Given that the 1st user is the admin and the blog owner. It would make sense if we could link the blog posts they write to their user in the database. In the future, maybe we will want to invite other users to write posts in the blog and grant them the admin privileges.

So we need to create a relationship between the User table and the BlogPost table to link them together. So we can see which BlogPosts a User has written. Or see which User is the author of a particular BlogPost.

If we were just writing Python code, you could imagine creating a User object which has a property called posts that contains a List of BlogPost objects.

e.g.

class User:
    def __init__(self, name, email, password):
         self.name = name
         self.email = email
         self.password = password
         self.posts = []
class BlogPost:
    def __init__(self, title, subtitle, body):
         self.title = title
         self.subtitle = subtitle
         self.body = body
new_user = User(
    name="Angela",
    email="angela@email.com",
    password=123456,
    posts=[
        BlogPost(
            title="Life of Cactus",
            subtitle="So Interesting",
            body="blah blah"
        )
    ]        
}

This would make it easy to find all the BlogPosts a particular user has written. But what about the other way around? How can you find the author of a particular BlogPost object? This is why we're using a database instead of just simple Python data structures.

In relational databases such as SQLite, MySQL or Postgresql we're able to define a relationship between tables using a ForeignKey and a relationship() method.

e.g. If we wanted to create a One to Many relationship between the User Table and the BlogPost table, where One User can create Many BlogPost objects, we can use the SQLAlchemy docs to achieve this.

https://docs.sqlalchemy.org/en/13/orm/basic_relationships.html

CHALLENGE 1: See if you can modify the User (Parent) and BlogPost (Child) class code to create a bidirectional One-to-Many relationship between the two tables. You should be able to easily locate the BlogPosts a User has written and also the User of any BlogPost object.

Hint:

SOLUTION

Re-creating the Database after changes to the Schema

If you re-run your blog at this point you'll get an error:

OperationalError: (sqlite3.OperationalError) no such column: blog_posts.author_id

The reason is that our new code in the main.py modifies our database model by adding a new column into our database that was not present in the original blog.db  from the starter code:

author_id = db.Column(db.Integer, db.ForeignKey("users.id"))

We don't have any valuable data at this point that we'd like to preserve, so the easiest way to simply delete the existing blog.db entirely and to use the line db.create_all() to re-create all the tables from scratch. Remember, this means you also have to register your user again and create a post since we've just wiped our database.

Now if you refresh your Blog website, you'll see the author name disappear from the index.html and page.html pages.

CHALLENGE 2: Modify the index.html and post.html pages so that the author name is still displayed in the right places.

HINT: the author property of BlogPost is now a User object.

SOLUTION