So I have these two simple classes inside the same python file which I'm trying to map with SQLAlchemy, where User column 'password_id' is a foreign key to the table Password 'password_id' column as well
from sqlalchemy.orm import relationship, declarative_base
from sqlalchemy import Column, Integer, String
from sqlalchemy.sql.schema import ForeignKey
Base = declarative_base()
class User(Base):
__tablename__ = 'users'
user_id = Column(Integer, primary_key=True)
name = Column(String)
email = Column(String)
password_id = Column(Integer, ForeignKey('password.password_id'))
parent = relationship("Password", back_populates="users")
class Password(Base):
__tablename__ = 'passwords'
password_id = Column(Integer, primary_key=True)
password = Column(String)
last_change_date = Column(DateTime)
valid_until = Column(DateTime)
child = relationship("User", back_populates="passwords", uselist=False)
Here's the db schema for context:
I'm following this guide from sql alchemy but for some reason I keep getting the error from the title 'Ensure that referencing columns are associated with a ForeignKey or ForeignKeyConstraint, or specify a 'primaryjoin' expression.', which is strange because accorind to what I see in the guide, my classes have been mapped correctly so I can't understand why this error is happening.
Thank you in advance
I think the issue is in the following line (the table name in ForeignKey)...
password_id = Column(Integer, ForeignKey('password.password_id'))
should be passwords instead of password.
Is it possible to build a SQLAlchemy relationship that will accept any instance that inherits from the same Base? Ideally, it maintains one foreign key for the instance's ID and a second foreign key for the instance's type.
As an example, this Variable class has a relationship applies_to between itself and the Car class.
# some overhead, for completeness
from sqlalchemy import Column, String, Integer, ForeignKey
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship, Session
from sqlalchemy import create_engine
engine = create_engine('sqlite:///:memory:')
session = Session(bind=engine)
Base = declarative_base()
# defining my classes
class Variable(Base):
__tablename__ = 'variables'
id = Column(Integer, primary_key=True)
name = Column(String)
value = Column(Integer)
applies_to_id = Column(Integer, ForeignKey('cars.id'))
applies_to = relationship('Car')
class Car(Base):
__tablename__ = 'cars'
id = Column(Integer, primary_key=True)
class Truck(Base):
__tablename__ = 'trucks'
id = Column(Integer, primary_key=True)
Base.metadata.create_all(engine)
With it, I can make a Car and a Variable for that car and commit to the database.
car = Car()
var1 = Variable(name="speed", value=50, applies_to=car)
session.add(car)
session.add(var1)
session.commit()
But I cannot do a similar thing with Truck and Variable.
truck = Truck()
var2 = Variable(name="speed", value=50, applies_to=truck)
session.add(truck)
session.add(var2)
session.commit()
AssertionError: Attribute 'applies_to' on class '<class '__main__.Variable'>' doesn't handle objects of type '<class '__main__.Truck'>'
I would like to edit the Variable class to support Truck and any other classes that inherit from Base through the applies_to relationship.
EDIT: Similar to sqlalchemy generic foreign key (like in django ORM), but that solution requires all classes to inherit from the HasAddresses mixin and handles a backref via addresses on those classes. Is there a solution that only edits the Variable class and doesn't maintain a backref to the other classes?
i'm using sqlalchemy + alembic + Flask and i can't map circular classes.
apps/users/models.py:
class User(Base):
__tablename__ = 'users'
id = Column(Integer, primary_key=True)
email = Column(String)
password = Column(String)
session = relationship("Session", back_populates='user', cascade='all,delete', lazy='dynamic')
notes = relationship('Note2User', back_populates='user', cascade='all,delete', lazy='dynamic')
apps/notes/models.py:
class Note2User(Base):
__tablename__ = 'notes_users_m2m'
id = Column(Integer, primary_key=True)
user_id = Column(Integer, ForeignKey('users.id', ondelete='CASCADE'), nullable=False)
user = relationship('User', back_populates='notes')
note_id = Column(Integer, ForeignKey('notes.id', ondelete='CASCADE'), nullable=False)
note = relationship('Note', back_populates='users')
Table Note2User made for m2m relationship User <-> Notes, but when i start app and done some request, gets error:
InvalidRequestError: When initializing mapper Mapper|User|users,
expression 'Note2User' failed to locate a name ("name 'Note2User' is
not defined"). If this is a class name, consider adding this
relationship() to the class after
both dependent classes have been defined.
Initializing db in db/init.py: (dunder name)
from sqlalchemy import create_engine, MetaData
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import Session
engine = create_engine('postgresql+psycopg2://server:12345#localhost:5432/test')
Base = declarative_base()
meta = MetaData()
meta.reflect(bind=engine)
db_session = Session(bind=engine)
Add an import for Note2User class in apps/users/models.py file so this model gets defined first before initializing that relatioship in User class which refrences it.
like this
# file: apps/users/models.py
from ..notes.models import Note2User
You need to import the user.models module into the notes.model module and vice versa. It would look something like this:
# file app/users/models.py
import app.notes.models as notes
# use it like this
notes.Notes2User()
# file app/notes/models.py
import app.users.models as users
users.User()
The advantage to this is that you will avoid circular dependency problems as your program inevitably grows. I had so many problems with circular dependencies when I was creating an app with your same stack. The only solution was to ditch the
from . import Foo
and only use
import bar.foo as foo
It is considered best practice to use the import syntax for this reason.
Reference.
How does one generate the SQL/Migrate Code/Whatever with SQLAlchemy when using the Declarative Base?
from sqlalchemy import *
from sqlalchemy.orm import relationship
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
engine = create_engine('mysql://root:password#localhost/mydb_dev', echo=True)
metadata = MetaData(bind=engine)
class User(Base):
__table__ = Table("users", metadata, autoload=True)
id = Column(Integer, primary_key=True)
display_name = Column(String)
email = Column(String)
password = Column(String)
def __repr__(self):
return "<User(id='{}', display_name='{}', email='{}')>".format(self.id, self.display_name, self.email)
class Site(Base):
__table__ = Table("sites", metadata, autoload=True)
id = Column(Integer, primary_key=True)
name = Column(String)
urls = relationship("URL")
def __repr__(self):
return "<Site(id='{}', name='{}')>".format(self.id, self.name)
I have this so far, I'd like to see what SQLAlchemy would generate as a schema.
Or, does SQLAlchemy do this at all? Is this a case where I create and manage the database and it's schema separately, and I just update my entities to reflect it?
Do understand that I am used to Doctrine2, and im very new to SQLAlchemy
Thanks!
You can create the db models by calling:
metadata.create_all()
http://docs.sqlalchemy.org/en/latest/core/metadata.html#sqlalchemy.schema.MetaData.create_all
Note that this only works for the creation of previously non-existent models, and doesn't handle updates or downgrades. Check out alembic for even finer control:
http://alembic.zzzcomputing.com/en/latest/
Simplified, I have the following class structure (in a single file):
Base = declarative_base()
class Item(Base):
__tablename__ = 'item'
id = Column(BigInteger, primary_key=True)
# ... skip other attrs ...
class Auction(Base):
__tablename__ = 'auction'
id = Column(BigInteger, primary_key=True)
# ... skipped ...
item_id = Column('item', BigInteger, ForeignKey('item.id'))
item = relationship('Item', backref='auctions')
I get the following error from this:
sqlalchemy.exc.InvalidRequestError
InvalidRequestError: When initializing mapper Mapper|Auction|auction, expression
'Item' failed to locate a name ("name 'Item' is not defined"). If this is a
class name, consider adding this relationship() to the Auction class after
both dependent classes have been defined.
I'm not sure how Python cannot find the Item class, as even when passing the class, rather than the name as a string, I get the same error. I've been struggling to find examples of how to do simple relationships with SQLAlchemy so if there's something fairly obvious wrong here I apologise.
This all turned out to be because of the way I've set SQLAlchemy up in Pyramid. Essentially you need to follow this section to the letter and make sure you use the same declarative_base instance as the base class for each model.
I was also not binding a database engine to my DBSession which doesn't bother you until you try to access table metadata, which happens when you use relationships.
if it's a subpackage class, add Item and Auction class to __init__.py in the subpackage.
The SQLAlchemy documentation on Importing all SQLAlchemy Models states in part:
However, due to the behavior of SQLAlchemy's "declarative" configuration mode, all modules which hold active SQLAlchemy models need to be imported before those models can successfully be used. So, if you use model classes with a declarative base, you need to figure out a way to get all your model modules imported to be able to use them in your application.
Once I imported all of the models (and relationships), the error about not finding the class name was resolved.
Note: My application does not use Pyramid, but the same principles apply.
Case with me
Two models defined in separate files, one is Parent and the other is Child, related with a Foreign Key. When trying to use Child object in celery, it gave
sqlalchemy.exc.InvalidRequestError: When initializing mapper Mapper|Child|child, expression 'Parent' failed to locate a name ("name 'Parent' is not defined"). If this is a class name, consider adding this relationship() to the <class 'app.models.child'>
parent.py
from app.models import *
class Parent(Base):
__tablename__ = 'parent'
id = Column(BigInteger, primary_key=True, autoincrement=True)
name = Column(String(60), nullable=False, unique=True)
number = Column(String(45), nullable=False)
child.py
from app.models import *
class Child(Base):
__tablename__ = 'child'
id = Column(BigInteger, primary_key=True, autoincrement=True)
parent_id = Column(ForeignKey('parent.id'), nullable=False)
name = Column(String(60), nullable=False)
parent = relationship('Parent')
Solution
Add an import statement for Parent in beginning of child.py
child.py (modified)
from app.models import *
from app.models.parent import Parent # import Parent in child.py 👈👈
class Child(Base):
__tablename__ = 'child'
id = Column(BigInteger, primary_key=True, autoincrement=True)
parent_id = Column(ForeignKey('parent.id'), nullable=False)
name = Column(String(60), nullable=False)
parent = relationship('Parent')
Why this worked
The order in which models get loaded is not fixed in SQLAlchemy.
So, in my case, Child was being loaded before Parent. Hence, SQLAlchemy can't find what is Parent. So, we just imported Parent before Child gets loaded.
Namaste 🙏
I've solved the same error by inheriting a 'db.Model' instead of 'Base'... but I'm doing the flask
Eg:
from flask_sqlalchemy import SQLAlchemy
db = SQLAlchemy()
class someClass(db.Model):
someRelation = db.relationship("otherClass")
Also, even though this doesn't apply to the OP, for anyone landing here having gotten the same error, check to make sure that none of your table names have dashes in them.
For example, a table named "movie-genres" which is then used as a secondary in a SQLAlchemy relationship will generate the same error "name 'movie' is not defined", because it will only read as far as the dash. Switching to underscores (instead of dashes) solves the problem.
My Solution
One models file, or even further, if you need.
models.py
from sqlalchemy import Boolean, BigInteger, Column, DateTime, Float, ForeignKey, BigInteger, Integer, String
from sqlalchemy.orm import relationship
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
from .parent import Parent
from .child import Child
parent.py
from sqlalchemy import Boolean, BigInteger, Column, DateTime, Float, ForeignKey, BigInteger, Integer, String
from sqlalchemy.orm import relationship
from sqlalchemy.ext.declarative import declarative_base
#Base = declarative_base()
class Parent(Base):
__tablename__ = 'parent'
id = Column(BigInteger, primary_key=True, autoincrement=True)
name = Column(String(60), nullable=False, unique=True)
number = Column(String(45), nullable=False)
child.py
from sqlalchemy import Boolean, BigInteger, Column, DateTime, Float, ForeignKey, BigInteger, Integer, String
from sqlalchemy.orm import relationship
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class Child(Base):
__tablename__ = 'child'
id = Column(BigInteger, primary_key=True, autoincrement=True)
parent_id = Column(ForeignKey('parent.id'), nullable=False)
name = Column(String(60), nullable=False)
parent = relationship('Parent')
Why this worked
Same Deepam answer, but with just one models.py file to import another models
I had a different error, but the answers in here helped me fix it.
The error I received:
sqlalchemy.exc.InvalidRequestError: When initializing mapper mapped class Parent->parents, expression 'Child' failed to locate a name ('Child'). If this is a class name, consider adding this relationship() to the <class 'parent.Parent'> class after both dependent classes have been defined.
My set-up is similar toDeepam's answer.
Briefly what I do different:
I have multiple separate .py files for each db.Model.
I use a construct/fill database .py file that pre-fills db.Model objects in either Multi-threading or single threading way
What caused the error:
Only in multi-threaded set up the error occured
This construct/fill .py script did import Parent, but not Child.
What fixed it:
Adding an import to Child fixed it.
I had yet another solution, but this helped clue me in. I was trying to implement versioning, from https://docs.sqlalchemy.org/en/14/orm/examples.html#versioning-objects using the "history_mapper" class.
I got this same error. All I had to do to fix it was change the order in which my models were imported.
Use back_populates for relationship mapping in both models.
Also keep in mind to import both the models in the models/__init__.py
Base = declarative_base()
class Item(Base):
__tablename__ = 'item'
id = Column(BigInteger, primary_key=True)
# ... skip other attrs ...
auctions = relationship('Auction', back_populates='item')
class Auction(Base):
__tablename__ = 'auction'
id = Column(BigInteger, primary_key=True)
# ... skipped ...
item_id = Column('item', BigInteger, ForeignKey('item.id'))
item = relationship('Item', back_populates='auctions')