Improve errors for unions (ExpediaGroup#1273)

Author: smyrick

generator/graphql-kotlin-schema-generator/src/main/kotlin/com/expediagroup/graphql/generator/exceptions/InvalidUnionException.kt

@@ -0,0 +1,19 @@
+/*
+ * Copyright 2021 Expedia, Inc
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.expediagroup.graphql.generator.exceptions
+
+class InvalidUnionException(type: String) : GraphQLKotlinException("The type $type was specified as a union but had no implementations. A Union type must define one or more member types.")

generator/graphql-kotlin-schema-generator/src/main/kotlin/com/expediagroup/graphql/generator/internal/types/generateUnion.kt

@@ -18,6 +18,7 @@ package com.expediagroup.graphql.generator.internal.types
 
 import com.expediagroup.graphql.generator.SchemaGenerator
 import com.expediagroup.graphql.generator.annotations.GraphQLUnion
+import com.expediagroup.graphql.generator.exceptions.InvalidUnionException
 import com.expediagroup.graphql.generator.extensions.unwrapType
 import com.expediagroup.graphql.generator.internal.extensions.getGraphQLDescription
 import com.expediagroup.graphql.generator.internal.extensions.getSimpleName
@@ -49,7 +50,7 @@ private fun generateUnionFromAnnotation(generator: SchemaGenerator, unionAnnotat
 
     val possibleTypes = unionAnnotation.possibleTypes.toList()
 
-    return createUnion(generator, builder, possibleTypes)
+    return createUnion(unionName, generator, builder, possibleTypes)
 }
 
 private fun generateUnionFromKClass(generator: SchemaGenerator, kClass: KClass<*>): GraphQLUnionType {
@@ -66,10 +67,14 @@ private fun generateUnionFromKClass(generator: SchemaGenerator, kClass: KClass<*
 
     val types = generator.classScanner.getSubTypesOf(kClass)
 
-    return createUnion(generator, builder, types)
+    return createUnion(name, generator, builder, types)
 }
 
-private fun createUnion(generator: SchemaGenerator, builder: GraphQLUnionType.Builder, types: List<KClass<*>>): GraphQLUnionType {
+private fun createUnion(typeName: String, generator: SchemaGenerator, builder: GraphQLUnionType.Builder, types: List<KClass<*>>): GraphQLUnionType {
+    if (types.isEmpty()) {
+        throw InvalidUnionException(typeName)
+    }
+
     types.map { generateGraphQLType(generator, it.createType()) }
         .forEach {
             when (val unwrappedType = it.unwrapType()) {

generator/graphql-kotlin-schema-generator/src/test/kotlin/com/expediagroup/graphql/generator/internal/types/GenerateUnionTest.kt

@@ -20,6 +20,7 @@ import com.expediagroup.graphql.generator.annotations.GraphQLDescription
 import com.expediagroup.graphql.generator.annotations.GraphQLName
 import com.expediagroup.graphql.generator.annotations.GraphQLUnion
 import com.expediagroup.graphql.generator.exceptions.InvalidGraphQLNameException
+import com.expediagroup.graphql.generator.exceptions.InvalidUnionException
 import com.expediagroup.graphql.generator.test.utils.SimpleDirective
 import graphql.schema.GraphQLObjectType
 import graphql.schema.GraphQLUnionType
@@ -84,7 +85,7 @@ class GenerateUnionTest : TypeTestHelper() {
 
     @Test
     fun `unused unions throw exception`() {
-        assertFailsWith(graphql.AssertException::class) {
+        assertFailsWith(InvalidUnionException::class) {
             generateUnion(generator, UnusedUnion::class)
         }
     }
@@ -133,7 +134,7 @@ class GenerateUnionTest : TypeTestHelper() {
     @Test
     fun `custom union annotation throws if possible types is empty`() {
         val annotation = AnnotationUnion::emptyUnion.annotations.first() as GraphQLUnion
-        assertFailsWith(graphql.AssertException::class) {
+        assertFailsWith(InvalidUnionException::class) {
             generateUnion(generator, Any::class, annotation)
         }
     }