Hey when generating an arbitrary domain model instance how can one lock some properties? Suppose I h...

# kotest

[André Martins]

Hey when generating an arbitrary domain model instance how can one lock some properties?
Suppose I have a Student data class with an id, number, name and some other fields.
If I have the following generator

fun Arb.Companion.student(): Arb<Student> = arbitrary { rs ->
    Student(
        id = Arb.uuid(V4,false).next(rs)
        name = Arb.string().next(rs),
        number = <http://Arb.int|Arb.int>().next(rs), ...
    )
}

What can I add here to lock the number I which my generated student have?

Arb.student().<here>?
// Something like
Arb.student().with(number, 123)

Anyway I can do this?

[Emil Kantis]

Arb.student().copy(number = 123) perhaps?

[André Martins]

I need to call next first, but that way I’m generating a random of everything and then locking it

[André Martins]

I’ve seen this in other frameworks wasn’t sure if for kotlin thats the way to go

[phldavies]

I’ve been taking the approach similar to this:

fun Arb.Companion.student(
    idArb: Arb<UUID> = uuid(V4, false),
    nameArb: Arb<String> = string(),
    numberArb: Arb<Int> = int()
): Arb<Student> = arbitrary {
    Student(
        id = idArb.bind(),
        name = nameArb.bind(),
        number = numberArb.bind(),
    )
}

val myStudent = Arb.student(numberArb = Arb.constant(123)).single()

[phldavies]

which also allows for more control over fields than just locking in values:

val namedStudent = Arb.student(nameArb = Arb.name().map { (first, last) -> "$last, $first" }).single()

[André Martins]

ya seems nice!

[phldavies]

Also, personal preference maybe, but I’ve been adding these arbitraries onto my domain model companions rather than onto Arb, i.e.

fun Student.Companion.arbitrary(...) = ...

allowing for

Student.arbitrary()

rather than

Arb.student()

It’s just kept Arb a little cleaner as base arbitraries

[André Martins]

thx for the suggestions 😄

[Emil Kantis]

In case the type is unique, you could also use Arb.data<Student>(mapOf(Int::class to Arb.constant(123)) to generate the entire thing.. 🙂 Arb.data<Student>() will create your first exmaple there, though I’m unsure if uuid binding will work without adding it explicitly.. Phil’s approach gives nice flexibility from use sites though, gonna try that out myself 🙂

[André Martins]

Ya I’ve seen that with bind too

[LeoColman]

I think you could also map an arb

[LeoColman]

Arb.student().map { it.copy(number = 123) }