Relationships between models in SQLAlchemy are links between two or more models that allow models to reference each other automatically. This allows naturally related data, such as comments on posts, to be easily retrieved from the database with its related data. This is where the R in RDBMS comes from, and it gives this type of database a large amount of power.
Let's create our first relation. Our blogging website is going to need some blog posts. Each blog post is going to be written by one user, so it makes sense to link posts back to the user who wrote them so that we can easily get all the posts by a user. This is an example of a one-to-many relationship, as shown in the following code:
SQLite and MySQL/MyISAM engines do not enforce relationship constraints. This might cause problems if you are using SQLite on your development environment...