Problem with mongodb

I am doing the Back End Development and APIs Projects - URL Shortener Microservice | Learn | freeCodeCamp.org challenge.

Logic- If the given URL previously exists in database, then return its short_url, else find the last saved short_url, increase it by 1 and set this number as the short_url to this URL, save it and send it back to the user.

The problem is my app gives the same short_url for every URL given. Below is my code,

const urlSchema = new Schema({
  original_url: String,
  short_url: Number
});
let URLs = mongoose.model("URLs", urlSchema);
app.post("/api/shorturl", function(req, res, next) {
  const url = req.body.url.match(/https?:\/\/w{0,3}\.?([^\/]+)\/?.*/)[1];

  dns.lookup(url, (err) => {
    if (err) {
      res.json({ error: 'invalid url' });
      return
    }

    // Check url in database
    URLs.findOne({ original_url: req.body.url }, function(err, doc) {
      if (err) {
        res.send(err);
        return
      }

      // If exists, send that shorturl, 
      if (doc) {
        res.json({ original_url: doc.original_url, short_url: doc.short_url });
        return
      }

      // Else send the Increament of the last short by 1 and store it with this url
      next()
    })
  })
}, function(req, res) {

  URLs.findOne().sort({ created_at: -1 }).exec(function(err, doc) {
    const newDoc = new URLs({ original_url: req.body.url, short_url: doc ? (doc.short_url + 1) : 1 });
    newDoc.save(function(err, data) {
      if (err) {
        res.send(err);
        return
      }

      res.json({ original_url: data.original_url, short_url: data.short_url })
    })
  })
  
});

This problem is solved