类构造函数参数中的打字稿泛型类型

Question

我正在尝试使用 facebook 数据加载器实现存储库通用模式。

此代码用于 GraphQL API。

抽象基类，我得到错误了吗：

import { Collection, Db, InsertOneWriteOpResult, ObjectId } from "mongodb";
import { RepositoryGeneric } from "../../types/database";

export default abstract class BaseRepository<TEntity, TLoaders> implements RepositoryGeneric<TEntity> {
    protected mongoCollection: Collection<TEntity>;
    protected dataLoaders: TLoaders;

    public constructor(database: Db, collection: string, loaders: TLoaders) {
        this.mongoCollection = database.collection(collection);
        this.dataLoaders = loaders;
    }

    public async findOne(id: ObjectId | string): Promise<TEntity | null> {
        if (typeof id === "string") {
            id = ObjectId.createFromHexString(id);
        }

        // This line is producing the error
        return this.dataLoaders.id.load(id);
    }

    public async create(entity: TEntity): Promise<TEntity> {
        const result: InsertOneWriteOpResult = await this.mongoCollection.insertOne(entity);

        if (!result.result.ok) {
            throw new Error();
        }

        return result.ops[0];
    }
}

类扩展抽象基类：

import { MongoClient } from "mongodb";
import { EntityBusiness } from "../../types/database";
import BusinessLoaders from "../loaders/businessLoaders";
import BaseRepository from "./baseRepository";

export default class BusinessRepository extends BaseRepository<EntityBusiness, BusinessLoaders> {
    public constructor(mongoClient: MongoClient, businessLoaders: BusinessLoaders) {
        super(mongoClient.db(), "business", businessLoaders);

        // tslint:disable-next-line: no-console
        console.log(businessLoaders.id);
    }
}

实现数据加载器的类：

import DataLoader from "dataloader";
import { AggregationCursor, Collection, MongoClient, ObjectId } from "mongodb";
import { EntityBusiness, LoaderCommon } from "../../types/database";

export default class BusinessLoaders implements LoaderCommon<EntityBusiness> {
    private idLoader: DataLoader<ObjectId, EntityBusiness>;

    public constructor(mongoClient: MongoClient) {
        this.idLoader = new DataLoader((keys) => this.idBatch(mongoClient, keys), {
            cacheKeyFn: (key) => key.toHexString(),
        });
    }

    public get id(): DataLoader<ObjectId, EntityBusiness> {
        return this.idLoader;
    }

    private async idBatch(mongoClient: MongoClient, keys: ObjectId[]): Promise<EntityBusiness[]> {
        const collection: Collection<EntityBusiness> = mongoClient.db().collection("business");
        const aggregation: AggregationCursor<EntityBusiness> = collection.aggregate([
            { $match: { _id: { $in: keys } } },
            { $addFields: { __order: { $indexOfArray: [keys, "$_id"] } } },
            { $sort: { __order: 1 } },
            { $project: { __order: 0 } },
        ]);

        return aggregation.toArray();

    }
}

我对打字稿没有太多经验，但我希望不会出错。相反，我得到错误：

错误 TS2339：“TLoaders”类型上不存在属性“id”。

Answer 1

您需要指定泛型类型有一些约束，特别是它有一个id 属性。您可以使用泛型类型定义中的 extends 关键字来做到这一点：

interface ILoadersInterface<TEntity> {
  id: DataLoader<ObjectId, TEntity | null>
}

export default abstract class BaseRepository<TEntity, TLoaders extends ILoadersInterface<TEntity>> 
  implements RepositoryGeneric<TEntity> {
    // ...
}

更一般地说，一个类需要在其内部有足够的类型信息才能正确解析所有代码使用，因为在这种情况下，TypeScript 无法通过使用推断TLoaders 将具有id 属性。从概念上讲，你可以想象一个泛型类型参数大致是unknown，除非你指定了一个约束，比如extends Interface、extends string等等。