在映射函数中使用 async await

Question

In my Node Express App, I want to get all comments for a lesson and modify each comment by adding a fullname field to each comment. For getting full name of a user, I have defined findFullNameByUserId function in UserController.js . I use findAllCommentsByLessonId() in CourseController.js as follows. However, when I'm using it, I always get an empty array. How can I use findFullNameByUserId in findAllCommentsByLessonId() so that fullname field can be added to each comment object?

CourseController.js

findAllCommentsByLessonId: async (courseId,lessonId,callback) => {
    Course.find({_id: courseId}, function(err, course) {
      if (err) {
        callback(err, null);
      } else {
        const lesson = course[0].lessons.filter(l => l._id !== lessonId)
        const comments = lesson[0].comments.map( async c => {
          try{
            const name = await UserController.findFullNameById(c.user)
            return (
              {
                userId: c.user,
                name: name,
                content: c.content
              }
            )
          }
          catch(err){
console.log(err)
          }
        })

// console.log(comments) --> This always prints [{}]
        callback(null, comments)   
    }
    });
  }

UserController.js

module.exports = {
    findFullNameById: async (userId) => {
        await User.find({_id: userId}, function(err, user) {
            if (err) {
                console.log(err)
            } else {
                return  user[0].name+" "+( user[0].lname ? user[0].lname : "") 

            }
          });
    }
}

Answer 1

in CourseController.js either you can use async-await or you can use callback

async-await way :

findAllCommentsByLessonId: async (courseId,lessonId) => {
    let course = await Course.findOne({_id: courseId});
    if (course){
        let lesson = course.lessons.find(l => l._id == lessonId);
        let comments = [];

        for(let comment of lesson.comments){
               let name = await UserController.findFullNameById(comment.user);
               comments.push({userId: comment.user,name: name,content: comment.content});
        }
      return comments;
    }else{
      return [];
      }
  }

callback way :

findAllCommentsByLessonId: (courseId,lessonId,callback) => {
       Course.findOne({_id: courseId},function(err, course) {
          if (err) {
            callback(err, null);
          } else {
            let lesson = course.lessons.find(l => l._id == lessonId);
            let comments = lesson.comments;
                comments.map((comment)=>{

                  UserController.findFullNameById(comment.user).then(name=>{
                      return {userId: comment.user,name: name,content: comment.content};
                  });

               });
              callback(null, comments);
            }
        });
 }

Answer 2

Start by dropping callbacks and actually using promises for await . You haven't specified what find is, but chances are it's a modern library supporting promises. So write

async function findAllCommentsByLessonId(courseId, lessonId) {
  const [course] = Course.find({_id: courseId};
  const lesson = course.lessons.find(l => l._id !== lessonId);
  const comments = lesson.comments.map(async c => {
    const name = await UserController.findFullNameById(c.user)
    return {
      userId: c.user,
      name,
      content: c.content
    };
  });
}

module.exports.findFullNameById = async (userId) => {
  const [user] = await User.find({_id: userId});
  return user.name + " " + (user.lname ? user.lname : "");
};

Then you'll notice that comments is not an array of users, but rather an array of promises for users, so wrap it in a await Promise.all(…) call.

Answer 3

As @SuleymanSah commented, I tried to use .populate and it worked well. I think this is the correct approach as for the exact reasons he's pointed out. The following is how I did it:

Lesson.findOne({ _id: lessonId }).
populate('comments.user').
exec(function(err, lesson) {
  if (err) {
    console.log(err);
    return callback(err, null);
  }
  if (!lesson) {
    console.log("No record found");
    return callback(err, null);
  }
  return callback(null, lesson.comments);
});