| name | add-dto-mapper |
| description | Add a mapper that converts a remote DTO into a data-layer model. Use when asked to "map a dto", "add a mapper", "convert <X>Dto to a model", or whenever a data source's DTO needs turning into the data layer's own model type. |
Add a DTO → model mapper
The mapper is the boundary that decouples the data layer's models from the wire
format. Past the mapper, nothing knows about DTOs — so a JSON/field change on a
DTO ripples no further than its mapper. Scope this skill to one concept's mapper.
Where things live
| Thing | Module | Package | Notes |
|---|
| DTO (wire format) | :datasources:remote | ...remote.dto | @Serializable, mirrors the JSON |
| Data model | :data:core | ...data.model | plain data class, no serialization annotations |
| Mapper | :data:core | ...data.mappers | internal extension fun Dto.toData() |
:data:core depends on :datasources:remote, so the mapper can see the DTO; the DTO
never depends on the model (the dependency points one way, inward).
Steps
1. DTO (already exists, or add it)
In :datasources:remote, ...remote.dto — see the add-remote-datasource
skill:
@Serializable
data class CharacterDto(val id: Int, val name: String)
2. Data model
:data:core, ...data.model/Character.kt — the data layer's own type, free of any
wire concerns:
data class Character(val id: Int, val name: String)
3. Mapper
:data:core, ...data.mappers/CharacterMapper.kt — one extension function per
concept. Always name it toData() — every DTO → model mapper follows this
one convention, so the call site reads the same everywhere (dto.toData()):
internal fun CharacterDto.toData() = Character(
id = id,
name = name,
)
Keep mapping logic total and pure: provide defaults for nullable/absent DTO
fields here rather than leaking nullability into the model.
4. Use it at the result boundary
In the repository, feed the mapper into toDataResult (see add-repository):
remoteDatasource.getCharacter(id).toDataResult { it.toData() }
Verify
./gradlew :data:core:compileDebugSources --console=plain
(Set JAVA_HOME to a JDK 21 — e.g. an SDKMAN *-jbr — if the shell can't find
Java.)
Checklist
