在Vitest的ESM环境中Mock node-postgres时遇到的问题求助

初始实现与局限 #

我在函数中通过命名导入使用pg的Pool类：

import {Pool} from 'pg';
...
const pool = new Pool({connectionString});
const client = await pool.connect();
await client.query('BEGIN');
...

对应Mock方式如下：

vi.mock('pg', () => {
    const Pool = vi.fn();
    const Client = vi.fn();
    return {Pool, Client};
});

function queryMockImplementation(migrationTableExists: boolean, migratedFiles?: string[]) {
    return (queryString: string) => {
        if (queryString === `SELECT EXISTS (SELECT FROM pg_tables WHERE schemaname = 'administration' AND tablename = 'migrations')`) {
            return Promise.resolve({rows: [{exists: migrationTableExists}]});
        }
        return Promise.resolve({rows: []});
    }
}

function connectMockImplementation(clientQueries: string[], throwOnSqlQuery?: string) {
    return async() => {
        return {
            query: vi.fn().mockImplementation((queryString: string) => {
                if (throwOnSqlQuery === queryString) {
                    throw new Error('Error thrown by mock for client query.');
                }
                clientQueries.push(queryString);
            }),
            release: vi.fn(),
        };
    };
}

测试时的调用代码：

it('Should mock', async () => {
    const {Pool} = await import('pg');
    const clientQueries: string[] = [];
    Pool.prototype.query = vi.fn().mockImplementation(queryMockImplementation(false));
    Pool.prototype.connect = vi.fn().mockImplementation(connectMockImplementation(clientQueries));
    const pool = new Pool();
});

这种方式虽能运行，但存在明显局限：

• 必须注入clientQueries数组到Mock实现中，才能监听client.query的调用内容
• 无法直接Mock客户端实例
• 命名导入的方式会给消费该库的ESM包带来兼容性问题

切换导入方式后的错误 #

当改为默认导入pg再解构Pool时：

import pg from 'pg';
const {Pool} = pg;

直接触发Vitest错误：

Error: [vitest] No "default" export is defined on the "pg" mock. Did you forget to return it from "vi.mock"?
If you need to partially mock a module, you can use "vi.importActual" inside:

vi.mock("pg", async () => {
const actual = await vi.importActual("pg")
return {
...actual,
// your mocked methods
},
})

调整Mock后的新问题 #

按照提示改用vi.importActual调整Mock：

vi.mock('pg', async () => {
    const actual = <Record<string, any>>await vi.importActual("pg")
    const Pool = vi.fn();
    const Client = vi.fn();
    return {...actual, Pool, Client};
});

但此时出现两个新问题：

• 测试必须依赖真实数据库和有效的连接字符串才能运行
• 之前配置的客户端查询Mock完全失效

内容的提问来源于stack exchange，提问作者Florat