Flask SQLAlchemy: many to many relationship error

Question

I am trying to set up many-to-many relationship in SQLAlchemy but I am getting the error:

from shopapp import db
db.create_all()

sqlalchemy.exc.NoReferencedTableError: Foreign key associated with column 'shoppinglists_products.shoppinglist_id_v2' could not find table 'shoppinglist' with which to generate a foreign key to target column 'id'

My code:

from sqlalchemy import ForeignKey
from shopapp import db


shoppinglists_products = db.Table("shoppinglists_products",
                                  db.Column("shoppinglist_id", db.Integer, ForeignKey("shoppinglist.id")),
                                  db.Column("product_id", db.Integer, ForeignKey("product.id")))

class ShoppingList(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(20), unique=True, nullable=False)
    products = db.relationship('Product', back_populates="shoppinglists", secondary="shoppinglists_products")


class Product(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(20), unique=True, nullable=False)

Where is the problem?

Answer 1

It seems like Flask-SQLAlchemy has problem finding the table for foreign key reference . Based on your code, here are the two ways you can fix this:

1) Fix shoppinglists_products table:

Flask-SQLAlchemy often converts the CamelCased model names into a syntax similar to this: camel_cased . In your case, ShoppingList will be referred to as shopping_list . Therefore, changing the ForeignKey("shoppinglist.id") to ForeignKey("shopping_list.id") will do the trick.

shoppinglists_products = db.Table("shoppinglists_products",
             db.Column("shoppinglist_id", db.Integer, ForeignKey("shopping_list.id")), # <-- fixed
                                  

2) Change the model names:

If you'd like, you could go ahead and change the model name from ShoppingList to Shopping and later refer to this as shopping . This would prevent any confusion from rendering further. Usually, developers don't quite often go for a class name which is combined of two words, especially for the ORM cases. This is because various frameworks has different ways of interpreting the class names to create tables.

Answer 2

Expanding on @P0intMaN's answer - explicitly providing the SQL Alchemy table name with __tablename__ = "ShoppingList" (for example) lets you use your preferred case style and prevents SQLAlchemy from 'helping' you by changing the name of something kind of important without telling you.

class ShoppingList(db.Model):
    __tablename__ = "ShoppingList"
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(20), unique=True, nullable=False)
    products = db.relationship('Product', back_populates="shoppinglists", secondary="shoppinglists_products")

In many/most Flask tutorials and books, simplistic table names (eg posts, comments, users) are used, which elide this issue. Thus a trap awaits for those of us who insist on meaningful CamelCased class names. This is mentioned somewhat casually in the documentation here: https://flask-sqlalchemy.palletsprojects.com/en/2.x/models/

Some parts that are required in SQLAlchemy are optional in Flask-SQLAlchemy. For instance the table name is automatically set for you unless overridden. It's derived from the class name converted to lowercase and with “CamelCase” converted to “camel_case”. To override the table name, set the tablename class attribute.