Inferring Implicit Mappings

Mappie infers implicit mappings by name, type, default arguments, getter- and setter methods, and other mappers that are defined. An implicit mapping for a target property is inferred automatically if it has the same name as a source property, and it is assignable from that source property. If it is not assignable, Mappie will check if there is a single mapper defined that can map the source type to the target type, and will automatically apply it. Mappie comes with several mappers out of the box. See Built-in Mappers.

For example, suppose we have a data class Person and a data class PersonDto

data class Person(val name: String, val age: Int)

data class PersonDto(val name: String, val age: Int)

The properties of Person match the parameters of the primary constructor of PersonDto, and as such, no explicit mappings have to be defined. We can simply construct such a mapper by writing

object PersonMapper : ObjectMappie<Person, PersonDto>()

which will generate a mapper which calls the primary constructor of PersonDto assigned to the fields of Person.

Mapper Generation #

Mappie can also generate mappers automatically. When a source type and a target type do not have an existing mapper, and one can be written without any explicit mappings, it will be generated automatically.

For example, suppose we have the data classes Person and PersonDto containing Gender and GenderDto enum classes

data class Person(val name: String, val gender: Gender)
enum class Gender { MALE, FEMALE, OTHER; }

data class PersonDto(val name: String, val gender: GenderDto)
enum class GenderDto { MALE, FEMALE, OTHER; }

We can generate a mapper from Person to PersonDto by writing

class PersonMapper : ObjectMappie<Person, PersonDto>()

and the nested mapper from Gender to GenderDto will be generated automatically as they both contain the same enum entries.

Default Arguments #

Mappie also considers default arguments as a possibility.

For example, suppose PersonDto is defined as

data class PersonDto(
    val name: String, 
    val age: Int, 
    val hasChildren: Boolean = false,
)

Mappie will use the default argument false for hasChildren if no explicit mapping is defined. This is enabled by default and can be disabled by setting the configuration option useDefaultArguments to false.