enhance(utils/astFromSchema): support GraphQLSchema instances without operation types defined

.changeset/fine-jobs-love.md

@@ -0,0 +1,26 @@
+---
+'@graphql-tools/utils': major
+---
+
+Support "federation/subgraph style" schemas in `astFromSchema` and `printSchemaWithDirectives`
+
+If a `GraphQLSchema` doesn't have any defined operation types, we should print the schema definition as an extension rather than omitting it entirely.
+They are not a valid schema on their own, but they are valid subgraph schemas in a federation setup, and it is possible to build such schemas with `assumeValid` options.
+
+```ts
+// A schema without defined root types
+buildSchema(
+  /* GraphQL */ `
+    extend schema @link(url: "https://specs.apollo.dev/federation/v2.0", import: ["@key"])
+
+    type User @key(fields: "id") {
+      id: ID!
+      username: String
+    }
+  `,
+  { assumeValid: true, assumeValidSDL: true }
+)
+```
+
+**POTENTIAL BREAKING CHANGE**:
+This can be a breaking change because now the schema above will be printed as the input, previously `extend schema` was converted to `schema {}`.

packages/utils/src/print-schema-with-directives.ts

@@ -179,7 +179,7 @@ export function astFromSchema(
   }
 
   const schemaNode: SchemaDefinitionNode | SchemaExtensionNode = {
-    kind: operationTypes != null ? Kind.SCHEMA_DEFINITION : Kind.SCHEMA_EXTENSION,
+    kind: operationTypes.length ? Kind.SCHEMA_DEFINITION : Kind.SCHEMA_EXTENSION,
     operationTypes,
     // ConstXNode has been introduced in v16 but it is not compatible with XNode so we do `as any` for backwards compatibility
     directives: directives as any,

packages/utils/tests/print-schema-with-directives.spec.ts

@@ -13,6 +13,7 @@ import {
   GraphQLString,
   printSchema,
   specifiedDirectives,
+  stripIgnoredCharacters,
 } from 'graphql';
 import { GraphQLJSON } from 'graphql-scalars';
 import { makeExecutableSchema } from '@graphql-tools/schema';
@@ -402,4 +403,17 @@ describe('printSchemaWithDirectives', () => {
       expect(output).toContain('input OneOfInput @oneOf');
     }
   });
+  it('prints a federation-style subgraph schema correctly', () => {
+    const sdl = stripIgnoredCharacters(/* GraphQL */ `
+      extend schema @link(url: "https://specs.apollo.dev/federation/v2.0", import: ["@key"])
+
+      type User @key(fields: "id") {
+        id: ID!
+        username: String
+      }
+    `);
+    const userSubgraph = buildSchema(sdl, { assumeValid: true, assumeValidSDL: true });
+    const printedSchema = stripIgnoredCharacters(printSchemaWithDirectives(userSubgraph));
+    expect(printedSchema).toBe(sdl);
+  });
 });