无法从Mongoose连接获取MongoClient实例，为何client为undefined？

根据Mongoose官方API文档，Connection的client属性应为MongoClient实例，但运行以下代码时触发错误：something bad happened in dbconnect: TypeError: Cannot read properties of undefined (reading 'db')，原因是client为undefined。

const connection = await mongoose.connect("mongodb://0.0.0.0:27017");
console.log('connected to Mongo');
  
const client = connection.client; // 预期获取MongoClient实例
  
try {
  const db = client.db("admin");
  console.log(`connected to database ${db.databaseName}`);
  const collections = await db.collections();
  collections.forEach(c => {console.log(`got collection ${c.collectionName}`)})
} catch(exception) {
  console.log(`something bad happened in dbconnect: ${exception}`); // 触发错误
} finally {
  console.log("closing database connection");
  connection.disconnect();
}

解决方案 #

问题出在对mongoose.connect()返回值的理解上：mongoose.connect()返回的并不是直接的Connection实例，而是一个包含connection属性的包装对象。你把整个返回对象当成了Connection，自然找不到client属性。

修正方式有两种：

方式一：使用mongoose.connection获取连接实例 #

await mongoose.connect("mongodb://0.0.0.0:27017");
console.log('connected to Mongo');

const connection = mongoose.connection;
const client = connection.client; // 现在能正确获取MongoClient实例

try {
  const db = client.db("admin");
  console.log(`connected to database ${db.databaseName}`);
  const collections = await db.collections();
  collections.forEach(c => {console.log(`got collection ${c.collectionName}`)})
} catch(exception) {
  console.log(`something bad happened in dbconnect: ${exception}`);
} finally {
  console.log("closing database connection");
  connection.disconnect();
}

方式二：从connect返回值中解构connection #

const { connection } = await mongoose.connect("mongodb://0.0.0.0:27017");
console.log('connected to Mongo');

const client = connection.client; // 正确获取MongoClient实例

try {
  const db = client.db("admin");
  console.log(`connected to database ${db.databaseName}`);
  const collections = await db.collections();
  collections.forEach(c => {console.log(`got collection ${c.collectionName}`)})
} catch(exception) {
  console.log(`something bad happened in dbconnect: ${exception}`);
} finally {
  console.log("closing database connection");
  connection.disconnect();
}

修改后就能正确获取到MongoClient实例，调用client.db()方法也不会再触发错误。

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