- update to add pre-coercion of OneOf errors, following this pending PR to graphql-js: graphql/graphql-js#4195

Author: jasonbahl

src/Utils/Value.php

@@ -88,7 +88,7 @@ public static function coerceInputValue($value, InputType $type, ?array $path =
                     $coercedItem = self::coerceInputValue(
                         $itemValue,
                         $itemType,
-                        [...$path ?? [], $index]
+                        $path === null ? [$index] : [...$path, $index]
                     );
 
                     if (isset($coercedItem['errors'])) {
@@ -132,7 +132,7 @@ public static function coerceInputValue($value, InputType $type, ?array $path =
                 $coercedField = self::coerceInputValue(
                     $fieldValue,
                     $field->getType(),
-                    [...$path ?? [], $fieldName],
+                    $path === null ? [$fieldName] : [...$path, $fieldName],
                 );
 
                 if (isset($coercedField['errors'])) {
@@ -171,6 +171,38 @@ public static function coerceInputValue($value, InputType $type, ?array $path =
             );
         }
 
+        // Validate OneOf constraints if this is a OneOf input type
+        if ($type->isOneOf()) {
+            $providedFieldCount = 0;
+            $nullFieldName = null;
+            
+            foreach ($coercedValue as $fieldName => $fieldValue) {
+                if ($fieldValue !== null) {
+                    $providedFieldCount++;
+                } else {
+                    $nullFieldName = $fieldName;
+                }
+            }
+            
+            // Check for null field values first (takes precedence)
+            if ($nullFieldName !== null) {
+                $errors = self::add(
+                    $errors,
+                    CoercionError::make("OneOf input object \"{$type->name}\" field \"{$nullFieldName}\" must be non-null.", $path, $value)
+                );
+            } elseif ($providedFieldCount === 0) {
+                $errors = self::add(
+                    $errors,
+                    CoercionError::make("OneOf input object \"{$type->name}\" must specify exactly one field.", $path, $value)
+                );
+            } elseif ($providedFieldCount > 1) {
+                $errors = self::add(
+                    $errors,
+                    CoercionError::make("OneOf input object \"{$type->name}\" must specify exactly one field.", $path, $value)
+                );
+            }
+        }
+
         return $errors === []
             ? self::ofValue($type->parseValue($coercedValue))
             : self::ofErrors($errors);

tests/Type/OneOfInputObjectTest.php

@@ -8,6 +8,7 @@
 use GraphQL\Type\Definition\ObjectType;
 use GraphQL\Type\Definition\Type;
 use GraphQL\Type\Schema;
+use GraphQL\Utils\Value;
 use PHPUnit\Framework\TestCase;
 
 class OneOfInputObjectTest extends TestCase
@@ -198,4 +199,39 @@ public function testOneOfIntrospection(): void
         self::assertTrue($oneOfType['isOneOf']);
         self::assertFalse($regularType['isOneOf']); // Should be false for regular input objects
     }
+
+    public function testOneOfCoercionValidation(): void
+    {
+        $oneOfType = new InputObjectType([
+            'name' => 'OneOfInput',
+            'fields' => [
+                'stringField' => Type::string(),
+                'intField' => Type::int(),
+            ],
+            'isOneOf' => true,
+        ]);
+
+        // Test valid input (exactly one field)
+        $validResult = Value::coerceInputValue(['stringField' => 'test'], $oneOfType);
+        $this->assertNull($validResult['errors']);
+        $this->assertEquals(['stringField' => 'test'], $validResult['value']);
+
+        // Test invalid input (no fields)
+        $noFieldsResult = Value::coerceInputValue([], $oneOfType);
+        $this->assertNotNull($noFieldsResult['errors']);
+        $this->assertCount(1, $noFieldsResult['errors']);
+        $this->assertEquals('OneOf input object "OneOfInput" must specify exactly one field.', $noFieldsResult['errors'][0]->getMessage());
+
+        // Test invalid input (multiple fields)
+        $multipleFieldsResult = Value::coerceInputValue(['stringField' => 'test', 'intField' => 42], $oneOfType);
+        $this->assertNotNull($multipleFieldsResult['errors']);
+        $this->assertCount(1, $multipleFieldsResult['errors']);
+        $this->assertEquals('OneOf input object "OneOfInput" must specify exactly one field.', $multipleFieldsResult['errors'][0]->getMessage());
+
+        // Test invalid input (null field value)
+        $nullFieldResult = Value::coerceInputValue(['stringField' => null], $oneOfType);
+        $this->assertNotNull($nullFieldResult['errors']);
+        $this->assertCount(1, $nullFieldResult['errors']);
+        $this->assertEquals('OneOf input object "OneOfInput" field "stringField" must be non-null.', $nullFieldResult['errors'][0]->getMessage());
+    }
 }