Swagger shows Mongo ObjectId as complex JSON instead of String - spring

Question

Project setup
I have a Kotlin Spring Boot 2.0 project that exposes a #RestController API that returns MongoDB models. For example, this model and controller:
#RestController
#RequestMapping("/api/accounts")
class AccountsController() {
#GetMapping
fun list(): List<Account> {
return listOf(Account(ObjectId(), "Account 1"), Account(ObjectId(), "Account 2"), Account(ObjectId(), "Account 3"))
}
}
#Document
data class Account(
#Id val id: ObjectId? = null,
val name: String
)
These models have ObjectId identifiers, but in the API I want them to be treated as plain String (i.e. instead of a complex JSON, the default behaviour).
To achieve this, I created these components to configure Spring Boot parameter binding and JSON parsing:
#JsonComponent
class ObjectIdJsonSerializer : JsonSerializer<ObjectId>() {
override fun serialize(value: ObjectId?, gen: JsonGenerator?, serializers: SerializerProvider?) {
if (value == null || gen == null) return
gen.writeString(value.toHexString())
}
}
#JsonComponent
class ObjectIdJsonDeserializer : JsonDeserializer<ObjectId>() {
override fun deserialize(p: JsonParser?, ctxt: DeserializationContext?): ObjectId? {
if (p == null) return null
val text = p.getCodec().readTree<TextNode>(p).textValue()
return ObjectId(text)
}
}
#Component
class StringToObjectIdConverter : Converter<String, ObjectId> {
override fun convert(source: String): ObjectId? {
return ObjectId(source)
}
}
So far this works as intended, calls to the API return this JSON:
[
{
"id": "5da454f4307b0a8b30838839",
"name": "Account 1"
},
{
"id": "5da454f4307b0a8b3083883a",
"name": "Account 2"
},
{
"id": "5da454f4307b0a8b3083883b",
"name": "Account 3"
}
]
Issue
The problem comes when integrating Swagger into the project, the documentation shows that calling this method returns a complex JSON instead of a plain String as the id property:
Adding #ApiModelProperty(dataType = "string") to the id field made no difference, and I can't find a way to solve it without changing all the id fields in the project to String. Any help would be appreciated.

Answer 1

I couldn't get #ApiModelProperty(dataType = "") to work, but I found a more convenient way configuring a direct substitute in the Swagger configuration using directModelSubstitute method of the Docket instance in this response.
#Configuration
#EnableSwagger2
class SwaggerConfig() {
#Bean
fun api(): Docket {
return Docket(DocumentationType.SWAGGER_2)
.directModelSubstitute(ObjectId::class.java, String::class.java)
}
}
Java equivalent:
#Bean
public Docket api() {
return new Docket(DocumentationType.SWAGGER_2)
.directModelSubstitute(ObjectId.class, String.class);
}

Answer 2

For OpenApi (Swagger 3.0) and SpringDoc the following global configuration could be used.
static {
SpringDocUtils.getConfig().replaceWithSchema(ObjectId.class, new StringSchema());
}