dropCollection执行无效：删除后仍查询到集合的问题排查

我编写了一段用于删除数据库中所有集合的代码，预期执行后调用collections()会返回空集合，但实际仍能查询到集合。

代码示例 #

const collectionsBeforeDrop = await db.collections();
collectionsBeforeDrop.forEach(async (c) => {
  console.log(`got collection name : ${c.collectionName}`);
  await db.dropCollection(c);
});
console.log(`checking after cleanup`)
const collectionsAfterDrop = await db.collections();
collectionsAfterDrop.forEach(c => {
  console.log(`got collection name after claanup! THIS SHOULDN'T HAVE HAPPENED : ${util.inspect(c.collectionName)}`); //THIS CODE HITS
  throw new Error("couldn't clean up after test case");
});

执行输出 #

cleaning up                                                                                                                

      at Object.log (test/question.test.js:61:13)

  console.log                                                                                                                  
    got db [object Object]                                                                                                     

      at Object.log (test/question.test.js:64:13)

  console.log                                                                                                                  
    truncating collections of db Questions                                                                                     

      at Object.log (test/question.test.js:67:13)

  console.log
    got collection name : areas

      at log (test/question.test.js:72:15)
          at Array.forEach (<anonymous>)

  console.log                                                                                                                  
    checking after cleanup                                                                                                     

      at Object.log (test/question.test.js:75:13)

  console.log                                                                                                                  
    got collection name after claanup! THIS SHOULDN'T HAVE HAPPENED : 'areas'

      at log (test/question.test.js:78:15)
          at Array.forEach (<anonymous>)

问题原因 #

核心问题是**Array.forEach不支持等待异步函数完成**。你在forEach里传入了async回调函数，forEach会立刻遍历执行所有回调，但不会等待这些异步的删除操作全部完成，就直接跳到后续的集合查询代码。所以你查询集合的时候，删除操作可能还没跑完，自然能看到残留的集合。

修复方案 #

可以用两种方式解决：

方案1：使用for...of循环 #

for...of可以配合await，确保每个删除操作完成后再执行下一个：

const collectionsBeforeDrop = await db.collections();
// 用for...of逐个等待删除完成
for (const c of collectionsBeforeDrop) {
  console.log(`got collection name : ${c.collectionName}`);
  await db.dropCollection(c);
}
console.log(`checking after cleanup`)
const collectionsAfterDrop = await db.collections();
collectionsAfterDrop.forEach(c => {
  console.log(`got collection name after cleanup! THIS SHOULDN'T HAVE HAPPENED : ${util.inspect(c.collectionName)}`);
  throw new Error("couldn't clean up after test case");
});

方案2：使用Promise.all #

把所有删除操作包装成Promise数组，用Promise.all等待全部操作完成：

const collectionsBeforeDrop = await db.collections();
// 批量等待所有删除操作完成
await Promise.all(collectionsBeforeDrop.map(async (c) => {
  console.log(`got collection name : ${c.collectionName}`);
  await db.dropCollection(c);
}));
console.log(`checking after cleanup`)
const collectionsAfterDrop = await db.collections();
collectionsAfterDrop.forEach(c => {
  console.log(`got collection name after cleanup! THIS SHOULDN'T HAVE HAPPENED : ${util.inspect(c.collectionName)}`);
  throw new Error("couldn't clean up after test case");
});

内容的提问来源于stack exchange，提问作者Manu Chadha