How can I access the data of a property that's in another table? (postgres, express)

I'm creating a blog app. A user signs in and they can create a new post. To do this the data is being sent to the controller like so:

//blog-controller.js

blogController.create = (req, res) => {
  Blog.create({
      title: req.body.title,
      content: req.body.content,
      author_id: req.body.author_id,
      // user_id: req.body.user.id //added user_id object 
    })
    .then(blog => {
      res.redirect(`/blog/${blog.id}`)
    }).catch(err => {
      console.log(err)
      res.status(400).json(err);
    });
};

I would like to assign the author of the blogpost to be the user that is currently signed in, the info that is created from my users controller:

//users-controller.js
usersController.create = (req, res) => {
   User.create({
    username: req.body.username,
    email: req.body.email,
    password_digest: hash,
  })

How can I set the info of the user.username to the blog.author_id? (i.e. from two different tables)

Thanks in advance!