How do I define relationships between models in Flask ORM?

How do I define relationships between models in Flask ORM?

In Flask ORM, you can define relationships using the relationship() function from SQLAlchemy. For example, if you have a User model and a Post model, you can define a one-to-many relationship like this:

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    posts = db.relationship('Post', backref='author')

class Post(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    user_id = db.Column(db.Integer, db.ForeignKey('user.id'))

What types of relationships can I define?

You can define several types of relationships in Flask ORM, including one-to-one, one-to-many, and many-to-many. Each type has its own use case and implementation.

Can you give an example of a many-to-many relationship?

Sure! For a many-to-many relationship, you typically create an association table. For example:

post_tags = db.Table('post_tags',
    db.Column('post_id', db.Integer, db.ForeignKey('post.id'), primary_key=True),
    db.Column('tag_id', db.Integer, db.ForeignKey('tag.id'), primary_key=True)
)

class Post(db.Model):
    tags = db.relationship('Tag', secondary=post_tags, backref='posts')

class Tag(db.Model):
    id = db.Column(db.Integer, primary_key=True)

How do I query related models?

You can query related models using the relationship attribute. For example, to get all posts by a user, you can do:

user = User.query.get(user_id)
user_posts = user.posts