deletMany and save behaving differently in Mongoose

I am using mongoose in express.js. My requirement is when a form is submitted using post, i need to delete the existing records and then save the new one.

When I use myModel.deleteMany({}), its not deleting. However it saves when I use myModel.save()

However if I use myModel.deleteMany({}).the(result=>{console.log(result)}), it deletes.

My doubt why I does deleteMany expect the promise to be consumed, for it to delete while save doesn't need such things

async function DoitFn(){
    Feedback.deleteMany({});
    console.log("Deleted");
}
app.post("/Process", function(req,res) {
    DoitFn();
    //Feedback.deleteMany({}).then((arg)=>{console.log(arg)});
    console.log("Post request");
    myFeedback = new Feedback ({
        Id: req.body.count,
        Rating:req.body.myRating,
        myView: req.body.myView
    })
    console.log(myFeedback.Rating);
    console.log(myFeedback.myView);
    myFeedback.save();
        result = Feedback.find().then(result=>{console.log(result);
        });
        return 1;
})

Solution

Mongoose queries are not promises, they have a then() function which will execute the query for you or you can use the async/await pattern. If you chain exec() when using async/await you will get better debugging capability because exec() will return a promise giving you a full stack trace in the event of an error.

In your DoitFn() your Feedback.deleteMany({}); just prepares the query, nothing is executed.

Whereas when you do Feedback.deleteMany({}).then((arg)...) the query is executed by the then() and the returned result is stored in the arg variable.

I would recommend the following changes:

async function DoitFn(){
   try{
      return await Feedback.deleteMany({}); //< use await keyword and return the result
   }catch(err){
      console.log('Error in DoitFn', err);
      throw err;
   }
}


app.post("/Process", async function(req, res) { //< mark callback as async
   console.log("Post request");
   try{
      const result = await DoitFn(); //< await the result
      console.log('Result of DoitFn = ', result);
      const myFeedback = new Feedback({
         Id: req.body.count,
         Rating: req.body.myRating,
         myView: req.body.myView
      });
      console.log(myFeedback.Rating);
      console.log(myFeedback.myView);
      await myFeedback.save(); //< need to use await here too
      const doc = await Feedback.find(); //< need to use await here too
      console.log('Found: ', doc);
      return res.status(200).json({
         message: 'Delete and create complete'
      })
   }catch(err){
      console.log('Error in app.post(/Process)', err);
      return res.status(500).json({
         message: 'Error on server'
      })
   }
})