Overview
In this tutorial, we are going to learn what is and how to use JetBrains/Exposed framework, the DSL approach especially.
In another blog post we did the custom implementation in advance and we extended Exposed framework. Now, we need to make a step back and have a quick look on the Exposed itself.
Setting up a test application
NOTE: For the sake of the test, we use Ktor - the easiest way to do so is to use initializer.
Once we go through the form, the application frame is ready to work with. Now, we need to add serialization functionality (because we want to return a JSON object as the response).
build.gradle.ts
plugins {
...
kotlin("plugin.serialization") version "1.8.10"
}
depenendencies {
...
implementation("io.ktor:ktor-server-content-negotiation:2.2.4")
implementation("io.ktor:ktor-serialization-kotlinx-json:2.2.4")
}
We may need H2 database:
build.gradle.ts
dependencies {
implementation("com.h2database:h2:$h2Version")
}
Adding the Exposed
Now, we can add our persistence layer - Exposed ORM:
build.gradle.ts
dependencies {
implementation("org.jetbrains.exposed:exposed-core:0.40.1")
implementation("org.jetbrains.exposed:exposed-jdbc:0.40.1")
}
In order to create a database connection and perform the initial db insert, I created Ktor's plugin:
Data.kt
package pl.brightinventions.plugins
import pl.brightinventions.dto.CreatePersonDto
import pl.brightinventions.exposed.Database
import pl.brightinventions.persistance.PersonDaoImpl
import pl.brightinventions.persistance.table.PersonTable
import io.ktor.server.application.*
import org.jetbrains.exposed.sql.SchemaUtils
import org.jetbrains.exposed.sql.transactions.transaction
import pl.brightinventions.dto.CreateAddressDto
import pl.brightinventions.persistance.table.AddressTable
fun Application.configureData() {
Database.register()
// @TODO("More logic incoming")
}
and register it on the application startup:
Application.kt
import pl.pl.brightinventionsugins.configureData
import pl.pl.brightinventionsugins.configureRouting
import io.ktor.serialization.kotlinx.json.*
import io.ktor.server.application.*
import io.ktor.server.engine.*
import io.ktor.server.netty.*
import io.ktor.server.plugins.contentnegotiation.*
fun main() {
embeddedServer(Netty, port = 8080, host = "0.0.0.0") {
install(ContentNegotiation) {
json()
}
configureData()
}
.start(wait = true)
}
Persistence layer logic
Once we added Exposed into our environment, it's time to introduce the table's model and some DTOs.
In Exposed, the table's representation is an object:
PersonTable.kt
import org.jetbrains.exposed.sql.Table
object PersonTable : Table("person") {
val id = integer("id").autoIncrement().uniqueIndex()
val name = text("name")
val surname = text("surname")
val age = integer("age")
}
Your object table needs to extend from the Exposed Table. The content of it is a group of defined columns. Additionally, I want to show you the relations, so we need one more table:
AddressTable.kt
package pl.brightinventions.persistance.table
import org.jetbrains.exposed.sql.Table
object AddressTable : Table("address") {
val id = integer("id").autoIncrement().uniqueIndex()
val personId = reference("person_id", PersonTable.id)
val street = text("street")
val house = text("house")
val apartment = text("apartment")
val city = text("city")
val postalCode = text("postal_code")
}
besides "normal" columns, we defined a reference here: personId, which relates to our newly created PersonTable
Let's create our Repository with the methods as follows:
PersonRepositoryImpl.kt (I skip the Repository interface part - you can check it on the Github repo page)
get a list of records
override fun findAll(): List<FoundPersonDto> = transaction {
PersonTable.selectAll().map {
FoundPersonDto(
it[PersonTable.id],
it[PersonTable.name],
it[PersonTable.surname],
it[PersonTable.age]
)
}
}
get a single record
this is the most questionable one because it can be solved in many ways. For the sake of this article, I want to propose the simplest, two-queries solution:
override fun find(id: PersonId): FoundPersonWithAddressDto? = transaction {
val addresses = AddressTable
.select { AddressTable.personId eq id }
.map {
FoundPersonAddressDto(
it[AddressTable.street],
it[AddressTable.house],
it[AddressTable.apartment],
it[AddressTable.city],
it[AddressTable.postalCode]
)
}
PersonTable.select { PersonTable.id eq id }.firstOrNull()?.let {
FoundPersonWithAddressDto(
it[PersonTable.id],
it[PersonTable.name],
it[PersonTable.surname],
it[PersonTable.age],
addresses
)
}
}
as you can see, in the first query we did select all the addresses that belong to a particular person by personId relation field. In the next query/mapping, we just assigned fetched list to the new FoundPersonWithAddressDto instance.
create the record
It's a simple operation, but after the execution, we want to return id of created records. Because PersonTable extends Table, we cannot use insertAndGetId (which is a part of DAO API which will be explained in the next episode of the series), so we need to do a little trick:
override fun create(person: CreatePersonDto): PersonId = transaction {
PersonTable.insert {
it[name] = person.name
it[surname] = person.surname
it[age] = person.age
}.resultedValues!!.map { it[PersonTable.id] }.first()
}
update the record
Pretty straight-forward - update with where clause
override fun update(id: PersonId, person: UpdatePersonDto) {
PersonTable.update({ PersonTable.id eq id }) {
it[age] = person.age
it[name] = person.name
it[surname] = person.surname
}
}
delete the record
Another simple and self-explanatory - delete with the where clause
override fun delete(id: PersonId): Unit = transaction {
PersonTable.deleteWhere {
PersonTable.id eq id
}
}
add child relations records
because we use DSL (instead of DAO), we need to take care of inserting child rows by hand:
override fun addAddress(personId: PersonId, address: CreateAddressDto) {
AddressTable.insert {
it[AddressTable.personId] = personId
it[street] = address.street
it[city] = address.city
it[house] = address.house
it[postalCode] = address.postalCode
it[apartment] = address.apartment
}
}
Initial insert
Once we have Table and Repository ready, we can go with implementing further our Data class:
Data.kt
fun Application.configureData() {
Database.register()
val repository = PersonRepositoryImpl()
transaction {
SchemaUtils.create(PersonTable)
SchemaUtils.create(AddressTable)
val john = repository.create(CreatePersonDto("John", "Doe", 33))
repository.addAddress(john, CreateAddressDto(
"ul. Jana Matejki", "12", "1", "Gdansk", "80-232"
))
repository.addAddress(john, CreateAddressDto(
"ul. Jana Matejki", "13", "1", "Gdansk", "80-232"
))
repository.create(CreatePersonDto("George", "Smith", 34))
repository.create(CreatePersonDto("Megan", "Miller", 22))
}
}
What we did here? In transaction block.
Every database access using Exposed is started by obtaining a connection and creating a transaction.
We created the tables in the database (SchemaUtils.create call) and filled up the DB with initial records. For the first created Person record, we put two addresses with reference to John.
REST endpoints
Right, we have the database filled up, we can use our DAO to create a REST endpoint:
Routing.kt
package pl.brightinventions.plugins
import io.ktor.http.*
import io.ktor.server.application.*
import io.ktor.server.request.*
import io.ktor.server.response.*
import io.ktor.server.routing.*
import pl.brightinventions.persistance.PersonRepositoryImpl
fun Application.configureRouting() {
val repository = PersonRepositoryImpl()
routing {
route("/person") {
get {
call.respond(repository.findAll())
}
get("/{id}") {
val found = repository.find(call.parameters["id"]?.toInt()!!)
found?.let { call.respond(it) } ?: call.respond(HttpStatusCode.NotFound)
}
post {
call.respond(repository.create(call.receive()))
}
delete("/{id}") {
call.respond(repository.delete(call.parameters["id"]?.toInt()!!))
}
put("/{id}") {
call.respond(repository.update(call.parameters["id"]?.toInt()!!, call.receive()))
}
}
}
}
What we did here? We (once again) created Ktor's plugin for registering the routing here. On the /person prefix, we register REST endpoints with a call to the PersonRepository.
but we need to register it in the application:
Application.kt
...
fun main() {
embeddedServer(Netty, port = 8080, host = "0.0.0.0") {
install(ContentNegotiation) {
json()
}
configureRouting()
configureData()
}
.start(wait = true)
}
Run it, baby!
GET http://localhost:8080/person \
should respond with a list of three persons: John, George, and Megan. Those will be represented by the header only (meaning, there will be no information about addresses yet).
So if we want to receive rich data about a particular person, we need to call\
GET http://localhost:8080/person/1,\
which will respond with fat Person DTO.
Conclusion
In this article, we've learned how to add JetBrains/Exposed to our project, and how to implement basic CRUD actions. In the next episode, I will show you, how to migrate from DSL to DAO approach - we will work with Entities, relations, and more! Stay tuned.
You can find the complete code over GitHub.
Read the next part of the series: Exposed in Your Project - Part 2 - DAO.
By Patryk Szlagowski, Senior Backend Developer @ Bright Inventions
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