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Inheritance

The main idea of TypeGraphQL is to create GraphQL types based on TypeScript classes.

In object-oriented programming it is common to compose classes using inheritance. Hence, TypeGraphQL supports composing type definitions by extending classes.

Types inheritance

One of the most known principles of software development is DRY - Don't Repeat Yourself - which is about avoiding code redundancy.

While creating a GraphQL API, it's a common pattern to have pagination args in resolvers, like skip and take. So instead of repeating ourselves, we declare it once:

@ArgsType()
class PaginationArgs {
  @Field(type => Int)
  skip: number = 0;

  @Field(type => Int)
  take: number = 25;
}

and then reuse it everywhere:

@ArgsType()
class GetTodosArgs extends PaginationArgs {
  @Field()
  onlyCompleted: boolean = false;
}

This technique also works with input type classes, as well as with object type classes:

@ObjectType()
class Person {
  @Field()
  age: number;
}

@ObjectType()
class Student extends Person {
  @Field()
  universityName: string;
}

Note that both the subclass and the parent class must be decorated with the same type of decorator, like @ObjectType() in the example Person -> Student above. Mixing decorator types across parent and child classes is prohibited and might result in a schema building error, e.g. we can't decorate the subclass with @ObjectType() and the parent with @InputType().

Resolver Inheritance

A special kind of inheritance in TypeGraphQL is resolver class inheritance. This pattern allows us e.g. to create a base CRUD resolver class for our resource/entity, so we don't have to repeat common boilerplate code.

Since we need to generate unique query/mutation names, we have to create a factory function for our base class:

function createBaseResolver() {
  abstract class BaseResolver {}

  return BaseResolver;
}

Be aware that with some tsconfig.json settings (like declarations: true) we might receive a [ts] Return type of exported function has or is using private name 'BaseResolver' error - in this case we might need to use any as the return type or create a separate class/interface describing the class methods and properties.

This factory should take a parameter that we can use to generate the query/mutation names, as well as the type that we would return from the resolvers:

function createBaseResolver<T extends ClassType>(suffix: string, objectTypeCls: T) {
  abstract class BaseResolver {}

  return BaseResolver;
}

It's very important to mark the BaseResolver class using the @Resolver decorator:

function createBaseResolver<T extends ClassType>(suffix: string, objectTypeCls: T) {
  @Resolver()
  abstract class BaseResolver {}

  return BaseResolver;
}

We can then implement the resolver methods as usual. The only difference is that we can use the name decorator option for @Query, @Mutation and @Subscription decorators to overwrite the name that will be emitted in schema:

function createBaseResolver<T extends ClassType>(suffix: string, objectTypeCls: T) {
  @Resolver()
  abstract class BaseResolver {
    protected items: T[] = [];

    @Query(type => [objectTypeCls], { name: `getAll${suffix}` })
    async getAll(@Arg("first", type => Int) first: number): Promise<T[]> {
      return this.items.slice(0, first);
    }
  }

  return BaseResolver;
}

Now we can create a specific resolver class that will extend the base resolver class:

const PersonBaseResolver = createBaseResolver("person", Person);

@Resolver(of => Person)
export class PersonResolver extends PersonBaseResolver {
  // ...
}

We can also add specific queries and mutations in our resolver class, as always:

const PersonBaseResolver = createBaseResolver("person", Person);

@Resolver(of => Person)
export class PersonResolver extends PersonBaseResolver {
  @Mutation()
  addPerson(@Arg("input") personInput: PersonInput): Person {
    this.items.push(personInput);
    return personInput;
  }
}

And that's it! We just need to normally register PersonResolver in buildSchema and the extended resolver will work correctly.

We must be aware that if we want to overwrite the query/mutation/subscription from the parent resolver class, we need to generate the same schema name (using the name decorator option or the class method name). It will overwrite the implementation along with the GraphQL args and return types. If we only provide a different implementation of the inherited method like getOne, it won't work.

Examples

More advanced usage examples of type inheritance (and interfaces) can be found in the example folder.

For a more advanced resolver inheritance example, please go to this example folder.