7.1 KiB
Overview
The DAO (Data Access Object) API of Exposed, is similar to ORM frameworks like Hibernate with specific Kotlin API.
A DB table is represented by an object inherited from org.jetbrains.exposed.sql.Table like that:
object StarWarsFilms : Table() {
val id: Column<Int> = integer("id").autoIncrement().primaryKey()
val sequelId: Column<Int> = integer("sequel_id").uniqueIndex()
val name: Column<String> = varchar("name", 50)
val director: Column<String> = varchar("director", 50)
}
Tables that contains Int id with the name id can be declared like that:
object StarWarsFilms : IntIdTable() {
val sequelId: Column<Int> = integer("sequel_id").uniqueIndex()
val name: Column<String> = varchar("name", 50)
val director: Column<String> = varchar("director", 50)
}
Note that these Column types will be defined automatically, so you can also just leave them away. This would produce the same result as the example above:
object StarWarsFilms : IntIdTable() {
val sequelId = integer("sequel_id").uniqueIndex()
val name = varchar("name", 50)
val director = varchar("director", 50)
}
An entity instance or a row in the table is defined as a class instance:
class StarWarsFilm(id: EntityID<Int>) : IntEntity(id) {
companion object : IntEntityClass<StarWarsFilm>(StarWarsFilms)
var sequelId by StarWarsFilms.sequelId
var name by StarWarsFilms.name
var director by StarWarsFilms.director
}
Basic CRUD operations
Create
val movie = StarWarsFilm.new {
name = "The Last Jedi"
sequelId = 8
director = "Rian Johnson"
}
Read
To get entities use one of the following
val movies = StarWarsFilm.all()
val movies = StarWarsFilm.find {StarWarsFilms.sequelId eq 8}
val movie = StarWarsFilm.findById(5)
- For a list of avaialable predicates see DSL Where expression.
Read a value from a property similar to any property in a Kotlin class:
val name = movie.name
Sort (Order-by)
val movies = StarWarsFilm.all().sortedBy { it.sequelId }
Update
Update a value of a property similar to any property in a Kotlin class:
movie.name = "Episode VIII – The Last Jedi"
- Note: Exposed doesn't make an immediate update when you set a new value for Entity, it just store it on the inner map. "Flushing" values to the database occurs at the end of the transaction or before next "select *" from the database.
Delete
movie.delete()
Referencing
many-to-one reference
Let's say you have this table:
object Users: IntIdTable() {
val name = varchar("name", 50)
}
class User(id: EntityID<Int>): IntEntity(id) {
companion object : IntEntityClass<User>(Users)
var name by Users.name
}
And now you want to add a table referencing this table (and other tables!):
object UserRatings: IntIdTable() {
val value = long("value")
val film = reference("film", StarWarsFilms)
val user = reference("user", Users)
}
class UserRating(id: EntityID<Int>): IntEntity(id) {
companion object : IntEntityClass<UserRating>(UserRatings)
var value by UserRatings.value
var film by StarWarsFilm referencedOn UserRatings.film // use referencedOn for normal references
var user by User referencedOn UserRatings.user
}
Now you can get the film for a rating in the same way you would get any other field:
filmRating.film // returns a StarWarsFilm object
Now if you wanted to get all the ratings for a film, you could do that by using the FilmRating.find function, but what is much easier is to just add a referrersOn field to the StarWarsFilm class:
class StarWarsFilm(id: EntityID<Int>) : IntEntity(id) {
companion object : IntEntityClass<StarWarsFilm>(StarWarsFilms)
...
val ratings by UserRating referrersOn UserRatings.film // make sure to use val and referrersOn
...
}
You can call:
movie.ratings // returns all UserRating objects with this movie as film
Optional reference
You can also add an optional reference:
object UserRatings: IntIdTable() {
...
val secondUser = reference("second_user", Users).nullable() // this reference is nullable!
...
}
class UserRating(id: EntityID<Int>): IntEntity(id) {
companion object : IntEntityClass<UserRating>(UserRatings)
...
var secondUser by User optionalReferencedOn UserRatings.secondUser // use optionalReferencedOn for nullable references
...
}
Now secondUser will be a nullable field.
Of course you can still use referrersOn.
many-to-many reference
In some cases a many-to-many reference may be required. Let's assume you want to add a reference to the following actors table to your StarWarsFilm DAO:
object Actors: IntIdTable() {
val firstname = varchar("firstname", 50)
val lastname = varchar("lastname", 50)
}
class Actor(id: EntityID<Int>): IntEntity(id) {
companion object : IntEntityClass<Actor>(Actors)
var firstname by Actors.firstname
var lastname by Actors.lastname
}
Create an additional intermediate table to store the references:
object StarWarsFilmActors : Table() {
val starWarsFilm = reference("starWarsFilm", StarWarsFilms).primaryKey(0)
val actor = reference("actor", Actors).primaryKey(1)
}
Add a reference to StarWarsFilm:
class StarWarsFilm(id: EntityID<Int>) : IntEntity(id) {
companion object : IntEntityClass<StarWarsFilm>(StarWarsFilms)
...
var actors by Actor via StarWarsFilmActors
...
}
Note: Creating the entity and the reference in the same transaction is not supported yet.
The creation needs to be done before setting the reference:
// create film
val film = transaction {
StarWarsFilm.new {
name = "The Last Jedi"
sequelId = 8
director = "Rian Johnson"
}
}
//create actor
val actor = transaction {
Actor.new {
firstname = "Daisy"
lastname = "Ridley"
}
}
//add reference
transaction {
film.actors = SizedCollection(listOf(actor))
}
Advanced CRUD operations
Read entity with a join to another table
Lets imagine that you want to find all users who rated second SW film with more then 5. First of all we should write that query using Exposed DSL.
val query = Users.innerJoin(UserRatings).innerJoin(StarWarsFilm)
.slice(Users.columns)
.select {
StarWarsFilms.sequelId eq 2 and (UserRatings.value gt 5)
}.withDistinct()
After that all we have to do is to "wrap" a result with User entity:
val users = User.wrapRows(query).toList()