe not. This change already breaks the code:

[error] /Users/juliano.alves/development/opensource/quill/quill-core/src/main/scala/io/getquill/norm/ExpandReturning.scala:23:11: match may not be exhaustive.
[error] It would fail on the following input: OutputClauseSupported
[error]     idiom.idiomReturningCapability match {
[error]           ^
[error] /Users/juliano.alves/development/opensource/quill/quill-core/src/main/scala/io/getquill/quotation/Parsing.scala:831:38: match may not be exhaustive.
[error] It would fail on the following input: OutputClauseSupported
[error]     def verifyAst(returnBody: Ast) = capability match {
[error]                                      ^
[error] /Users/juliano.alves/development/opensource/quill/quill-core/src/main/scala/io/getquill/quotation/Parsing.scala:875:7: match may not be exhaustive.
[error] It would fail on the following input: OutputClauseSupported
[error]       idiomReturnCapability match {
[error]       ^
[error] three errors found
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Let's take care of ExpandReturning first. The error happens because OutputClauseSupported is not being taken in consideration during the pattern matching. We are basing the new implementation on ReturningClauseSupported, so it's reasonable to simply mirror its behaviour:

// line 23
idiom.idiomReturningCapability match {
  case ReturningClauseSupported | OutputClauseSupported =>
    ReturnAction.ReturnRecord
  case ReturningMultipleFieldSupported =>
  ...
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Similar changes in Parsing, for both errors:

// line 831
def verifyAst(returnBody: Ast) = capability match {
  case OutputClauseSupported =>
  case ReturningClauseSupported =>
  // Only .returning(r => r.prop) or .returning(r => OneElementCaseClass(r.prop1..., propN)) or .returning(r => (r.prop1..., propN)) (well actually it's prop22) is allowed.
  case ReturningMultipleFieldSupported =>
    returnBody match {
  ...

// line 875
idiomReturnCapability match {
  case ReturningMultipleFieldSupported | ReturningClauseSupported | OutputClauseSupported =>
  case ReturningSingleFieldSupported =>
    c.fail(s"The 'returning' clause is not supported by the ${currentIdiom.getOrElse("specified")} idiom. Use 'returningGenerated' instead.")
  case ReturningNotSupported =>
    c.fail(s"The 'returning' or 'returningGenerated' clauses are not supported by the ${currentIdiom.getOrElse("specified")} idiom.")
}
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There is something in this snippet that deserves attention, the idiomReturnCapability being matched. It is defined in Parsing as well:

private[getquill] def idiomReturnCapability: ReturningCapability = {
  val returnAfterInsertType =
    currentIdiom
      .toSeq
      .flatMap(_.members)
      .collect {
        case ms: MethodSymbol if (ms.name.toString == "idiomReturningCapability") => Some(ms.returnType)
      }
      .headOption
      .flatten

  returnAfterInsertType match {
    case Some(returnType) if (returnType =:= typeOf[ReturningClauseSupported]) => ReturningClauseSupported
    case Some(returnType) if (returnType =:= typeOf[ReturningSingleFieldSupported]) => ReturningSingleFieldSupported
    case Some(returnType) if (returnType =:= typeOf[ReturningMultipleFieldSupported]) => ReturningMultipleFieldSupported
    case Some(returnType) if (returnType =:= typeOf[ReturningNotSupported]) => ReturningNotSupported
    // Since most SQL Dialects support returing a single field (that is auto-incrementing) allow a return
    // of a single field in the case that a dialect is not actually specified. E.g. when SqlContext[_, _]
    // is used to define `returning` clauses.
    case other => ReturningSingleFieldSupported
  }
}
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Long story short, returnAfterInsertType has the type information about ReturningCapability being used by the idiom. Then, the pattern matching returns the corresponding object of that type, or a ReturningSingleFieldSupported otherwise. So OutputClauseSupported has to be included as an option:

  returnAfterInsertType match {
    case Some(returnType) if (returnType =:= typeOf[ReturningClauseSupported]) => ReturningClauseSupported
    case Some(returnType) if (returnType =:= typeOf[OutputClauseSupported]) => OutputClauseSupported
    ...
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Code compiling, tests passing. Time to write some tests to the new feature.

Adapting the SQL Idiom

Let's start simple, adding a test similar to returning clause - single but generating an OUTPUT clause:

"output clause - single" in testContext.withDialect(MirrorSqlDialectWithOutputClause) { ctx =>
  import ctx._
  val q = quote {
    qr1.insert(lift(TestEntity("s", 0, 1L, None))).returning(_.l)
  }

  val mirror = ctx.run(q)
  mirror.string mustEqual "INSERT INTO TestEntity (s,i,l,o) OUTPUT INSERTED.l VALUES (?, ?, ?, ?)"
  mirror.returningBehavior mustEqual ReturnRecord
}
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It fails, for obvious reasons. Time to work on SqlIdiom and figure how to generate the expected result. Remember those two pattern clauses related to idiomReturningCapability?

case r @ ReturningAction(Insert(table: Entity, Nil), alias, prop) =>
  idiomReturningCapability match {
  ...

case r @ ReturningAction(action, alias, prop) =>
  idiomReturningCapability match {
  ...
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In order to identify which of the ReturningActions clauses we should look at, we will start dealing with Quill's ASTs!

Quill's Abstract Syntax Trees

This is my favourite part of the process, using the console to explore the AST. In the sbt console:

scala> import io.getquill._
scala> import io.getquill.ast._
scala> val ctx = new SqlMirrorContext(MirrorSqlDialectWithOutputClause, Literal)
scala> import ctx._

scala> case class TestEntity(s: String, i: Int, l: Long, o: Option[Int])
scala> val qr1 = quote { query[TestEntity] }

scala> val q = quote {
         qr1.insert(lift(TestEntity("s", 0, 1L, None))).returning(_.l)
       }
q: ctx.Quoted[ctx.ActionReturning[TestEntity,Long]]{def quoted: io.getquill.ast.Returning; def ast: io.getquill.ast.Returning; def id380689071(): Unit; val liftings: AnyRef{val TestEntity.apply("s", 0, 1L, scala.None).s: io.getquill.quotation.ScalarValueLifting[String,String]; val TestEntity.apply("s", 0, 1L, scala.None).i: io.getquill.quotation.ScalarValueLifting[Int,Int]; val TestEntity.apply("s", 0, 1L, scala.None).l: io.getquill.quotation.ScalarValueLifting[Long,Long]; val TestEntity.apply("s", 0, 1L, scala.None).o: io.getquill.quotation.ScalarValueLifting[Option[Int],Option[Int]]}} = $anon$1@374e427b
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To make visualization of the AST easy, Quill integrates the awesome PPrint module:

scala> pprint.pprintln(q.ast, 200)
Returning(
  Insert(
    Entity("TestEntity", List()),
    List(
      Assignment(Ident("v"), Property(Ident("v"), "s"), ScalarValueLift("$line9.$read.$iw.$iw.$iw.$iw.$iw.$iw.TestEntity.apply(\"s\", 0, 1L, scala.None).s", "s", MirrorEncoder(<function3>))),
      Assignment(Ident("v"), Property(Ident("v"), "i"), ScalarValueLift("$line9.$read.$iw.$iw.$iw.$iw.$iw.$iw.TestEntity.apply(\"s\", 0, 1L, scala.None).i", 0, MirrorEncoder(<function3>))),
      Assignment(Ident("v"), Property(Ident("v"), "l"), ScalarValueLift("$line9.$read.$iw.$iw.$iw.$iw.$iw.$iw.TestEntity.apply(\"s\", 0, 1L, scala.None).l", 1L, MirrorEncoder(<function3>))),
      Assignment(Ident("v"), Property(Ident("v"), "o"), ScalarValueLift("$line9.$read.$iw.$iw.$iw.$iw.$iw.$iw.TestEntity.apply(\"s\", 0, 1L, scala.None).o", None, MirrorEncoder(<function3>)))
    )
  ),
  Ident("x1"),
  Property(Ident("x1"), "l")
)
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Apparently we are dealing with the second of those two cases. Let's double check:

scala> q.ast match {
         case ReturningAction(Insert(entity: Entity, Nil), _, prop) => "first"
         case ReturningAction(action, alias, prop) => "second"
       }
res18: Boolean = second
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Changing SqlIdiom

Following the same approach as ReturningClauseSupported, but using OUTPUT instead of RETURNING, we have:

case r @ ReturningAction(action, alias, prop) =>
  idiomReturningCapability match {
    ...
    case ReturningClauseSupported =>
      stmt"${action.token} RETURNING ${returnListTokenizer.token(ExpandReturning(r)(this, strategy).map(_._1))}"
    case OutputClauseSupported =>
      stmt"${action.token} OUTPUT ${returnListTokenizer.token(ExpandReturning(r)(this, strategy).map(_._1))}"
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That change generates:

scala> ctx.run(q).string
<console>:23: INSERT INTO TestEntity (s,i,l,o) VALUES (?, ?, ?, ?) OUTPUT l
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Okay, ${action.token} becomes INSERT INTO TestEntity (s,i,l,o) VALUES (?, ?, ?, ?), so it's necessary to extract some information from action in order to generate the insert clause. Quill already does that:

// line 432
case Insert(entity: Entity, assignments) =>
  val table = insertEntityTokenizer.token(entity)
  val columns = assignments.map(_.property.token)
  val values = assignments.map(_.value)
  stmt"INSERT $table${actionAlias.map(alias => stmt" AS ${alias.token}").getOrElse(stmt"")} (${columns.mkStmt(",")}) VALUES (${values.map(scopedTokenizer(_)).mkStmt(", ")})"
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Combining this code with our knowledge about the existing RETURNING clause to generate the OUTPUT clause, we have:

case OutputClauseSupported => action match {
  case Insert(entity: Entity, assignments) =>
    val table = insertEntityTokenizer.token(entity)
    val columns = assignments.map(_.property.token)
    val values = assignments.map(_.value)
    stmt"INSERT $table${actionAlias.map(alias => stmt" AS ${alias.token}").getOrElse(stmt"")} (${columns.mkStmt(",")}) OUTPUT ${returnListTokenizer.token(ExpandReturning(r)(this, strategy).map(_._1))} VALUES (${values.map(scopedTokenizer(_)).mkStmt(", ")})"
  case other =>
    fail(s"Action ast can't be translated to sql: '$other'")
}
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Back to sbt console:

scala> ctx.run(q).string
<console>:20: INSERT INTO TestEntity (s,i,l,o) OUTPUT l VALUES (?, ?, ?, ?)
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Almost there! We now have to find a way to include INSERTED. before every single element of returnListTokenizer. ExpandReturning is the object handling that:

def apply(returning: ReturningAction)(idiom: Idiom, naming: NamingStrategy): List[(Ast, Statement)] = {
  val ReturningAction(_, alias, properties) = returning

  val dePropertized =
    Transform(properties) {
      case `alias` => ExternalIdent(alias.name)
    }
  ...
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And I'll be entirely honest here, I have no idea what to do. I've never seen this part of the code.

Asking for help

When contributing to open source, never be afraid to ask for help

And that's what I did. I actually came up with a solution before deusaquilus's review, but it hadn't used the code already in place to solve that problem - and I knew that. Living and learning!

def apply(returning: ReturningAction, renameAlias: Option[String] = None)(idiom: Idiom, naming: NamingStrategy): List[(Ast, Statement)] = {
  val ReturningAction(_, alias, properties) = returning

  val dePropertized = renameAlias match {
    case Some(newName) =>
      BetaReduction(properties, alias -> Ident(newName))
    case None =>
      BetaReduction(properties, alias -> ExternalIdent(alias.name))
  }
  ...
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My knowledge regarding BetaReduction is limited, so let's trust deusaquilus advice here. Let's make use of the new resource in SqlIdiom, but we should extract the duplicated code first:

case Insert(entity: Entity, assignments) =>
  val (table, columns, values) = insertInfo(insertEntityTokenizer, entity, assignments)
  stmt"INSERT $table${actionAlias.map(alias => stmt" AS ${alias.token}").getOrElse(stmt"")} (${columns.mkStmt(",")}) VALUES (${values.map(scopedTokenizer(_)).mkStmt(", ")})"
...

private def insertInfo(insertEntityTokenizer: Tokenizer[Entity], entity: Entity, assignments: List[Assignment])(implicit astTokenizer: Tokenizer[Ast]) = {
  val table = insertEntityTokenizer.token(entity)
  val columns = assignments.map(_.property.token)
  val values = assignments.map(_.value)
  (table, columns, values)
}
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And finally, let's introduce INSERTED:

case OutputClauseSupported => action match {
  case Insert(entity: Entity, assignments) =>
    val (table, columns, values) = insertInfo(insertEntityTokenizer, entity, assignments)
    stmt"INSERT $table${actionAlias.map(alias => stmt" AS ${alias.token}").getOrElse(stmt"")} (${columns.mkStmt(",")}) OUTPUT ${returnListTokenizer.token(ExpandReturning(r, Some("INSERTED"))(this, strategy).map(_._1))} VALUES (${values.map(scopedTokenizer(_)).mkStmt(", ")})"
  case other =>
    fail(s"Action ast can't be translated to sql: '$other'")
}
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Now, in the console we have:

scala> ctx.run(q).string
<console>:20: INSERT INTO TestEntity (s,i,l,o) OUTPUT INSERTED.l VALUES (?, ?, ?, ?)
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And the tests are green again. Let's cover the other possibilities for returning:

"output clause - multi" in testContext.withDialect(MirrorSqlDialectWithOutputClause) { ctx =>
  import ctx._
  val q = quote {
    qr1.insert(lift(TestEntity("s", 0, 1L, None))).returning(r => (r.i, r.l))
  }
  val mirror = ctx.run(q)
  mirror.string mustEqual "INSERT INTO TestEntity (s,i,l,o) OUTPUT INSERTED.i, INSERTED.l VALUES (?, ?, ?, ?)"
  mirror.returningBehavior mustEqual ReturnRecord
}
"output clause - operation" in testContext.withDialect(MirrorSqlDialectWithOutputClause) { ctx =>
  import ctx._
  val q = quote { qr1.insert(lift(TestEntity("s", 0, 1L, None))).returning(r => (r.i, r.l + 1)) }
  val mirror = ctx.run(q)

  mirror.string mustEqual "INSERT INTO TestEntity (s,i,l,o) OUTPUT INSERTED.i, INSERTED.l + 1 VALUES (?, ?, ?, ?)"
}
"output clause - record" in testContext.withDialect(MirrorSqlDialectWithOutputClause) { ctx =>
  import ctx._
  val q = quote {
    qr1.insert(lift(TestEntity("s", 0, 1L, None))).returning(r => r)
  }
  val mirror = ctx.run(q)
  mirror.string mustEqual "INSERT INTO TestEntity (s,i,l,o) OUTPUT INSERTED.s, INSERTED.i, INSERTED.l, INSERTED.o VALUES (?, ?, ?, ?)"
  mirror.returningBehavior mustEqual ReturnRecord
}
"output clause - embedded" - {
  "embedded property" in testContext.withDialect(MirrorSqlDialectWithOutputClause) { ctx =>
    import ctx._
    val q = quote {
      qr1Emb.insert(lift(TestEntityEmb(Emb("s", 0), 1L, None))).returning(_.emb.i)
    }
    val mirror = ctx.run(q)
    mirror.string mustEqual "INSERT INTO TestEntity (s,i,l,o) OUTPUT INSERTED.i VALUES (?, ?, ?, ?)"
    mirror.returningBehavior mustEqual ReturnRecord
  }
  "two embedded properties" in testContext.withDialect(MirrorSqlDialectWithOutputClause) { ctx =>
    import ctx._
    val q = quote {
      qr1Emb.insert(lift(TestEntityEmb(Emb("s", 0), 1L, None))).returning(r => (r.emb.i, r.emb.s))
    }
    val mirror = ctx.run(q)
    mirror.string mustEqual "INSERT INTO TestEntity (s,i,l,o) OUTPUT INSERTED.i, INSERTED.s VALUES (?, ?, ?, ?)"
    mirror.returningBehavior mustEqual ReturnRecord
  }
}
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Great, everything works! Now we have to handle a fundamental difference between OUTPUT and RETURNING.

Stopping invalid actions

The next test is "with returning clause - query", with this code:

val q = quote {
  qr1
    .insert(lift(TestEntity("s", 0, 1L, None)))
    .returning(r => (query[Dummy].map(d => d.i).max))
}
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Unlike Postgres, SQL Server doesn't allow an output clause in a select statement, meaning that our implementation has to fail the compilation when a query is present. Let's start with a test:

"output clause - should fail on query" in testContext.withDialect(MirrorSqlDialectWithOutputClause) { ctx =>
  """import ctx._; quote { qr4.insert(lift(TestEntity4(1L))).returning(r => query[TestEntity4].filter(t => t.i == r.i)) }""" mustNot compile
}
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Recapping the beginning of our session:

Parsing verifies the type of the returning clause and can fail the compilation if necessary

That is exactly what we need:

// line 831
def verifyAst(returnBody: Ast) = capability match {
  case OutputClauseSupported =>
  case ReturningClauseSupported =>
  // Only .returning(r => r.prop) or .returning(r => OneElementCaseClass(r.prop1..., propN)) or .returning(r => (r.prop1..., propN)) (well actually it's prop22) is allowed.
  case ReturningMultipleFieldSupported =>
    returnBody match {
  ...
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In order to figure the right moment to stop the compilation, we need to understand that AST better. Back to the console:

val q = quote { qr4.insert(lift(TestEntity4(1L))).returning(r => query[TestEntity4].filter(t => t.i == r.i)) }

scala> pprint.pprintln(q.ast, 200)
Returning(
  Insert(
    Entity("TestEntity4", List()),
    List(Assignment(Ident("v"), Property(Ident("v"), "i"), ScalarValueLift("$line8.$read.$iw.$iw.$iw.$iw.$iw.$iw.TestEntity4.apply(1L).i", 1L, MirrorEncoder(<function3>))))
  ),
  Ident("r"),
  Filter(Entity("TestEntity4", List()), Ident("t"), BinaryOperation(Property(Ident("t"), "i"), ==, Property(Ident("r"), "i")))
)
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The last line is the returnBody we are looking for. Having .map instead of .filter it generates:

val q2 = quote { qr4.insert(lift(TestEntity4(1L))).returning(r => query[TestEntity4]).map(t => t.i) }

scala> pprint.pprintln(q2.ast, 200)
Returning(
  ...
  Map(Entity("TestEntity4", List()), Ident("x1"), Property(Ident("x1"), "i"))
)
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None of these can be allowed to compile. Many similar operations extend Query, so let's consider that to define a limitation in Parsing:

implicit class InsertReturnCapabilityExtension(capability: ReturningCapability) {
  def verifyAst(returnBody: Ast) = capability match {
    case OutputClauseSupported =>
      returnBody match {
        case _: Query =>
          c.fail(s"${currentIdiom.map(n => s"The dialect $n does").getOrElse("Unspecified dialects do")} not allow queries in 'returning' clauses.")
        case _ =>
      }
    case ReturningClauseSupported =>
    ...
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After this change, the tests are happy and green again. We need the same tests for .returningGenerated, but the good news are that we need a quite similar change, which is trivial at this point:

// line 450
case r @ ReturningAction(Insert(table: Entity, Nil), alias, prop) =>
  idiomReturningCapability match {
    ...
    case ReturningClauseSupported =>
      stmt"INSERT INTO ${table.token} ${defaultAutoGeneratedToken(prop.token)} RETURNING ${returnListTokenizer.token(ExpandReturning(r)(this, strategy).map(_._1))}"
    case OutputClauseSupported =>
      stmt"INSERT INTO ${table.token} OUTPUT ${returnListTokenizer.token(ExpandReturning(r, Some("INSERTED"))(this, strategy).map(_._1))} ${defaultAutoGeneratedToken(prop.token)}"
    ...
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It concludes the changes around the sql idiom.

Changing SQLServerDialect

The dialect will extend CanOutputClause from now on:

trait SQLServerDialect
  extends SqlIdiom
  with QuestionMarkBindVariables
  with ConcatSupport
  with CanOutputClause {
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We have to ensure that the SQL generated is valid against the database. Let's add some tests to JdbcContextSpec:

"Insert with returning with multiple columns" in {
  testContext.run(qr1.delete)
  val inserted = testContext.run {
    qr1.insert(lift(TestEntity("foo", 1, 18L, Some(123)))).returning(r => (r.i, r.s, r.o))
  }
  (1, "foo", Some(123)) mustBe inserted
}

"Insert with returning with multiple columns and operations" in {
  testContext.run(qr1.delete)
  val inserted = testContext.run {
    qr1.insert(lift(TestEntity("foo", 1, 18L, Some(123)))).returning(r => (r.i + 100, r.s, r.o.map(_ + 100)))
  }
  (1 + 100, "foo", Some(123 + 100)) mustBe inserted
}

"Insert with returning with multiple columns and query embedded" in {
  testContext.run(qr1Emb.delete)
  testContext.run(qr1Emb.insert(lift(TestEntityEmb(Emb("one", 1), 18L, Some(123)))))
  val inserted = testContext.run {
    qr1Emb.insert(lift(TestEntityEmb(Emb("two", 2), 18L, Some(123)))).returning(r =>
      (r.emb.i, r.o))
  }
  (2, Some(123)) mustBe inserted
}

"Insert with returning with multiple columns - case class" in {
  case class Return(id: Int, str: String, opt: Option[Int])
  testContext.run(qr1.delete)
  val inserted = testContext.run {
    qr1.insert(lift(TestEntity("foo", 1, 18L, Some(123)))).returning(r => Return(r.i, r.s, r.o))
  }
  Return(1, "foo", Some(123)) mustBe inserted
}
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What can go wrong?

[info] - Insert with returning with multiple columns *** FAILED ***
[info]   com.microsoft.sqlserver.jdbc.SQLServerException: A result set was generated for update.
[info]   at com.microsoft.sqlserver.jdbc.SQLServerException.makeFromDriverError(SQLServerException.java:227)
[info]   at com.microsoft.sqlserver.jdbc.SQLServerPreparedStatement.doExecutePreparedStatement(SQLServerPreparedStatement.java:592)
[info]   at com.microsoft.sqlserver.jdbc.SQLServerPreparedStatement$PrepStmtExecCmd.doExecute(SQLServerPreparedStatement.java:508)
[info]   at com.microsoft.sqlserver.jdbc.TDSCommand.execute(IOBuffer.java:7233)
[info]   at com.microsoft.sqlserver.jdbc.SQLServerConnection.executeCommand(SQLServerConnection.java:2869)
[info]   at com.microsoft.sqlserver.jdbc.SQLServerStatement.executeCommand(SQLServerStatement.java:243)
[info]   at com.microsoft.sqlserver.jdbc.SQLServerStatement.executeStatement(SQLServerStatement.java:218)
[info]   at com.microsoft.sqlserver.jdbc.SQLServerPreparedStatement.executeUpdate(SQLServerPreparedStatement.java:461)
[info]   at com.zaxxer.hikari.pool.ProxyPreparedStatement.executeUpdate(ProxyPreparedStatement.java:61)
[info]   at com.zaxxer.hikari.pool.HikariProxyPreparedStatement.executeUpdate(HikariProxyPreparedStatement.java)
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This is an interesting error, which actually happens in ProductJdbcSpec as well. Luckly, it has been reported in the issue:

Also note that SQL Server requires prep.executeQuery() instead of a combination of preparedStatement.executeUpdate() and preparedStatement.getGeneratedKeys()

Good to know. JdbcContextBase is the responsible for handling jdbc connections. According to the description above, the method we are looking for is executeActionReturning:

def executeActionReturning[O](sql: String, prepare: Prepare = identityPrepare, extractor: Extractor[O], returningBehavior: ReturnAction): Result[O] =
  withConnectionWrapped { conn =>
    val (params, ps) = prepare(prepareWithReturning(sql, conn, returningBehavior))
    logger.logQuery(sql, params)
    ps.executeUpdate()
    handleSingleResult(extractResult(ps.getGeneratedKeys, extractor))
  }
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SqlServerJdbcContextBase extends JdbcContextBase, defining SQLServerDialect as idiom:

trait SqlServerJdbcContextBase[N <: NamingStrategy] extends JdbcContextBase[SQLServerDialect, N]
  with BooleanObjectEncoding
  with UUIDStringEncoding {

  val idiom = SQLServerDialect
}
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SQL Server demands an exceptional behaviour regarding preparedStatement, so we need to override executeActionReturning:

trait SqlServerJdbcContextBase[N <: NamingStrategy] extends JdbcContextBase[SQLServerDialect, N]
  with BooleanObjectEncoding
  with UUIDStringEncoding {

  val idiom = SQLServerDialect

  override def executeActionReturning[O](sql: String, prepare: Prepare = identityPrepare, extractor: Extractor[O], returningBehavior: ReturnAction): Result[O] =
    withConnectionWrapped { conn =>
      val (params, ps) = prepare(prepareWithReturning(sql, conn, returningBehavior))
      logger.logQuery(sql, params)
      handleSingleResult(extractResult(ps.executeQuery, extractor))
    }
}
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Obviously we have to add more tests to SQLServerDialectSpec, but this post is already long enough to paste those tests here. With that we finish our contribution to Quill.

That was fun, maybe you should be the pilot next time! Let me know when we should have the next session in the comments!

Originally posted on my blog

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