fix(visitor-plugin-common): avoid reading from null values (#9709)

* fix(visitor-plugin-common): avoid reading from null values

* Add e2e test

.changeset/perfect-forks-flash.md

@@ -0,0 +1,5 @@
+---
+'@graphql-codegen/visitor-plugin-common': patch
+---
+
+Avoid reading from null values when selection sets only contain fragments.

dev-test/codegen.ts

@@ -221,6 +221,17 @@ const config: CodegenConfig = {
       preset: 'client',
       presetConfig: { fragmentMasking: true },
     },
+    './dev-test/test-null-value/result.d.ts': {
+      schema: './dev-test/test-null-value/schema.graphql',
+      documents: ['./dev-test/test-null-value/query.ts'],
+      plugins: ['typescript', 'typescript-operations'],
+      config: {
+        // The combination of these two flags caused the following issue:
+        // https://github.com/dotansimha/graphql-code-generator/pull/9709
+        skipTypename: true,
+        mergeFragmentTypes: true,
+      },
+    },
   },
 };
 

dev-test/test-null-value/query.ts

@@ -0,0 +1,16 @@
+export const MY_QUERY = /* GraphQL */ `
+  fragment CartLine on CartLine {
+    id
+    quantity
+  }
+
+  query Test {
+    cart {
+      lines {
+        nodes {
+          ...CartLine
+        }
+      }
+    }
+  }
+`;

dev-test/test-null-value/result.d.ts

@@ -0,0 +1,50 @@
+export type Maybe<T> = T | null;
+export type InputMaybe<T> = Maybe<T>;
+export type Exact<T extends { [key: string]: unknown }> = { [K in keyof T]: T[K] };
+export type MakeOptional<T, K extends keyof T> = Omit<T, K> & { [SubKey in K]?: Maybe<T[SubKey]> };
+export type MakeMaybe<T, K extends keyof T> = Omit<T, K> & { [SubKey in K]: Maybe<T[SubKey]> };
+export type MakeEmpty<T extends { [key: string]: unknown }, K extends keyof T> = { [_ in K]?: never };
+export type Incremental<T> = T | { [P in keyof T]?: P extends ' $fragmentName' | '__typename' ? T[P] : never };
+/** All built-in and custom scalars, mapped to their actual values */
+export type Scalars = {
+  ID: { input: string; output: string };
+  String: { input: string; output: string };
+  Boolean: { input: boolean; output: boolean };
+  Int: { input: number; output: number };
+  Float: { input: number; output: number };
+};
+
+export type BaseCartLine = {
+  id: Scalars['String']['output'];
+  quantity: Scalars['Int']['output'];
+};
+
+export type BaseCartLineConnection = {
+  id: Scalars['String']['output'];
+  nodes: Array<BaseCartLine>;
+};
+
+export type Cart = {
+  id: Scalars['String']['output'];
+  lines: BaseCartLineConnection;
+};
+
+export type CartLine = BaseCartLine & {
+  id: Scalars['String']['output'];
+  quantity: Scalars['Int']['output'];
+};
+
+export type ComponentizableCartLine = BaseCartLine & {
+  id: Scalars['String']['output'];
+  quantity: Scalars['Int']['output'];
+};
+
+export type QueryRoot = {
+  cart?: Maybe<Cart>;
+};
+
+export type CartLineFragment = { id: string; quantity: number };
+
+export type TestQueryVariables = Exact<{ [key: string]: never }>;
+
+export type TestQuery = { cart?: { lines: { nodes: Array<{ id: string; quantity: number }> } } | null };

dev-test/test-null-value/schema.graphql

@@ -0,0 +1,32 @@
+schema {
+  query: QueryRoot
+}
+
+interface BaseCartLine {
+  id: String!
+  quantity: Int!
+}
+
+type BaseCartLineConnection {
+  id: String!
+  nodes: [BaseCartLine!]!
+}
+
+type Cart {
+  id: String!
+  lines: BaseCartLineConnection!
+}
+
+type CartLine implements BaseCartLine {
+  id: String!
+  quantity: Int!
+}
+
+type ComponentizableCartLine implements BaseCartLine {
+  id: String!
+  quantity: Int!
+}
+
+type QueryRoot {
+  cart: Cart
+}

packages/plugins/other/visitor-plugin-common/src/selection-set-to-object.ts

@@ -714,7 +714,6 @@ export class SelectionSetToObject<Config extends ParsedDocumentsConfig = ParsedD
     return (
       Object.keys(grouped)
         .map(typeName => {
-          const hasUnions = grouped[typeName].filter(s => typeof s !== 'string' && s.union).length > 0;
           const relevant = grouped[typeName].filter(Boolean);
 
           if (relevant.length === 0) {
@@ -729,6 +728,8 @@ export class SelectionSetToObject<Config extends ParsedDocumentsConfig = ParsedD
             })
             .join(' & ');
 
+          const hasUnions = grouped[typeName].filter(s => typeof s !== 'string' && s.union).length > 0;
+
           return relevant.length > 1 && !hasUnions ? `( ${res} )` : res;
         })
         .filter(Boolean)