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To establish a one-to-one relationship through a linking table using Kotlin and exposed, you can follow these steps:

1. Define the entities: Create two entities, let's say User and Profile, which will be linked through a linking table (user_profile). Define their properties as per your requirement.

data class User(
    val id: Int,
    val name: String,
    ...
)

data class Profile(
    val id: Int,
    val bio: String,
    ...
)

1. Define the linking table: Create a table in your database schema to link the User and Profile entities together. You can define this table using Exposed's Table class and add two foreign key columns, one for User and another for Profile.

object UserProfiles: Table() {
    val userId = integer("userId").references(Users.id)
    val profileId = integer("profileId").references(Profiles.id)
}

1. Define the DAOs: Create two DAOs, one for User and another for Profile, to interact with the database. In these DAOs, define the necessary functions to insert, update, delete, and retrieve data from the database.

class UserDao {
    fun getUserById(id: Int): User = ...
    fun saveUser(user: User): User = ...
}

class ProfileDao {
    fun getProfileById(id: Int): Profile = ...
    fun saveProfile(profile: Profile): Profile = ...
}

1. Define the mapping: Use Exposed's join() function to define a mapping between the User and Profile entities using the linking table. This will allow you to easily retrieve the linked entity from either side of the relationship.

val userToProfile = UserProfiles.join(Users, JoinType.INNER, UserProfiles.userId, Users.id).join(
    Profiles,
    JoinType.INNER,
    UserProfiles.profileId,
    Profiles.id
)

val profileToUser = UserProfiles.join(Profiles, JoinType.INNER, UserProfiles.profileId, Profiles.id).join(
    Users,
    JoinType.INNER,
    UserProfiles.userId,
    Users.id
)

1. Use the mapping: You can now use the defined mapping to retrieve the linked entity from either side of the relationship. For example, to retrieve the profile of a user, you can use the following code snippet:

val user = userDao.getUserById(userId)
val profile = userToProfile.select { Users.id eq userId }.map { it[Profiles] }.single()

This will retrieve the user from the database, then use the userToProfile mapping to retrieve the profile of that user. Similarly, you can retrieve the user of a profile using the profileToUser mapping.