[Objection2] Documentation request for plugin in TypeScript

[willsoto]

There is currently a "recipe" of how to write a plugin (essentially a mixin) using regular JavaScript but I think it would be nice to have a TypeScript example just like you do for the query builder.

[koskimas]

It's basically impossible to type plugins unfortunately. Especially if multiple plugins are used. There's no way to extend an existing dynamic type in typescript. A plugin is a function that takes a model class constructor and returns a new constructor inherited from the input class:

function somePlugin<T extends Model>(modelClass: ModelClass<T>): ??? {
  return class extends modelClass {
    ...
  }
}

The ??? part is impossible to type so that the plugin works for both

class Person extends somePlugin(Model) {

}

and

class Person extends somePlugin(someOtherPlugin(Model)) {

}

If the plugin doesn't add any new methods to neither the QueryBuilder nor the Model, the typing is trivial: just return the input type:

function somePlugin<T extends typeof Model>(modelClass: T): T {
  ...
}

[willsoto]

That is, unfortunately, what I have come to realize after many hours of trying to type them. Even after lowering my expectations and peppering a few @ts-ignores around, it doesn't seem doable in a way that isn't totally gross. I was hoping with the addition of mixin support in TS it would be doable, but it does seem like anything that is non-trivial is just not doable cleanly.

---

As an aside, one thing I was using with some level of success before was ModelClass, which now seems to be an empty interface.

Is it no longer required to have that duplication? I believe it was there before to deal with static class members.

[koskimas]

The old ModelClass is now replaced with typeof Model in most places.

[willsoto]

That is what I thought. Thanks.

I'd like to write some docs anyway with some examples along with the drawbacks. Would that be okay?

[koskimas]

hmm... I was able to create this.. Maybe it's possible nowadays after all

import { Model, QueryBuilder, Page } from './typings/objection';

type Constructor<T extends Model> = new (...args: any[]) => T;

class CustomQueryBuilder<M extends Model, R = M[]> extends QueryBuilder<M, R> {
  ArrayQueryBuilderType!: CustomQueryBuilder<M, M[]>;
  SingleQueryBuilderType!: CustomQueryBuilder<M, M>;
  NumberQueryBuilderType!: CustomQueryBuilder<M, number>;
  PageQueryBuilderType!: CustomQueryBuilder<M, Page<M>>;

  someCustomMethod(): this {
    return this;
  }
}

function mixin<T extends Constructor<Model>>(ModelClass: T) {
  return class extends ModelClass {
    static QueryBuilder = QueryBuilder;
    QueryBuilderType: CustomQueryBuilder<this, this[]>;

    mixinMethod() {}
  };
}

class Person extends Model {}

const MixinPerson = mixin(Person);

async () => {
  const z = await MixinPerson.query()
    .whereIn('id', [1, 2])
    .someCustomMethod()
    .where('foo', 1)
    .someCustomMethod();

  z[0].mixinMethod();
};

[koskimas]

This was with typescript 3.7.2

[willsoto]

That is actually really close to what I have. I'll update my stuff to reflect your example and let you know if there are any issues. I am also on 3.7.2.

[willsoto]

So here is the example I am working with. I'll annotate with the errors I am seeing:

import { Model } from 'objection';

export type ModelConstructor<T extends Model = Model> = new (
  ...args: any[]
) => T;

export function Mixin(options = {}) {
  return function<T extends ModelConstructor>(Base: T) {
    class CustomQueryBuilder<M extends Model, R = M[]> extends QueryBuilder<M, R> {
      ArrayQueryBuilderType!: CustomQueryBuilder<M, M[]>;
      SingleQueryBuilderType!: CustomQueryBuilder<M, M>;
      NumberQueryBuilderType!: CustomQueryBuilder<M, number>;
      PageQueryBuilderType!: CustomQueryBuilder<M, Page<M>>;

      // Class 'QueryBuilder<M, R>' defines instance member property 'delete', but extended class 'CustomQueryBuilder<M, R>' defines it as instance member function.ts(2425)
      delete() {
        const value = new Date();

        // Argument of type '{ [x: string]: unknown; }' is not assignable to parameter of type 'PartialModelObject<this["ModelType"]>'.ts(2345)
        return this.patch({
          "deleted_at": value
        });
      }

      kept(): this {
        // Property 'ref' does not exist on type 'T'.ts(2339)
        return this.whereNull(Base.ref(column));
      }

      discarded(): this {
        // Property 'ref' does not exist on type 'T'.ts(2339)
        return this.whereNotNull(Base.ref(column));
      }
    }

    return class extends Base {
      static QueryBuilder = CustomQueryBuilder;

      QueryBuilderType!: CustomQueryBuilder<this, this[]>;

      static get modifiers(): Modifiers<CustomQueryBuilder<Model>> {
        return {
          // Property 'modifiers' does not exist on type 'T'.ts(2339)
          ...super.modifiers,
          discarded(builder) {
            builder.discarded();
          },
          kept(builder) {
            builder.kept();
          }
        };
      }
    };
  };
}

Based on these errors, it feels like TypeScript just can't figure out the correct type for T here. It can't figure out any static or instance properties. When I type Base. this is the autocomplete I get - basically just function methods and properties. It has no additional Objection properties or methods.

[koskimas]

The ModelConstructor doesn't have any properties or methods so that's completely understandable. How about

export function Mixin(options = {}) {
  return function<T extends typeof Model>(Base: T) {

[willsoto]

I was using T extends typeof Model before your example, and that got rid of most of the errors there. The remaining errors were these two:

// Class 'QueryBuilder<M, R>' defines instance member property 'delete', but extended class 'CustomQueryBuilder<M, R>' defines it as instance member function.ts(2425)
delete() {
  const value = new Date();

  // Argument of type '{ [x: string]: unknown; }' is not assignable to parameter of type 'PartialModelObject<this["ModelType"]>'.ts(2345)
  return this.patch({
    "deleted_at": value
  });
}

The second error ts(2345) makes sense to me I think, but I am not sure how to resolve it.

[koskimas]

The first error is now fixed in 2.0.3

[willsoto]

Thank you!

I ended up doing an assertion for the second error:

this.patch({
  "deleted_at": value
} as PartialModelObject<T>);

Not the worst compromise to me...

[koskimas]

That error makes complete sense. There is no deleted_at property. You can define it for your model, then you shouldn't need the cast

[willsoto]

@koskimas Yes, you are right. I got mixed up since I simplified it for the example. In the code I am using, that column is dynamic based on options passed to the decorator. And I have yet to find a way to dynamically add properties to classes in that way via a decorator.

[olavim]

Fyi @koskimas, generating declaration files ("declaration": true in tsconfig.json) with the examples discussed here does not work, because CustomQueryBuilder must be exported. I've gone into further detail here if you want to give your opinion/insight: olavim/objection-cursor#12