Tracker.autorun only runs once

Here is what I am currently working with

class FooComponent extends Component {

  constructor(...args) {
    super(...args);

    this.state = {
      model: this.getModel()
    };
  }

  componentWillUnmount() {
    this._unmounted = true;
    this._modelComputation && this._modelComputation.stop();
    super.componentWillUnmount && super.componentWillUnmount();
  }

  getModel() {
    const model = {};

    this._modelComputation && this._modelComputation.stop();

    this._modelComputation = Tracker.autorun((computation) => {
      const { id } = this.props;
      const data = id && Collection.findOne(id);

      if (data) {
        Object.assign(model, data);
        !this._unmounted && this.forceUpdate();
      }
    });

    return model;
  }

  ...

}

Unfortunately, the reactive model does not work, and Tracker.autorun does not execute the function when the model is updated in the database. From the documentation, Collection.findOne should be reactive, right?

I am not sure what I am doing wrong. Why isn't Tracker monitoring the DB model? Why isn't the function re-evaluating Collection.findOne when the DB changes?

** Edit **

When updating the DB, I do see the collection change through meteortoys:allthings, but autorun is not re-executed.

Looking at how tracker-react implements it, I changed my code like so

class FooComponent extends Component {

  constructor(...args) {
    super(...args);

    this.state = {
      model: this.getModel()
    };
  }

  componentWillUnmount() {
    this._unmounted = true;
    this._modelComputation && this._modelComputation.stop();
    super.componentWillUnmount && super.componentWillUnmount();
  }

  getModel() {
    const model = {};

    this._modelComputation && this._modelComputation.stop();

    this._modelComputation = Tracker.nonreactive(() => {
      return Tracker.autorun((computation) => {
        const { id } = this.props;
        const data = id && Collection.findOne(id);

        if (data) {
          Object.assign(model, data);
          !this._unmounted && this.forceUpdate();
        }
      });
    });

    return model;
  }

  ...

}

I don't fully understand why, but it works, now.