SKILL.md
Kotlin Exposed Patterns
Comprehensive patterns for database access with JetBrains Exposed ORM, including DSL queries, DAO, transactions, and production-ready configuration.
When to Use
- Setting up database access with Exposed
- Writing SQL queries using Exposed DSL or DAO
- Configuring connection pooling with HikariCP
- Creating database migrations with Flyway
- Implementing the repository pattern with Exposed
- Handling JSON columns and complex queries
How It Works
Exposed provides two query styles: DSL for direct SQL-like expressions and DAO for entity lifecycle management. HikariCP manages a pool of reusable database connections configured via HikariConfig. Flyway runs versioned SQL migration scripts at startup to keep the schema in sync. All database operations run inside newSuspendedTransaction blocks for coroutine safety and atomicity. The repository pattern wraps Exposed queries behind an interface so business logic stays decoupled from the data layer and tests can use an in-memory H2 database.
Examples
DSL Query
suspend fun findUserById(id: UUID): UserRow? =
newSuspendedTransaction {
UsersTable.selectAll()
.where { UsersTable.id eq id }
.map { it.toUser() }
.singleOrNull()
}DAO Entity Usage
suspend fun createUser(request: CreateUserRequest): User =
newSuspendedTransaction {
UserEntity.new {
name = request.name
email = request.email
role = request.role
}.toModel()
}HikariCP Configuration
val hikariConfig = HikariConfig().apply {
driverClassName = config.driver
jdbcUrl = config.url
username = config.username
password = config.password
maximumPoolSize = config.maxPoolSize
isAutoCommit = false
transactionIsolation = "TRANSACTION_READ_COMMITTED"
validate()
}Database Setup
HikariCP Connection Pooling
// DatabaseFactory.kt
object DatabaseFactory {
fun create(config: DatabaseConfig): Database {
val hikariConfig = HikariConfig().apply {
driverClassName = config.driver
jdbcUrl = config.url
username = config.username
password = config.password
maximumPoolSize = config.maxPoolSize
isAutoCommit = false
transactionIsolation = "TRANSACTION_READ_COMMITTED"
validate()
}
return Database.connect(HikariDataSource(hikariConfig))
}
}
data class DatabaseConfig(
val url: String,
val driver: String = "org.postgresql.Driver",
val username: String = "",
val password: String = "",
val maxPoolSize: Int = 10,
)Flyway Migrations
// FlywayMigration.kt
fun runMigrations(config: DatabaseConfig) {
Flyway.configure()
.dataSource(config.url, config.username, config.password)
.locations("classpath:db/migration")
.baselineOnMigrate(true)
.load()
.migrate()
}
// Application startup
fun Application.module() {
val config = DatabaseConfig(
url = environment.config.property("database.url").getString(),
username = environment.config.property("database.username").getString(),
password = environment.config.property("database.password").getString(),
)
runMigrations(config)
val database = DatabaseFactory.create(config)
// ...
}Migration Files
-- src/main/resources/db/migration/V1__create_users.sql
CREATE TABLE users (
id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
name VARCHAR(100) NOT NULL,
email VARCHAR(255) NOT NULL UNIQUE,
role VARCHAR(20) NOT NULL DEFAULT 'USER',
metadata JSONB,
created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
);
CREATE INDEX idx_users_email ON users(email);
CREATE INDEX idx_users_role ON users(role);Table Definitions
DSL Style Tables
// tables/UsersTable.kt
object UsersTable : UUIDTable("users") {
val name = varchar("name", 100)
val email = varchar("email", 255).uniqueIndex()
val role = enumerationByName<Role>("role", 20)
val metadata = jsonb<UserMetadata>("metadata", Json.Default).nullable()
val createdAt = timestampWithTimeZone("created_at").defaultExpression(CurrentTimestampWithTimeZone)
val updatedAt = timestampWithTimeZone("updated_at").defaultExpression(CurrentTimestampWithTimeZone)
}
object OrdersTable : UUIDTable("orders") {
val userId = uuid("user_id").references(UsersTable.id)
val status = enumerationByName<OrderStatus>("status", 20)
val totalAmount = long("total_amount")
val currency = varchar("currency", 3)
val createdAt = timestampWithTimeZone("created_at").defaultExpression(CurrentTimestampWithTimeZone)
}
object OrderItemsTable : UUIDTable("order_items") {
val orderId = uuid("order_id").references(OrdersTable.id, onDelete = ReferenceOption.CASCADE)
val productId = uuid("product_id")
val quantity = integer("quantity")
val unitPrice = long("unit_price")
}Composite Tables
object UserRolesTable : Table("user_roles") {
val userId = uuid("user_id").references(UsersTable.id, onDelete = ReferenceOption.CASCADE)
val roleId = uuid("role_id").references(RolesTable.id, onDelete = ReferenceOption.CASCADE)
override val primaryKey = PrimaryKey(userId, roleId)
}DSL Queries
Basic CRUD
// Insert
suspend fun insertUser(name: String, email: String, role: Role): UUID =
newSuspendedTransaction {
UsersTable.insertAndGetId {
it[UsersTable.name] = name
it[UsersTable.email] = email
it[UsersTable.role] = role
}.value
}
// Select by ID
suspend fun findUserById(id: UUID): UserRow? =
newSuspendedTransaction {
UsersTable.selectAll()
.where { UsersTable.id eq id }
.map { it.toUser() }
.singleOrNull()
}
// Select with conditions
suspend fun findActiveAdmins(): List<UserRow> =
newSuspendedTransaction {
UsersTable.selectAll()
.where { (UsersTable.role eq Role.ADMIN) }
.orderBy(UsersTable.name)
.map { it.toUser() }
}
// Update
suspend fun updateUserEmail(id: UUID, newEmail: String): Boolean =
newSuspendedTransaction {
UsersTable.update({ UsersTable.id eq id }) {
it[email] = newEmail
it[updatedAt] = CurrentTimestampWithTimeZone
} > 0
}
// Delete
suspend fun deleteUser(id: UUID): Boolean =
newSuspendedTransaction {
UsersTable.deleteWhere { UsersTable.id eq id } > 0
}
// Row mapping
private fun ResultRow.toUser() = UserRow(
id = this[UsersTable.id].value,
name = this[UsersTable.name],
email = this[UsersTable.email],
role = this[UsersTable.role],
metadata = this[UsersTable.metadata],
createdAt = this[UsersTable.createdAt],
updatedAt = this[UsersTable.updatedAt],
)Advanced Queries
// Join queries
suspend fun findOrdersWithUser(userId: UUID): List<OrderWithUser> =
newSuspendedTransaction {
(OrdersTable innerJoin UsersTable)
.selectAll()
.where { OrdersTable.userId eq userId }
.orderBy(OrdersTable.createdAt, SortOrder.DESC)
.map { row ->
OrderWithUser(
orderId = row[OrdersTable.id].value,
status = row[OrdersTable.status],
totalAmount = row[OrdersTable.totalAmount],
userName = row[UsersTable.name],
)
}
}
// Aggregation
suspend fun countUsersByRole(): Map<Role, Long> =
newSuspendedTransaction {
UsersTable
.select(UsersTable.role, UsersTable.id.count())
.groupBy(UsersTable.role)
.associate { row ->
row[UsersTable.role] to row[UsersTable.id.count()]
}
}
// Subqueries
suspend fun findUsersWithOrders(): List<UserRow> =
newSuspendedTransaction {
UsersTable.selectAll()
.where {
UsersTable.id inSubQuery
OrdersTable.select(OrdersTable.userId).withDistinct()
}
.map { it.toUser() }
}
// LIKE and pattern matching — always escape user input to prevent wildcard injection
private fun escapeLikePattern(input: String): String =
input.replace("\\", "\\\\").replace("%", "\\%").replace("_", "\\_")
suspend fun searchUsers(query: String): List<UserRow> =
newSuspendedTransaction {
val sanitized = escapeLikePattern(query.lowercase())
UsersTable.selectAll()
.where {
(UsersTable.name.lowerCase() like "%${sanitized}%") or
(UsersTable.email.lowerCase() like "%${sanitized}%")
}
.map { it.toUser() }
}Pagination
data class Page<T>(
val data: List<T>,
val total: Long,
val page: Int,
val limit: Int,
) {
val totalPages: Int get() = ((total + limit - 1) / limit).toInt()
val hasNext: Boolean get() = page < totalPages
val hasPrevious: Boolean get() = page > 1
}
suspend fun findUsersPaginated(page: Int, limit: Int): Page<UserRow> =
newSuspendedTransaction {
val total = UsersTable.selectAll().count()
val data = UsersTable.selectAll()
.orderBy(UsersTable.createdAt, SortOrder.DESC)
.limit(limit)
.offset(((page - 1) * limit).toLong())
.map { it.toUser() }
Page(data = data, total = total, page = page, limit = limit)
}Batch Operations
// Batch insert
suspend fun insertUsers(users: List<CreateUserRequest>): List<UUID> =
newSuspendedTransaction {
UsersTable.batchInsert(users) { user ->
this[UsersTable.name] = user.name
this[UsersTable.email] = user.email
this[UsersTable.role] = user.role
}.map { it[UsersTable.id].value }
}
// Upsert (insert or update on conflict)
suspend fun upsertUser(id: UUID, name: String, email: String) {
newSuspendedTransaction {
UsersTable.upsert(UsersTable.email) {
it[UsersTable.id] = EntityID(id, UsersTable)
it[UsersTable.name] = name
it[UsersTable.email] = email
it[updatedAt] = CurrentTimestampWithTimeZone
}
}
}DAO Pattern
Entity Definitions
// entities/UserEntity.kt
class UserEntity(id: EntityID<UUID>) : UUIDEntity(id) {
companion object : UUIDEntityClass<UserEntity>(UsersTable)
var name by UsersTable.name
var email by UsersTable.email
var role by UsersTable.role
var metadata by UsersTable.metadata
var createdAt by UsersTable.createdAt
var updatedAt by UsersTable.updatedAt
val orders by OrderEntity referrersOn OrdersTable.userId
fun toModel(): User = User(
id = id.value,
name = name,
email = email,
role = role,
metadata = metadata,
createdAt = createdAt,
updatedAt = updatedAt,
)
}
class OrderEntity(id: EntityID<UUID>) : UUIDEntity(id) {
companion object : UUIDEntityClass<OrderEntity>(OrdersTable)
var user by UserEntity referencedOn OrdersTable.userId
var status by OrdersTable.status
var totalAmount by OrdersTable.totalAmount
var currency by OrdersTable.currency
var createdAt by OrdersTable.createdAt
val items by OrderItemEntity referrersOn OrderItemsTable.orderId
}DAO Operations
suspend fun findUserByEmail(email: String): User? =
newSuspendedTransaction {
UserEntity.find { UsersTable.email eq email }
.firstOrNull()
?.toModel()
}
suspend fun createUser(request: CreateUserRequest): User =
newSuspendedTransaction {
UserEntity.new {
name = request.name
email = request.email
role = request.role
}.toModel()
}
suspend fun updateUser(id: UUID, request: UpdateUserRequest): User? =
newSuspendedTransaction {
UserEntity.findById(id)?.apply {
request.name?.let { name = it }
request.email?.let { email = it }
updatedAt = OffsetDateTime.now(ZoneOffset.UTC)
}?.toModel()
}Transactions
Suspend Transaction Support
// Good: Use newSuspendedTransaction for coroutine support
suspend fun performDatabaseOperation(): Result<User> =
runCatching {
newSuspendedTransaction {
val user = UserEntity.new {
name = "Alice"
email = "alice@example.com"
}
// All operations in this block are atomic
user.toModel()
}
}
// Good: Nested transactions with savepoints
suspend fun transferFunds(fromId: UUID, toId: UUID, amount: Long) {
newSuspendedTransaction {
val from = UserEntity.findById(fromId) ?: throw NotFoundException("User $fromId not found")
val to = UserEntity.findById(toId) ?: throw NotFoundException("User $toId not found")
// Debit
from.balance -= amount
// Credit
to.balance += amount
// Both succeed or both fail
}
}Transaction Isolation
suspend fun readCommittedQuery(): List<User> =
newSuspendedTransaction(transactionIsolation = Connection.TRANSACTION_READ_COMMITTED) {
UserEntity.all().map { it.toModel() }
}
suspend fun serializableOperation() {
newSuspendedTransaction(transactionIsolation = Connection.TRANSACTION_SERIALIZABLE) {
// Strictest isolation level for critical operations
}
}Repository Pattern
Interface Definition
interface UserRepository {
suspend fun findById(id: UUID): User?
suspend fun findByEmail(email: String): User?
suspend fun findAll(page: Int, limit: Int): Page<User>
suspend fun search(query: String): List<User>
suspend fun create(request: CreateUserRequest): User
suspend fun update(id: UUID, request: UpdateUserRequest): User?
suspend fun delete(id: UUID): Boolean
suspend fun count(): Long
}Exposed Implementation
class ExposedUserRepository(
private val database: Database,
) : UserRepository {
override suspend fun findById(id: UUID): User? =
newSuspendedTransaction(db = database) {
UsersTable.selectAll()
.where { UsersTable.id eq id }
.map { it.toUser() }
.singleOrNull()
}
override suspend fun findByEmail(email: String): User? =
newSuspendedTransaction(db = database) {
UsersTable.selectAll()
.where { UsersTable.email eq email }
.map { it.toUser() }
.singleOrNull()
}
override suspend fun findAll(page: Int, limit: Int): Page<User> =
newSuspendedTransaction(db = database) {
val total = UsersTable.selectAll().count()
val data = UsersTable.selectAll()
.orderBy(UsersTable.createdAt, SortOrder.DESC)
.limit(limit)
.offset(((page - 1) * limit).toLong())
.map { it.toUser() }
Page(data = data, total = total, page = page, limit = limit)
}
override suspend fun search(query: String): List<User> =
newSuspendedTransaction(db = database) {
val sanitized = escapeLikePattern(query.lowercase())
UsersTable.selectAll()
.where {
(UsersTable.name.lowerCase() like "%${sanitized}%") or
(UsersTable.email.lowerCase() like "%${sanitized}%")
}
.orderBy(UsersTable.name)
.map { it.toUser() }
}
override suspend fun create(request: CreateUserRequest): User =
newSuspendedTransaction(db = database) {
UsersTable.insert {
it[name] = request.name
it[email] = request.email
it[role] = request.role
}.resultedValues!!.first().toUser()
}
override suspend fun update(id: UUID, request: UpdateUserRequest): User? =
newSuspendedTransaction(db = database) {
val updated = UsersTable.update({ UsersTable.id eq id }) {
request.name?.let { name -> it[UsersTable.name] = name }
request.email?.let { email -> it[UsersTable.email] = email }
it[updatedAt] = CurrentTimestampWithTimeZone
}
if (updated > 0) findById(id) else null
}
override suspend fun delete(id: UUID): Boolean =
newSuspendedTransaction(db = database) {
UsersTable.deleteWhere { UsersTable.id eq id } > 0
}
override suspend fun count(): Long =
newSuspendedTransaction(db = database) {
UsersTable.selectAll().count()
}
private fun ResultRow.toUser() = User(
id = this[UsersTable.id].value,
name = this[UsersTable.name],
email = this[UsersTable.email],
role = this[UsersTable.role],
metadata = this[UsersTable.metadata],
createdAt = this[UsersTable.createdAt],
updatedAt = this[UsersTable.updatedAt],
)
}JSON Columns
JSONB with kotlinx.serialization
// Custom column type for JSONB
inline fun <reified T : Any> Table.jsonb(
name: String,
json: Json,
): Column<T> = registerColumn(name, object : ColumnType<T>() {
override fun sqlType() = "JSONB"
override fun valueFromDB(value: Any): T = when (value) {
is String -> json.decodeFromString(value)
is PGobject -> {
val jsonString = value.value
?: throw IllegalArgumentException("PGobject value is null for column '$name'")
json.decodeFromString(jsonString)
}
else -> throw IllegalArgumentException("Unexpected value: $value")
}
override fun notNullValueToDB(value: T): Any =
PGobject().apply {
type = "jsonb"
this.value = json.encodeToString(value)
}
})
// Usage in table
@Serializable
data class UserMetadata(
val preferences: Map<String, String> = emptyMap(),
val tags: List<String> = emptyList(),
)
object UsersTable : UUIDTable("users") {
val metadata = jsonb<UserMetadata>("metadata", Json.Default).nullable()
}Testing with Exposed
In-Memory Database for Tests
class UserRepositoryTest : FunSpec({
lateinit var database: Database
lateinit var repository: UserRepository
beforeSpec {
database = Database.connect(
url = "jdbc:h2:mem:test;DB_CLOSE_DELAY=-1;MODE=PostgreSQL",
driver = "org.h2.Driver",
)
transaction(database) {
SchemaUtils.create(UsersTable)
}
repository = ExposedUserRepository(database)
}
beforeTest {
transaction(database) {
UsersTable.deleteAll()
}
}
test("create and find user") {
val user = repository.create(CreateUserRequest("Alice", "alice@example.com"))
user.name shouldBe "Alice"
user.email shouldBe "alice@example.com"
val found = repository.findById(user.id)
found shouldBe user
}
test("findByEmail returns null for unknown email") {
val result = repository.findByEmail("unknown@example.com")
result.shouldBeNull()
}
test("pagination works correctly") {
repeat(25) { i ->
repository.create(CreateUserRequest("User $i", "user$i@example.com"))
}
val page1 = repository.findAll(page = 1, limit = 10)
page1.data shouldHaveSize 10
page1.total shouldBe 25
page1.hasNext shouldBe true
val page3 = repository.findAll(page = 3, limit = 10)
page3.data shouldHaveSize 5
page3.hasNext shouldBe false
}
})Gradle Dependencies
// build.gradle.kts
dependencies {
// Exposed
implementation("org.jetbrains.exposed:exposed-core:1.0.0")
implementation("org.jetbrains.exposed:exposed-dao:1.0.0")
implementation("org.jetbrains.exposed:exposed-jdbc:1.0.0")
implementation("org.jetbrains.exposed:exposed-kotlin-datetime:1.0.0")
implementation("org.jetbrains.exposed:exposed-json:1.0.0")
// Database driver
implementation("org.postgresql:postgresql:42.7.5")
// Connection pooling
implementation("com.zaxxer:HikariCP:6.2.1")
// Migrations
implementation("org.flywaydb:flyway-core:10.22.0")
implementation("org.flywaydb:flyway-database-postgresql:10.22.0")
// Testing
testImplementation("com.h2database:h2:2.3.232")
}Quick Reference: Exposed Patterns
| Pattern | Description |
|---|---|
object Table : UUIDTable("name") | Define table with UUID primary key |
newSuspendedTransaction { } | Coroutine-safe transaction block |
Table.selectAll().where { } | Query with conditions |
Table.insertAndGetId { } | Insert and return generated ID |
Table.update({ condition }) { } | Update matching rows |
Table.deleteWhere { } | Delete matching rows |
Table.batchInsert(items) { } | Efficient bulk insert |
innerJoin / leftJoin | Join tables |
orderBy / limit / offset | Sort and paginate |
count() / sum() / avg() | Aggregation functions |
Remember: Use the DSL style for simple queries and the DAO style when you need entity lifecycle management. Always use newSuspendedTransaction for coroutine support, and wrap database operations behind a repository interface for testability.