allow querying fields on unions if the unions are contrained to implement the interface

changed TypeInfo behavior

Author: yaacovCR

src/utilities/TypeInfo.ts

@@ -27,6 +27,7 @@ import {
   isListType,
   isObjectType,
   isOutputType,
+  isUnionType,
 } from '../type/definition';
 import type { GraphQLDirective } from '../type/directives';
 import {
@@ -323,6 +324,14 @@ function getFieldDef(
   if (isObjectType(parentType) || isInterfaceType(parentType)) {
     return parentType.getFields()[name];
   }
+  if (isUnionType(parentType)) {
+    for (const iface of parentType.getInterfaces()) {
+      const field = iface.getFields()[name];
+      if (field) {
+        return field;
+      }
+    }
+  }
 }
 
 /**

src/validation/__tests__/FieldsOnCorrectTypeRule-test.ts

@@ -41,7 +41,9 @@ const testSchema = buildSchema(`
     meowVolume: Int
   }
 
-  union CatOrDog implements Pet = Cat | Dog
+  union CatOrDog = Cat | Dog
+
+  union ConstrainedCatOrDog implements Pet = Cat | Dog
 
   type Human {
     name: String
@@ -83,7 +85,7 @@ describe('Validate: Fields on correct type', () => {
 
   it('Union implementing interface field selection', () => {
     expectValid(`
-      fragment unionImplementingInterfaceFieldSelection on CatOrDog {
+      fragment unionImplementingInterfaceFieldSelection on ConstrainedCatOrDog {
         __typename
         name
       }
@@ -408,7 +410,7 @@ describe('Validate: Fields on correct type', () => {
         interface Feline implements Animal & Mammal { name: String }
         type Cat implements Animal & Mammal & Feline { name: String }
 
-        union CatOrDog implements Animal & Mammal = Cat | Dog
+        union CatOrDog = Cat | Dog
         type Query { catOrDog: CatOrDog }
       `);