Kotlin Exposed Patterns

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 = "[email protected]"
            }
            // 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", "[email protected]"))

        user.name shouldBe "Alice"
        user.email shouldBe "[email protected]"

        val found = repository.findById(user.id)
        found shouldBe user
    }

    test("findByEmail returns null for unknown email") {
        val result = repository.findByEmail("[email protected]")
        result.shouldBeNull()
    }

    test("pagination works correctly") {
        repeat(25) { i ->
            repository.create(CreateUserRequest("User $i", "[email protected]"))
        }

        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

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.