tpolecat

7. DDL, Inserting, and Updating

In this chapter we examine operations that modify data in the database, and ways to retrieve the results of these updates.

Setting Up

Again we set up a transactor and pull in YOLO mode, but this time we’re not using the world database.

import doobie.imports._
import scalaz._, Scalaz._
import scalaz.concurrent.Task
import scalaz.stream.Process

val xa = DriverManagerTransactor[Task](
  "org.postgresql.Driver", "jdbc:postgresql:world", "postgres", ""
)

import xa.yolo._

Data Definition

It is uncommon to define database structures at runtime, but doobie handles it just fine and treats such operations like any other kind of update. And it happens to be useful here!

Let’s create a new table, which we will use for the examples to follow. This looks a lot like our prior usage of the sql interpolator, but this time we’re using update rather than query. The .run method gives a ConnectionIO[Int] that yields the total number of rows modified, and the YOLO-mode .quick gives a Task[Unit] that prints out the row count.

val drop: Update0 = 
  sql"""
    DROP TABLE IF EXISTS person
  """.update

val create: Update0 = 
  sql"""
    CREATE TABLE person (
      id   SERIAL,
      name VARCHAR NOT NULL UNIQUE,
      age  SMALLINT
    )
  """.update

We can compose these and run them together.

scala> (drop.quick *> create.quick).run
  0 row(s) updated
  0 row(s) updated

Inserting

Inserting is straightforward and works just as with selects. Here we define a method that constructs an Update0 that inserts a row into the person table.

def insert1(name: String, age: Option[Short]): Update0 =
  sql"insert into person (name, age) values ($name, $age)".update

Let’s insert a few rows.

scala> insert1("Alice", Some(12)).quick.run
  1 row(s) updated

scala> insert1("Bob", None).quick.run
  1 row(s) updated

And read them back.

case class Person(id: Long, name: String, age: Option[Short])

scala> sql"select id, name, age from person".query[Person].quick.run
  Person(1,Alice,Some(12))
  Person(2,Bob,None)

Updating

Updating follows the same pattern. Here we update Alice’s age.

scala> sql"update person set age = 15 where name = 'Alice'".update.quick.run
  1 row(s) updated

scala> sql"select id, name, age from person".query[Person].quick.run
  Person(2,Bob,None)
  Person(1,Alice,Some(15))

Retrieving Results

When we insert we usually want the new row back, so let’s do that. First we’ll do it the hard way, by inserting, getting the last used key via lastVal(), then selecting the indicated row.

def insert2(name: String, age: Option[Short]): ConnectionIO[Person] =
  for {
    _  <- sql"insert into person (name, age) values ($name, $age)".update.run
    id <- sql"select lastval()".query[Long].unique
    p  <- sql"select id, name, age from person where id = $id".query[Person].unique
  } yield p

scala> insert2("Jimmy", Some(42)).quick.run
  Person(3,Jimmy,Some(42))

This is irritating but it is supported by all databases (although the “get the last used id” function will vary by vendor).

Some database (like H2) allow you to return [only] the inserted id, allowing the above operation to be reduced to two statements (see below for an explanation of withUniqueGeneratedKeys).

def insert2_H2(name: String, age: Option[Short]): ConnectionIO[Person] =
  for {
    id <- sql"insert into person (name, age) values ($name, $age)".update.withUniqueGeneratedKeys[Int]("id")
    p  <- sql"select id, name, age from person where id = $id".query[Person].unique
  } yield p

scala> insert2_H2("Ramone", Some(42)).quick.run
  Person(4,Ramone,Some(42))

Other databases (including PostgreSQL) provide a way to do this in one shot by returning multiple specified columns from the inserted row.

def insert3(name: String, age: Option[Short]): ConnectionIO[Person] = {
  sql"insert into person (name, age) values ($name, $age)"
    .update.withUniqueGeneratedKeys("id", "name", "age")
}

The withUniqueGeneratedKeys specifies that we expect exactly one row back (otherwise an exception will be thrown), and requires a list of columns to return. This isn’t the most beautiful API but it’s what JDBC gives us. And it does work.

scala> insert3("Elvis", None).quick.run
  Person(5,Elvis,None)

This mechanism also works for updates, for databases that support it. In the case of multiple row updates we omit unique and get a Process[ConnectionIO, Person] back.

val up = {
  sql"update person set age = age + 1 where age is not null"
    .update.withGeneratedKeys[Person]("id", "name", "age")
}

Running this process updates all rows with a non-NULL age and returns them.

scala> up.quick.run
  Person(1,Alice,Some(16))
  Person(3,Jimmy,Some(43))
  Person(4,Ramone,Some(43))

scala> up.quick.run // and again!
  Person(1,Alice,Some(17))
  Person(3,Jimmy,Some(44))
  Person(4,Ramone,Some(44))

Batch Updates

doobie supports batch updating via the updateMany and updateManyWithGeneratedKeys operations on the Update data type (which we haven’t seen before). An Update0, which is the type of a sql"..." expression, represents a parameterized statement where the arguments are known. An Update[A] is more general, and represents a parameterized statement where the composite argument of type A is not known.

// Given some values ...
val a = 1; val b = "foo"

// this expression ...
sql"... $a $b ..."

// is syntactic sugar for this one, which is an Update applied to (a, b)
Update[(Int, String)]("... ? ? ...").run((a, b))

By using an Update directly we can apply many sets of arguments to the same statement, and execute it as a single batch operation.

type PersonInfo = (String, Option[Short])

def insertMany(ps: List[PersonInfo]): ConnectionIO[Int] = {
  val sql = "insert into person (name, age) values (?, ?)"
  Update[PersonInfo](sql).updateMany(ps)
}

// Some rows to insert
val data = List[PersonInfo](
  ("Frank", Some(12)), 
  ("Daddy", None))

Running this program yields the number of updated rows.

scala> insertMany(data).quick.run
  2

For databases that support it (such as PostgreSQL) we can use updateManyWithGeneratedKeys to return a stream of updated rows.

def insertMany2(ps: List[PersonInfo]): Process[ConnectionIO, Person] = {
  val sql = "insert into person (name, age) values (?, ?)"
  Update[PersonInfo](sql).updateManyWithGeneratedKeys[Person]("id", "name", "age")(ps)
}

// Some rows to insert
val data = List[PersonInfo](
  ("Banjo",   Some(39)), 
  ("Skeeter", None), 
  ("Jim-Bob", Some(12)))

Running this program yields the updated instances.

scala> insertMany2(data).quick.run
  Person(8,Banjo,Some(39))
  Person(9,Skeeter,None)
  Person(10,Jim-Bob,Some(12))