I’m trying to write a REST-API using Ktor and Koin.
I already wrote the authentication part, but when I wonted to test it I couldn’t
get Koin to replace the user repository implementation, my Ktor app is using.
I know Koin has a section on there web side for Koin in tests and I also know that there have been
similar questions on stack overflow (see my research section below), but I just
couldn’t make it work although, I tried several things already. Now I’m confused and don’t know what to do.

Here are some of the materials I visited doing my research:

• Inject a koin database into a ktor testapplication
• Setting up a test with Koin in ktor
• Test Ktor Application with Koin (not embedded)
• https://towardsdev.com/how-to-use-the-koin-framework-in-ktor-applications-c4dc81e7b6bf
• https://insert-koin.io/docs/quickstart/ktor/
• https://youtrack.jetbrains.com/issue/KTOR-5859/How-to-setup-Koin-in-Ktor-using-testApplication
• https://github.com/thomasneuteboom/ktor-koin-example
• https://towardsdev.com/how-to-use-the-koin-framework-in-ktor-applications-c4dc81e7b6bf
• https://ktor.io/docs/server-testing.html
• https://insert-koin.io/docs/reference/koin-test/testing/

Here is also how my last try looks like:

class LoginTest1 : AnnotationSpec(), KoinTest {
    val userPaul = User("Paul", "pwd")
    val users: MutableSet<User> = HashSet<User>().apply { add(userPaul) }
    val userRepo = UserRepositoryImpl(users) // this is just a test in memory implementation using a hash set

    // something else I tried, in order to load the alternative koin module
    // @BeforeTest
    // fun setup(){
    //     startKoin{ modules(module { single<UserRepository> { userRepo } }) }
    // }

    // yet another thing I tried
    // @JvmField
    // @RegisterExtension
    // val koinTestExtension = KoinTestExtension.create {
    //     modules( module { single<UserRepository> { userRepo } } )
    // }

    @Test
    fun `login correctly`() {
        testApplication {
            application {
                koin { // instead of this koin context I also tried to use startKoin {...}
                    loadKoinModules( // I also tried modules instead of loadKoinModules
                        module { single<UserRepository> { userRepo } }
                    )
                }
            }

            /* btw. I can use inject<UserRepository> here inorder to access the alternative implementation declared above,
                but it is not injected into my application. */

            val client = createClient()

            users.forEach { user ->
                val response = client.sendLoginRequest(user.name, user.password)

                /* This actually works perfectly fine when I just hard code the user into my actual
                    repository instead of trying to replace the injection */
                response.status shouldBe HttpStatusCode.OK
                val loginResponse = Json.decodeFromString<LoginResponse>(response.bodyAsText())
                with(loginResponse) {
                    shouldNotBeNull()
                    token.shouldNotBeEmpty()
                }
            }
        }
    }
}

suspend fun HttpClient.sendLoginRequest(username: String, password: String): HttpResponse = post("/api/auth") {
    contentType(ContentType.Application.Json)
    setBody(LoginRequest(username, password))
}

fun ApplicationTestBuilder.createClient(): HttpClient {
    return createClient {
        install(ClientContentNegotiation) { json() }
    }
}