Bugfix: Complete field is always using the first field def instead of…

packages/apollo-gateway/src/__tests__/__fixtures__/schemas/accounts.ts

@@ -8,12 +8,19 @@ export const typeDefs = gql`
     me: User
   }
 
+  type Department {
+    id: String
+    name: String
+  }
+
   type PasswordAccount @key(fields: "email") {
     email: String!
+    department: Department
   }
 
   type SMSAccount @key(fields: "number") {
     number: String
+    department: Department
   }
 
   union AccountType = PasswordAccount | SMSAccount

packages/apollo-gateway/src/__tests__/buildQueryPlan.test.ts

@@ -790,4 +790,55 @@ describe('buildQueryPlan', () => {
       }
     `);
   });
+
+  describe(`for union types`, () => {
+    it(`include same type in a union with different fields`, () => {
+      const query = gql`
+        {
+          user(id: "2") {
+            account {
+              ... on SMSAccount {
+                number
+                department {
+                  id
+                }
+              }
+              ... on PasswordAccount {
+                department {
+                  name
+                }
+                email
+              }
+            }
+          }
+        }
+      `;
+      const queryPlan = buildQueryPlan(buildOperationContext(schema, query));
+      expect(queryPlan).toMatchInlineSnapshot(`
+        QueryPlan {
+          Fetch(service: "accounts") {
+            {
+              user(id: "2") {
+                account {
+                  __typename
+                  ... on SMSAccount {
+                    number
+                    department {
+                      id
+                    }
+                  }
+                  ... on PasswordAccount {
+                    department {
+                      name
+                    }
+                    email
+                  }
+                }
+              }
+            }
+          },
+        }
+      `);
+    });
+  });
 });

packages/apollo-gateway/src/buildQueryPlan.ts

@@ -409,7 +409,7 @@ function splitFields(
             parentType,
             group,
             path,
-            fieldsForResponseName,
+            fieldsForParentType,
           ),
         );
       } else {
@@ -446,7 +446,7 @@ function splitFields(
                 runtimeParentType,
                 group,
                 path,
-                fieldsForResponseName,
+                fieldsForParentType,
               ),
             );
           }