这是我可以使用提供的详细信息执行的操作。

如果您使用的是 Node.js v7.6 或更高版本，则可以利用 async/await 的使用并使此操作更轻松，重构如下。

async function yourFunctionName(req, res) {
    const obj = await Collection1.find({archived:false}).exec();

    obj = _.groupBy(obj, 'department');

    Object.keys(obj).forEach(key => {
        const object_collection = [];
        const frenchPromises = [];

        for (let i= 0; i <= obj[key].length; i ++) {
            // This will create an array of promises that will be resolved later.
            frenchPromises.push(
                // This will do the same as your callback, but returning a promise.
                French.find().exec().then(users => users[0])
            );
        }

        // When awaiting a Promise.all we will get an array containing the results
        // of all the promises insde the frenchPromises array.
        // Meaning, that we will get back the array of users from the previous code.
        object_collection = await Promise.all(frenchPromises);

        console.log(object_collection)
    });
}

如果不利用 async/await ，这里是相同的，但仅使用 Promises。

const obj = Collection1.find({ archived: false })
    .exec()
    .then(obj => {
        obj = _.groupBy(obj, 'department');
        const frenchPromises = [];

        Object.keys(obj).forEach(key => {
            for (let i = 0; i <= obj[key].length; i++) {
                // This will create an array of promises that will be resolved later.
                frenchPromises.push(
                    // This will do the same as your callback, but returning a promise.
                    French.find().exec().then(users => users[0])
                );
            }
        });

        // Returns an array of promises that will be resolved in the next .then() call
        return Promise.all(frenchPromises);
    }) // This will return the array of users
    .then(object_collection => {
        console.log(object_collection);
    });

希望对您有所帮助。