# Kotlin Exposed Patterns

> JetBrains Exposed ORM patterns including DSL queries, DAO pattern, transactions, HikariCP connection pooling, Flyway migrations, and repository pattern.

- Skill: `lidge-jun/kotlin-exposed-patterns` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add lidge-jun/kotlin-exposed-patterns`
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- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: lidge-jun (https://skillmd.com/u/lidge-jun)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/lidge-jun/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

```kotlin
suspend fun findUserById(id: UUID): UserRow? =
    newSuspendedTransaction {
        UsersTable.selectAll()
            .where { UsersTable.id eq id }
            .map { it.toUser() }
            .singleOrNull()
    }
```

### DAO Entity Usage

```kotlin
suspend fun createUser(request: CreateUserRequest): User =
    newSuspendedTransaction {
        UserEntity.new {
            name = request.name
            email = request.email
            role = request.role
        }.toModel()
    }
```

### HikariCP Configuration

```kotlin
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

```kotlin
// 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

```kotlin
// 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

```sql
-- 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

```kotlin
// 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

```kotlin
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

```kotlin
// 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],
)
```

### Pagination

See `references/code-examples.md` for full pagination implementation.

#### Quick Pagination Setup

```kotlin
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
}
```

### Batch Operations

```kotlin
// 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

```kotlin
// 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

See `references/code-examples.md` for full DAO repository examples.

Basic DAO find and create:

```kotlin
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()
    }
```

## Transactions

### Suspend Transaction Support

All database operations must use `newSuspendedTransaction` for coroutine support. This ensures operations are atomic and safe to call from suspending contexts:

```kotlin
suspend fun performDatabaseOperation(): Result<User> =
    runCatching {
        newSuspendedTransaction {
            val user = UserEntity.new {
                name = "Alice"
                email = "alice@example.com"
            }
            user.toModel()
        }
    }
```

Nested transactions with savepoints are also supported. See `references/code-examples.md` for isolation level examples.

## Repository Pattern

### Interface Definition

```kotlin
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

See `references/code-examples.md` for the complete `ExposedUserRepository` implementation.

## JSON Columns

See `references/code-examples.md` for full JSONB custom column type implementation.

Quick usage with PostgreSQL JSONB:

```kotlin
@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

See `references/code-examples.md` for a full in-memory H2 test suite example using KoTest.

Quick test setup:

```kotlin
database = Database.connect(
    url = "jdbc:h2:mem:test;DB_CLOSE_DELAY=-1;MODE=PostgreSQL",
    driver = "org.h2.Driver",
)
transaction(database) {
    SchemaUtils.create(UsersTable)
}
```

## Gradle Dependencies

```kotlin
// 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. For advanced queries, joins, aggregation, and testing patterns, see `references/code-examples.md`.