Allow nested properties as sources

[syh7]

I would like support for nested properties as sources. For example

data class Computer (
    val name : String,
    val info : Info,
)
data class Info (
    val ean: String,
    val buyDate: Instant,
)

data class ComputerDto(
    val name: String,
    val ean: String
)

@Konverter
interface ComputerMapper{
    @Konvert(mappings = [
        Mapping(target = "ean", source = "info.ean")
    ])
    fun toDto(source: Computer): ComputerDto
}

The trick is in Mapping(target = "ean", source = "info.ean"). I've often wanted to get a specific property from one of the subproperties, and right now I have a workaround of creating an extra function, like:

@Konverter
interface ComputerMapper {

    @Konvert(
        mappings = [
            Mapping(target = "ean", source = "info"),
        ],
    )
    fun toDto(source: Computer): ComputerDto
    
    fun mapEan(source: Info)=source.ean
}

[mcarleio]

I agree, that this could be a nice functionality, but I am not sure how easy this is to implement and how to handle nullables on the way... Will think about this, but add this to the backlog for now 🤔

FYI: You can also use @Konverter.Source and define further parameters in the function. This can then look like the following:

@Konvert
fun toDto(@Konverter.Source source: Computer, ean: String = source.info.ean): ComputerDto

// usage:
ComputerMapperImpl.toDto(computer)

This has the downside, that someone could pass a wrong EAN to the mapping (e.g. ComputerMapperImpl.toDto(computer, "fake"))

Another alternative is of course to use an expression:

@Konvert(mappings = [
    Mapping(target = "ean", expression = "it.info.ean")
])
fun toDto(source: Computer): ComputerDto

But expressions should be used with careful consideration. Konvert will not add any further mapping logic and just assumes the expressions are correct and leave the validation to the compiler.