Relationships in Flask SQL-Alchemy

What is Flask SQL-alchemy?

Flask SQL-alchemy is an extension of Flask web framework that simplifies the integration of relational databases with Flask application. SQLAlchemy is a Object-Relational-Mapping (ORM) library for Python. With SQLAlchemy, you can define database models as Python classes, and extensions take care of translating these models into SQL commands for database operations. One of the key benefits is the ability to support multiple databases on the backend. Flask-SQLAlchemy also offers features like database migrations, session management, and query building that streamline the process of database development. In Flask-SQLAlchemy, relationships can be modeled between databases using SQLAlchemy’s ORM. Relationships between databases can be represented as one-to-one, one-to-many, and many-to-many.

One-to-Many Relationship

In a one-to-many relationship, one record in the first table is associated with multiple records in the second table. For example, a simple blog application with two tables ‘User’ and ‘Post’. A user can create multiple posts, but each post is associated with only one user.

In the example above, the ‘User’ class defines a ‘posts’ attribute using the ‘db.relationship’ function. This establishes the one-to-many relationship with the ‘Post’ class.

Many-to-Many Relationship

In a many to many relationship, records in both tables are related to multiple records between ‘Song’ and ‘Playlist’. Where a song can be in multiple playlists and a playlist can have multiple songs.

We define a ‘playlists’ attribute in the ‘Song’ class to establish the many-to-many relationship with the ‘Playlist’. The ‘secondary’ argument specifies the association table that connects the two classes.

One-to-One Relationship

In a one-to-one relationship, one record in the first table is related to one record on the second table and vice versa. For example, ‘User’ have exactly one ‘UserProfile’ where they have a one-to-one relationship.

In the ‘User’ class, we define a ‘profile’ attribute using the ‘db.relationship’ function. To indicate that it’s a one-to-one relationship, we use the ‘uselist=False’. The ‘backred’ argument creates a virtual column name ‘user’ on the ‘UserProfile’ class, this allows access to the user associated with the profile.