OneToManySelfReferenced.md

Domain model

There are occasions where an entity should map to itself. I.e. consider a Person who has friends. Those friends are instances of Person too:

case class Person(val name: String, val friends: Set[Person])

Table

The table might look like this (ddl for postgresql):

create table Person (
	id serial not null,
	name varchar(100) not null,
	friend_id int,
	primary key (id),
	foreign key (friend_id) references Person(id) on delete cascade
)

The friend_id is a foreign key to the same table.

Mappings

We will map this as a one-to-many relationship between Person and Person:

object PersonEntity extends Entity[Int,SurrogateIntId, Person] {
	val aid = key("id") autogenerated (_.id)
	val name = column("name") to (_.name)
	val friends = onetomany(PersonEntity) foreignkey "friend_id" to (_.friends)

	def constructor(implicit m) = new Person(name, friends) with Stored {
		val id:Int = aid
	}
}

The interesting bit is the following:

val friends = onetomany(PersonEntity) foreignkey "friend_id" to (_.friends)

We created a one-to-many relationship from Person=>Person using the friend_id column.