Omit discriminator property for Kotlin serialization (#342)

Generating the discriminator property causes a name conflict with Kotlin Serialization's class discriminator.

Example: `Sealed class 'subclass' cannot be serialized as base class 'com.example.models.Superclass' because it has property name that conflicts with JSON class discriminator 'type'.`

Author: ulrikandersen

end2end-tests/models-kotlinx/openapi/api.yaml

@@ -71,18 +71,24 @@ components:
     LandlinePhone:
       type: object
       required:
+        - type
         - number
         - area_code
       properties:
+        type:
+          type: string
         number:
           type: string
         area_code:
           type: string
     MobilePhone:
       type: object
       required:
+        - type
         - number
       properties:
+        type:
+          type: string
         number:
           type: string
     Error:

src/main/kotlin/com/cjbooms/fabrikt/generators/GeneratorUtils.kt

@@ -219,4 +219,33 @@ object GeneratorUtils {
     }
 
     private fun isNullable(parameter: Parameter): Boolean = !parameter.isRequired && parameter.schema.default == null
+
+    /**
+     * Converts a TypeSpec to an object by copying over all properties, functions, etc.
+     */
+    fun TypeSpec.toObjectTypeSpec(): TypeSpec {
+        require(name != null) { "Name must be set to convert to object" }
+
+        val objectBuilder = TypeSpec.objectBuilder(name!!)
+            .addAnnotations(annotations)
+            .addModifiers(modifiers)
+            .superclass(superclass)
+            .addProperties(propertySpecs)
+            .addFunctions(funSpecs)
+            .addKdoc(kdoc)
+
+        for ((typeName, _) in superinterfaces) {
+            objectBuilder.addSuperinterface(typeName)
+        }
+
+        if (initializerBlock.isNotEmpty()) {
+            objectBuilder.addInitializerBlock(initializerBlock)
+        }
+
+        for (nestedType in typeSpecs) {
+            objectBuilder.addType(nestedType)
+        }
+
+        return objectBuilder.build()
+    }
 }

src/main/kotlin/com/cjbooms/fabrikt/generators/PropertyUtils.kt

@@ -127,18 +127,22 @@ object PropertyUtils {
             if (isDiscriminatorFieldWithSingleKnownValue(classSettings, schemaName)) {
                 this as PropertyInfo.Field
                 if (classSettings.polymorphyType in listOf(ClassSettings.PolymorphyType.SUB, ClassSettings.PolymorphyType.ONE_OF)) {
-                    property.initializer(name)
-                    serializationAnnotations.addParameter(property, oasKey)
-                    val constructorParameter: ParameterSpec.Builder = ParameterSpec.builder(name, wrappedType)
-                    val discriminators = maybeDiscriminator.getDiscriminatorMappings(schemaName)
-                    when (val discriminator = discriminators.first()) {
-                        is PropertyInfo.DiscriminatorKey.EnumKey ->
-                            constructorParameter.defaultValue("%T.%L", wrappedType, discriminator.enumKey)
-
-                        is PropertyInfo.DiscriminatorKey.StringKey ->
-                            constructorParameter.defaultValue("%S", discriminator.stringValue)
+                    if (!serializationAnnotations.supportsBackingPropertyForDiscriminator) {
+                        return // Skip adding the property to the class
+                    } else {
+                        property.initializer(name)
+                        serializationAnnotations.addParameter(property, oasKey)
+                        val constructorParameter: ParameterSpec.Builder = ParameterSpec.builder(name, wrappedType)
+                        val discriminators = maybeDiscriminator.getDiscriminatorMappings(schemaName)
+                        when (val discriminator = discriminators.first()) {
+                            is PropertyInfo.DiscriminatorKey.EnumKey ->
+                                constructorParameter.defaultValue("%T.%L", wrappedType, discriminator.enumKey)
+
+                            is PropertyInfo.DiscriminatorKey.StringKey ->
+                                constructorParameter.defaultValue("%S", discriminator.stringValue)
+                        }
+                        constructorBuilder.addParameter(constructorParameter.build())
                     }
-                    constructorBuilder.addParameter(constructorParameter.build())
                 }
             } else {
                 property.initializer(name)

src/main/kotlin/com/cjbooms/fabrikt/generators/model/ModelGenerator.kt

@@ -6,6 +6,7 @@ import com.cjbooms.fabrikt.cli.ModelCodeGenOptionType.SEALED_INTERFACES_FOR_ONE_
 import com.cjbooms.fabrikt.configurations.Packages
 import com.cjbooms.fabrikt.generators.ClassSettings
 import com.cjbooms.fabrikt.generators.GeneratorUtils.toClassName
+import com.cjbooms.fabrikt.generators.GeneratorUtils.toObjectTypeSpec
 import com.cjbooms.fabrikt.generators.MutableSettings
 import com.cjbooms.fabrikt.generators.PropertyUtils.addToClass
 import com.cjbooms.fabrikt.generators.PropertyUtils.isNullable
@@ -498,7 +499,14 @@ class ModelGenerator(
 
         serializationAnnotations.addClassAnnotation(classBuilder)
 
-        return classBuilder.build()
+        val classTypeSpec = classBuilder.build()
+
+        return if (classBuilder.propertySpecs.isNotEmpty()) {
+            classTypeSpec
+        } else {
+            // properties have been filtered out in generation process so return an object instead
+            classTypeSpec.toObjectTypeSpec()
+        }
     }
 
     private fun polymorphicSuperSubType(

src/main/kotlin/com/cjbooms/fabrikt/model/JacksonAnnotations.kt

@@ -10,6 +10,7 @@ import com.squareup.kotlinpoet.TypeName
 import com.squareup.kotlinpoet.TypeSpec
 
 object JacksonAnnotations : SerializationAnnotations {
+    override val supportsBackingPropertyForDiscriminator = true
     override val supportsAdditionalProperties = true
     override fun addIgnore(propertySpecBuilder: PropertySpec.Builder) =
         propertySpecBuilder.addAnnotation(JacksonMetadata.ignore)

src/main/kotlin/com/cjbooms/fabrikt/model/KotlinxSerializationAnnotations.kt

@@ -9,6 +9,13 @@ import kotlinx.serialization.SerialName
 import kotlinx.serialization.Serializable
 
 object KotlinxSerializationAnnotations : SerializationAnnotations {
+    /**
+     * Polymorphic class discriminators are added as annotations in kotlinx serialization.
+     * Including them in the class definition causes compilation errors since the property name
+     * will conflict with the class discriminator name.
+     */
+    override val supportsBackingPropertyForDiscriminator = false
+
     /**
      * Supporting "additionalProperties: true" for kotlinx serialization requires additional
      * research and work due to Any type in the map (val properties: MutableMap<String, Any?>)
@@ -18,6 +25,7 @@ object KotlinxSerializationAnnotations : SerializationAnnotations {
      * See also https://github.com/Kotlin/kotlinx.serialization/issues/1978
      */
     override val supportsAdditionalProperties = false
+
     override fun addIgnore(propertySpecBuilder: PropertySpec.Builder) =
         propertySpecBuilder // not applicable
 

src/main/kotlin/com/cjbooms/fabrikt/model/SerializationAnnotations.kt

@@ -6,6 +6,11 @@ import com.squareup.kotlinpoet.TypeName
 import com.squareup.kotlinpoet.TypeSpec
 
 sealed interface SerializationAnnotations {
+    /**
+     * Whether to include backing property for a polymorphic discriminator
+     */
+    val supportsBackingPropertyForDiscriminator: Boolean
+
     /**
      * Whether the annotation supports OpenAPI's additional properties
      * https://spec.openapis.org/oas/v3.0.0.html#model-with-map-dictionary-properties

src/test/resources/examples/discriminatedOneOf/models/kotlinx/StateA.kt

@@ -1,13 +1,8 @@
 package examples.discriminatedOneOf.models
 
-import javax.validation.constraints.NotNull
 import kotlinx.serialization.SerialName
 import kotlinx.serialization.Serializable
 
 @SerialName("a")
 @Serializable
-public data class StateA(
-  @SerialName("status")
-  @get:NotNull
-  public val status: Status = Status.A,
-) : State
+public object StateA : State

src/test/resources/examples/discriminatedOneOf/models/kotlinx/StateB.kt

@@ -10,7 +10,4 @@ public data class StateB(
   @SerialName("mode")
   @get:NotNull
   public val mode: StateBMode,
-  @SerialName("status")
-  @get:NotNull
-  public val status: Status = Status.B,
 ) : State