In Android development, Room is a library that provides an abstraction layer over SQLite for database operations. One of the key features of Room is the ability to define relationships between entities (tables) in the database. In this article, we will discuss the three types of relationships that can be defined in Room: one-to-many, many-to-many, and one-to-one.
One-to-Many Relationship
A one-to-many relationship is a relationship between two entities where one entity has a single record that is related to multiple records in the other entity. For example, in a database of employees, each employee has only one department, but each department can have multiple employees.
To define a one-to-many relationship in Room, we can use the @ForeignKey annotation on the child entity and the @Relation annotation on the parent entity.
Here is an example of how to define a one-to-many relationship between an Employee and a Department entity:
@Entity
data class Employee(
@PrimaryKey val id: Int,
val name: String,
val departmentId: Int
)
@Entity
data class Department(
@PrimaryKey val id: Int,
val name: String
)
data class EmployeeWithDepartment(
@Embedded val employee: Employee,
@Relation(
parentColumn = "departmentId"…