Mongoose Population returns null always

Question

So I'm kinda new to Relations and I'm trying to populate a Document.

Here is my Document Model:

const { Schema, model, Types } = require('mongoose');

const DmsSchema = new Schema(
  {
    description: {
      type: String,
      default: '',
    },

    price: {
      type: String,
      default: '',
    },

    date: {
      type: String,
      default: '',
    },

    document: {
      type: Types.ObjectId,
      ref: 'Upload',
      default: null,
    },

    type: {
      type: String,
      default: 'rechnung',
    },
  },
  {
    timestamps: true,
  }
);

module.exports = model('Dms', DmsSchema);

And here is the File Upload Schema:

const { Schema, model, Types } = require('mongoose');

const UploadSchema = new Schema(
  {
    name: {
      type: String,
      default: '',
    },

    stream: {
      type: String,
      default: '',
    },

    type: {
      type: String,
      default: '',
    },

    assigned: {
      type: Types.ObjectId,
      ref: 'Dms',
      default: null,
    },
  },
  {
    timestamps: true,
  }
);

module.exports = model('Upload', UploadSchema);

I have created relations between them using objects Id, and the upload does exist too, but when I fetch it it doesn't work and returns null on the filed document, I don't know why:

Here Is how i fetch the data:

fetchSingle: async (req, res) => {
    try {
      Dms.findById(req.params.id)
        .populate('document')
        .exec(function (err, data) {
          if (err) {
            console.log(err);
            res.status(404).send({
              success: false,
              message: 'Something went wrong fetching the documents.',
            });
          }
          res.status(200).send({
            success: true,
            data: data,
          });
        });
    } catch (error) {
      console.log(error);
      res.status(500).send({
        success: false,
        message: 'Something went wrong fetching the documents.',
      });
    }
  },