I came to a problem, where I can create conversations with multiple people 2 and so on. However, I can't understand why it doesn't store data to seperate User models.

Here is a code that you only need to know:

router.post(
  "/",
  auth,
  [
    check("conversators", "There should be at least two conversators").isLength(
      { min: 2 }
    ),
  ],
  async (req, res) => {
    const { conversators } = req.body;
    const errors = validationResult(req);

    if (!errors.isEmpty()) {
      return res.status(400).json({ errors: errors.array() });
    }

    try {
      let conversation = new Conversation({
        user: req.user.id,
        conversators: conversators,
      });

      await conversators.map(async (conversator) => {
        let user = await User.findById(conversator);
        let newData = user;
        newData.conversations.push(conversation.id);

        console.log('Created data', newData);

        let newUser = await User.findOneAndUpdate(
          { user: conversator },
          {
            $set: {
              newData,
            },
          },
          { new: true }
        );

        await newUser.save();
        console.log(newUser);
      });

      await conversation.save();
      res.status(200).json(conversation);
    } catch (error) {
      console.error(error.message);
      res.status(500).send("Server error.");
    }
  }
);

module.exports = router;

What I can assure is that this line: console.log('Created data', newData); prints the desired data. However, the next console: console.log(newUser); prints the same User model as the previous one.

const mongoose = require("mongoose");
const Schema = mongoose.Schema;

const UserSchema = new Schema({
  name: {
    type: String,
    required: true,
  },
  surname: {
    type: String,
    required: true,
  },
  email: {
    type: String,
    required: true,
  },
  password: {
    type: String,
    required: true,
  },
  conversations: [
    {
      type: mongoose.Schema.Types.ObjectId,
      ref: "conversation",
    },
  ],
  date: {
    type: Date,
    default: Date.now,
  },
});

module.exports = User = mongoose.model("user", UserSchema);