Schemas

This chapter describes how to work with database schemas in the Lifted Embedding API. This explains how you can write schema descriptions by hand. Instead you can also use the code generator to take this work off your hands.

Table Rows

In order to use the Lifted Embedding API for type-safe queries, you need to define Table row classes for your database schema. These describe the structure of the tables:

class Coffees(tag: Tag) extends Table[(String, Int, Double, Int, Int)](tag, "COFFEES") {
  def name = column[String]("COF_NAME", O.PrimaryKey)
  def supID = column[Int]("SUP_ID")
  def price = column[Double]("PRICE")
  def sales = column[Int]("SALES", O.Default(0))
  def total = column[Int]("TOTAL", O.Default(0))
  def * = (name, supID, price, sales, total)
}

All columns are defined through the column method. Each column has a Scala type and a column name for the database (usually in upper-case). The following primitive types are supported out of the box for JDBC-based databases in JdbcProfile (with certain limitations imposed by the individual database drivers):

• Numeric types: Byte, Short, Int, Long, BigDecimal, Float, Double
• LOB types: java.sql.Blob, java.sql.Clob, Array[Byte]
• Date types: java.sql.Date, java.sql.Time, java.sql.Timestamp
• Boolean
• String
• Unit
• java.util.UUID

Nullable columns are represented by Option[T] where T is one of the supported primitive types. Note that all operations on Option values are currently using the database’s null propagation semantics which may differ from Scala’s Option semantics. In particular, None === None evaluates to None. This behaviour may change in a future major release of Slick.

After the column name, you can add optional column options to a column definition. The applicable options are available through the table’s O object. The following ones are defined for JdbcProfile:

PrimaryKey

Mark the column as a (non-compound) primary key when creating the DDL statements.

Default[T](defaultValue: T)

Specify a default value for inserting data into the table without this column. This information is only used for creating DDL statements so that the database can fill in the missing information.

DBType(dbType: String)

Use a non-standard database-specific type for the DDL statements (e.g. DBType("VARCHAR(20)") for a String column).

AutoInc

Mark the column as an auto-incrementing key when creating the DDL statements. Unlike the other column options, this one also has a meaning outside of DDL creation: Many databases do not allow non-AutoInc columns to be returned when inserting data (often silently ignoring other columns), so Slick will check if the return column is properly marked as AutoInc where needed.

NotNull, Nullable

Explicitly mark the column as nullable or non-nullable when creating the DDL statements for the table. Nullability is otherwise determined from the type (Option or non-Option). There is usually no reason to specify these options.

Every table requires a * method contatining a default projection. This describes what you get back when you return rows (in the form of a table row object) from a query. Slick’s * projection does not have to match the one in the database. You can add new columns (e.g. with computed values) or omit some columns as you like. The non-lifted type corresponding to the * projection is given as a type parameter to Table. For simple, non-mapped tables, this will be a single column type or a tuple of column types.

If your database layout requires schema names, you can specify the schema name for a table in front of the table name, wrapped in Some():

class Coffees(tag: Tag)
  extends Table[(String, Int, Double, Int, Int)](tag, Some("MYSCHEMA"), "COFFEES") {
  //...
}

Table Query

Alongside the Table row class you also need a TableQuery value which represents the actual database table:

val coffees = TableQuery[Coffees]

The simple TableQuery[T] syntax is a macro which expands to a proper TableQuery instance that calls the table’s constructor (new TableQuery(new T(_))).

You can also extend TableQuery to use it as a convenient namespace for additional functionality associated with the table:

object coffees extends TableQuery(new Coffees(_)) {
  val findByName = this.findBy(_.name)
}

Mapped Tables

It is possible to define a mapped table that uses a custom type for its * projection by adding a bi-directional mapping with the <> operator:

case class User(id: Option[Int], first: String, last: String)

class Users(tag: Tag) extends Table[User](tag, "users") {
  def id = column[Int]("id", O.PrimaryKey, O.AutoInc)
  def first = column[String]("first")
  def last = column[String]("last")
  def * = (id.?, first, last) <> (User.tupled, User.unapply)
}
val users = TableQuery[Users]

It is optimized for case classes (with a simple apply method and an unapply method that wraps its result in an Option) but it can also be used with arbitrary mapping functions. In these cases it can be useful to call .shaped on a tuple on the left-hand side in order to get its type inferred properly. Otherwise you may have to add full type annotations to the mapping functions.

For case classes with hand-written companion objects, .tupled only works if you manually extend the correct Scala function type. Alternatively you can use (User.apply _).tupled. See SI-3664 and SI-4808.

Constraints

A foreign key constraint can be defined with a table’s foreignKey method. It takes a name for the constraint, the local column (or projection, so you can define compound foreign keys), the linked table, and a function from that table to the corresponding column(s). When creating the DDL statements for the table, the foreign key definition is added to it.

class Suppliers(tag: Tag) extends Table[(Int, String, String, String, String, String)](tag, "SUPPLIERS") {
  def id = column[Int]("SUP_ID", O.PrimaryKey)
  //...
}
val suppliers = TableQuery[Suppliers]

class Coffees(tag: Tag) extends Table[(String, Int, Double, Int, Int)](tag, "COFFEES") {
  def supID = column[Int]("SUP_ID")
  //...
  def supplier = foreignKey("SUP_FK", supID, suppliers)(_.id)
  // compiles to SQL:
  //   alter table "COFFEES" add constraint "SUP_FK" foreign key("SUP_ID")
  //     references "SUPPLIERS"("SUP_ID")
  //     on update NO ACTION on delete NO ACTION
}
val coffees = TableQuery[Coffees]

Independent of the actual constraint defined in the database, such a foreign key can be used to navigate to the linked data with a join. For this purpose, it behaves the same as a manually defined utility method for finding the joined data:

def supplier = foreignKey("SUP_FK", supID, suppliers)(_.id)
def supplier2 = suppliers.filter(_.id === supID)

A primary key constraint can be defined in a similar fashion by adding a method that calls primaryKey. This is useful for defining compound primary keys (which cannot be done with the O.PrimaryKey column option):

class A(tag: Tag) extends Table[(Int, Int)](tag, "a") {
  def k1 = column[Int]("k1")
  def k2 = column[Int]("k2")
  def * = (k1, k2)
  def pk = primaryKey("pk_a", (k1, k2))
  // compiles to SQL:
  //   alter table "a" add constraint "pk_a" primary key("k1","k2")
}

Other indexes are defined in a similar way with the index method. They are non-unique by default unless you set the unique parameter:

class A(tag: Tag) extends Table[(Int, Int)](tag, "a") {
  def k1 = column[Int]("k1")
  def k2 = column[Int]("k2")
  def * = (k1, k2)
  def idx = index("idx_a", (k1, k2), unique = true)
  // compiles to SQL:
  //   create unique index "idx_a" on "a" ("k1","k2")
}

All constraints are discovered reflectively by searching for methods with the appropriate return types which are defined in the table. This behavior can be customized by overriding the tableConstraints method.

Data Definition Language

DDL statements for a table can be created with its TableQuery“s ddl method. Multiple DDL objects can be concatenated with ++ to get a compound DDL object which can create and drop all entities in the correct order, even in the presence of cyclic dependencies between tables. The statements are executed with the create and drop methods:

val ddl = coffees.ddl ++ suppliers.ddl
db withDynSession {
  ddl.create
  //...
  ddl.drop
}

You can use the createStatements and dropStatements methods to get the SQL code:

ddl.createStatements.foreach(println)
ddl.dropStatements.foreach(println)