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Association API

Overview

The Association API makes it easy to navigate association objects from the EntityStore obtained by executing a query specifying include or includeAll.

Annotating an entity class (or entity definition class) with a dedicated annotation generates a utility function for navigating association objects.

Annotations

@KomapperOneToOne

Indicates a one-to-one relationship between the annotated entity class and the target entity class.

@KomapperEntity
@KomapperOneToOne(targetEntity = Address::class)
data class Employee(...)

The targetEntity property specifies the entity class (or entity definition class) associated with the annotated class. The specification is required.

The navigator property specifies the name of a utility function for navigating the association. If not specified, it is inferred from the value specified for the targetEntity property.

The following utility function is generated from the above definition:

fun example.Employee.`address`(
    store: org.komapper.core.dsl.query.EntityStore,
    source: example._Employee = org.komapper.core.dsl.Meta.`employee`,
    target: example._Address = org.komapper.core.dsl.Meta.`address`,
): example.Address? {
    return store.oneToOne(source, target)[this]
}

@KomapperManyToOne

Indicates a many-to-one relationship between the annotated entity class and the target entity class.

@KomapperEntity
@KomapperManyToOne(targetEntity = Department::class)
data class Employee(...)

The targetEntity property specifies the entity class (or entity definition class) associated with the annotated class. The specification is required.

The navigator property specifies the name of a utility function for navigating the association. If not specified, it is inferred from the value specified for the targetEntity property.

The following utility function is generated from the above definition:

fun example.Employee.`department`(
    store: org.komapper.core.dsl.query.EntityStore,
    source: example._Employee = org.komapper.core.dsl.Meta.`employee`,
    target: example._Department = org.komapper.core.dsl.Meta.`department`,
): example.Department? {
    return store.manyToOne(source, target)[this]
}

@KomapperOneToMany

Indicates a one-to-many relationship between the annotated entity class and the target entity class.

@KomapperEntity
@KomapperOneToMany(targetEntity = Employee::class, navigator = "employees")
data class Department(...)

The targetEntity property specifies the entity class (or entity definition class) associated with the annotated class. The specification is required.

The navigator property specifies the name of a utility function for navigating the association. If not specified, it is inferred from the value specified for the targetEntity property.

The following utility function is generated from the above definition:

fun example.Department.`employees`(
    store: org.komapper.core.dsl.query.EntityStore,
    source: example._Department = org.komapper.core.dsl.Meta.`department`,
    target: example._Employee = org.komapper.core.dsl.Meta.`employee`,
): Set<example.Employee> {
    return store.oneToMany(source, target)[this] ?: emptySet()
}

Indicates the associated source entity metamodel and the associated target entity metamodel. It can be specified in the link property of the following annotations:

  • @KomapperOneToOne
  • @KomapperManyToOne
  • @KomapperOneToMany

@KomapperAggregateRoot

Indicates that the annotated entity class (or entity definition class) is the aggregate root.

@KomapperEntity
@KomapperAggregateRoot(navigator = "departments")
data class Department(...)

The navigator property specifies the name of a utility function to retrieve the aggregate root. If not specified, it is inferred from the annotated class.

The above definition generates the following utility function:

fun org.komapper.core.dsl.query.EntityStore.`departments`(
    target: example._Department = org.komapper.core.dsl.Meta.`department`,
): Set<example.Department> {
    return this[target]
}

Example

Entity class definitions

Suppose we have the following entity class definitions:

@KomapperEntity
@KomapperOneToOne(targetEntity = Employee::class)
data class Address(
    @KomapperId
    val addressId: Int,
    val street: String,
)

@KomapperEntity
@KomapperAggregateRoot("departments")
@KomapperOneToMany(targetEntity = Employee::class, navigator = "employees")
data class Department(
    @KomapperId
    val departmentId: Int,
    val departmentName: String,
)

@KomapperEntity
@KomapperManyToOne(targetEntity = Department::class)
@KomapperOneToOne(targetEntity = Address::class)
data class Employee(
    @KomapperId
    val employeeId: Int,
    val employeeName: String,
    val departmentId: Int,
    val addressId: Int,
)

Navigation code

We can concisely retrieve the association objects from the EntityStore by using the generated utility functions:

val a = Meta.address
val e = Meta.employee
val d = Meta.department

val query = QueryDsl.from(a)
    .innerJoin(e) {
        a.addressId eq e.addressId
    }.innerJoin(d) {
        e.departmentId eq d.departmentId
    }.includeAll()

val store: EntityStore = db.runQuery(query)

for (department in store.departments()) {
    val employees = department.employees(store)
    for (employee in employees) {
        val address = employee.address(store)
        println("department=${department.departmentName}, employee=${employee.employeeName}, address=${address?.street}")
    }
}

Navigation code using context receiver

Enabling the komapper.enableEntityStoreContext option allows more concise navigation of association objects by eliminating the need to pass EntityStore arguments explicitly.

val a = Meta.address
val e = Meta.employee
val d = Meta.department

val query = QueryDsl.from(a)
    .innerJoin(e) {
        a.addressId eq e.addressId
    }.innerJoin(d) {
        e.departmentId eq d.departmentId
    }.includeAll()

val store: EntityStore = db.runQuery(query)

with(store.asContext()) {
    for (department in departments()) {
        val employees = department.employees()
        for (employee in employees) {
            val address = employee.address()
            println("department=${department.departmentName}, employee=${employee.employeeName}, address=${address?.street}")
        }
    }
}
Last modified February 23, 2023: Simplify sample code (99184db)