Update a post if the current user is the author.

def update(id):
    """Update a post if the current user is the author."""
    post = get_post(id)

    if request.method == "POST":
        title = request.form["title"]
        body = request.form["body"]
        error = None

        if not title:
            error = "Title is required."

        if error is not None:
            flash(error)
        else:
            post.title = title
            post.body = body
            db.session.commit()
            return redirect(url_for("blog.index"))

    return render_template("blog/update.html", post=post)