To retrieve posts that have been shared by a specific user, we need to add a route endpoint that will receive the request for these posts and respond accordingly to the requesting client- side.

On the backend, we will define another post-related route that will receive a query to return posts by a specific user, as follows.

mern-social/server/routes/post.routes.js:

router.route('/api/posts/by/:userId')
    .get(authCtrl.requireSignin, postCtrl.listByUser)

The listByUser controller method in post.controller.js will query the Post collection to find posts that have a matching reference in the postedBy field to the user specified in the userId param in the route. The listByUser controller method will look as follows.

mern-social/server/controllers/post.controller.js:

const listByUser = async (req, res) => {
  try {
    let posts = await Post.find({postedBy: req.profile._id})
                          .populate('comments.postedBy', '_id name')
                          .populate('postedBy', '_id name')
                          .sort('-created')
                          .exec()
    res.json(posts)
  } catch(err) {
    return res.status(400).json({
      error: errorHandler.getErrorMessage(err)
    })
  }
}

This query will return the list of posts that were created by a specific user. We need to call this API from the frontend, which we will do in the next section.