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

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

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()
}

// 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,
)

// 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)
    // ...
}

-- 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);

// 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")
}

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)
}

// 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],
)

// 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() }
    }

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 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
        }
    }
}

// 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
}

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()
    }

// 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
    }
}

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
    }
}

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
}

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],
    )
}