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SQLAlchemy Many to Many Relationships: An Example in Flask

Many-to-many relationships are fairly common in many (ahem) applications, and this is to remind myself how to create them - I always seem to forget! These are known as has_many :through associations in the Ruby on Rails world, and are needed whenever any Object A can have many Object B records, and any Object B can also be associated with many Object A records.

One real-world example would be an ecommerce site with database models for Users and Products - a User can have many Products through a secondary table (ie Orders), and a Product can be purchased by many Users through the same Orders table.

In this case, the concept of an "order" which associates users with products is fairly well known - but in situations where this does not apply, it is common to name this secondary table as a combination of both bodels (ie "UserProducts" if the concept of "Orders" is not commonly known). 

Here's how to define it in Flask / SQLAlchemy models.

1. Define your User model

This could look like:

from sqlalchemy.orm import relationship

class User(BaseModel):
    __tablename__ = 'users'

    id = db.Column(db.Integer, primary_key=True)
    email = db.Column(db.String(255))

    # ... any other fields

    create_dttm = db.Column(db.DateTime, server_default=db.func.current_timestamp())

    products = relationship("Product", secondary="orders")

2. Define the Product model

from sqlalchemy.orm import relationship

class Product(BaseModel):
    __tablename__ = 'products'

    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String(255))

    # ... any other fields

    create_dttm = db.Column(db.DateTime, default=datetime.utcnow)

    users = relationship("User", secondary="orders")

3. And finally, the association table

from sqlalchemy.orm import relationship, backref

class Order(BaseModel):
    __tablename__ = 'orders'
    id = db.Column(db.Integer, primary_key=True)
    user_id = db.Column(db.Integer, db.ForeignKey('users.id'))
    product_id = db.Column(db.Integer, db.ForeignKey('products.id'))

    # ... any other fields

    dttm = db.Column(db.DateTime, default=datetime.utcnow)

    user = relationship(User, backref=backref("orders", cascade="all, delete-orphan"))
    product = relationship(Product, backref=backref("orders", cascade="all, delete-orphan"))

4. Usage

Once these models have been defined and imported (and presumably your database tables have been created), you can use them easily:

user = User.query.first()
user.products  # List all products, eg [<productA>, <productB> ]
user.orders    # List all orders, eg [<order1>, <order2>]
user.orders[0].products  # List products from the first order

p1 = Product.query.first()
p1.users  # List all users who have bought this product, eg [<user1>, <user2>]
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