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CompileError with inherited parameter in sql-alchemy table class

I Have a SQLalchemy class (flask-sqlalchemy) which inherits from another class and looks like this
class Cat(db.Model, Category):
__tablename__ = 'Cat'
id = db.Column(db.String(1000), primary_key=True, nullable=False)
parent_id = db.Column(db.String(1000), db.ForeignKey('SocArxivCategory.id'))
parent_category = db.relationship('Cat',
backref=db.backref('children',
order_by=Category.name,
lazy='selectin'),
remote_side='Cat.id')
and the parent class is
class Category():
__tablename__ = 'Category'
name = db.Column(db.String(2000), nullable=False)
You see that the Cat table builds a relationship using the Category.name attribute. But this seems to cause an error
....
File "/usr/local/lib/python2.7/site-packages/sqlalchemy/sql/compiler.py", line 692, in _fallback_column_name
raise exc.CompileError("Cannot compile Column object until "
CompileError: Cannot compile Column object until its 'name' is assigned.
so it seems to try to build the relationship before inheriting the name attribute? I am not sure whether this is an SQLalchemy issue or a Python issue? Any idea how to fix this?

Ok after a bit of searching and more consulting of the sqlalchemy docs I found a solution. Changing the definition of the relationship to
parent_category = db.relationship('Cat',
backref=db.backref('children',
order_by=lambda: Cat.name,
lazy='selectin'),
remote_side='Cat.id')
did the trick

Creating second model upon instantiation of the first within sqlachemy

I am currently working with some legacy code that looks as follows:
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Unicode
from sqlalchemy.dialects.postgresql import ARRAY, TEXT
Base = declarative_base()
class Book(Base):
__tablename__ = 'book'
id = Column(Integer, primary_key=True)
title = Column(Unicode)
keywords = Column('keywords', ARRAY(TEXT), primary_key=False)
The keywords are currently being kept as an array, but I'd like to flatten this out and have them be in their own separate model
class Keyword():
__tablename__ = 'keyword'
id = Column(Integer, primary_key=True)
book_id = Column(Integer, ForeignKey('book.id', ondelete='cascade'),
nullable=False)
keyword = Column(Unicode)
How can I make it such that when a Book() is created, it also creates the
accompanying keywords? As an intermediate step for migrating the API, I'd like to keep the current array column, but also have the accompanying Keyword() instances be created.
I could do this within an __init__ method, but would need to know what the current Session() was, in order to run a commit. I could also perhaps use a property attribute, attached to keywords, but am not sure how that would work given that I am working with a class that inherits from SQLAlchemy's base, and not with a regular class that I have defined. What's the correct way to do this?

You can use object_session to find out the session of a given instance.
But if you define relationship between a Book and Keywords, you should not need even bother:
class Book(Base):
# ...
rel_keywords = relationship('Keyword', backref='book')
def init_keyword_relationship(self):
for kw in self.keywords:
self.rel_keywords.add(Keyword(keyword=kw))
sess = # ... get_session...
books = sess.query(Book).all()
for book in books:
book.init_keyword_relationship()
sess.commit()
However, I would do a migration once and get rid of the keywords array in order not to add a logic to keep those in sync.

SQLAlchemy Model Circular Import

I have two models in the same module named models. They are a 1-1 relationship and have been configured per the SQLAlchemy docs.
Vehicle.py
from models.AssetSetting import AssetSetting
class Vehicle(Base):
__tablename__ = 'vehicles'
vehicle_id = Column(Integer, primary_key=True)
...
settings = relationship('AssetSetting', backref=backref('asset_settings'))
AssetSetting.py
from models.Vehicle import Vehicle
class AssetSetting(Base):
__tablename__ = 'asset_settings'
asset_alert_setting_id = Column(Integer, primary_key=True, autoincrement=True)
...
vehicle = relationship('vehicles', foreign_keys=Column(ForeignKey('vehicles.vehicle_id')))
If I use the string relationship building (i.e. ForeignKey('vehicles.vehicle_id')) I get the error:
sqlalchemy.exc.InvalidRequestError:
When initializing mapper Mapper|AssetSetting|asset_settings, expression 'vehicles' failed to locate a name ("name 'vehicles' is not defined").
If this is a class name, consider adding this relationship() to the <class 'models.AssetSetting.AssetSetting'> class after both dependent classes have been defined.
If I use the class mapping, I get the classic circular import error:
Traceback (most recent call last):
File "tracking_data_runner.py", line 7, in <module>
from models.Tracker import Tracker
File "/.../models/Tracker.py", line 5, in <module>
from models.Vehicle import Vehicle
File "/.../models/Vehicle.py", line 13, in <module>
from models.Tracker import Tracker
ImportError: cannot import name 'Tracker'
I believe I could fix this issue by putting the files in the same package but would prefer to keep them separate. Thoughts?

To avoid circular import errors, you should use string relationship building, but both of your models have to use the same Base - the same declarative_base instance. Instantiate your Base once and use it when initializing both Vehicle and AssetSetting.
Or, you may explicitly map the table names and classes to help mapper relate your models:
Base = declarative_base(class_registry={"vehicles": Vehicle, "asset_settings": AssetSetting})

I discovered my problem was two fold:
I was referencing Vehicles improperly in my relationship. It should be relationship('Vehicle' not relationship('vehicles'
Apparently it is improper to declare the FK inside the relationship as I did in AssetSettings.py (foreign_keys=Column(ForeignKey('vehicles.vehicle_id'))). I had to declare the FK and then pass it in to the relationship.
My configurations look like this now:
Vehicle.py
class Vehicle(Base, IDiagnostable, IUsage, ITrackable):
__tablename__ = 'vehicles'
vehicle_id = Column(Integer, primary_key=True)_id = Column(Integer)
settings = relationship('AssetSetting', backref=backref('asset_settings'))
AssetSetting.py
class AssetSetting(Base):
__tablename__ = 'asset_settings'
asset_alert_setting_id = Column(Integer, primary_key=True, autoincrement=True)
vehicle_id = Column(ForeignKey('vehicles.vehicle_id'))
vehicle = relationship('Vehicle', foreign_keys=vehicle_id)

Your __tablename__ is referencing vehicles but your Foreign Key is referencing vehicle.vehicle_id

NoForeignKey error on relationship even though SQLAlchemy model specifies foreign key

I am having some difficulty setting up a one to one relationship between two models in my flask application. I have two models Employeeand `Photo'. An employee has only one photo associated with it and vice-versa.
This is the code that I have in my models.py file:
class Employee(db.Model):
__tablename__ = 'employees'
id = db.Column(db.Integer, primary_key=True)
photo = db.relationship("Photo", uselist=False, back_populates='employees')
class Photo(db.Model):
__tablename__ = 'photos'
id = db.Column(db.Integer, primary_key=True)
employee_id = db.Column(db.Integer, db.ForeignKey('employees.id'))
employee = db.relationship('Photo', back_populates='photo')
I've followed the instruction on the SQL Alchemy documentation found hereSQL Alchemy simple relationships. The error that I keep encountering is shown below:
sqlalchemy.exc.NoForeignKeysError: Could not determine join condition between parent/child tables on relationship Photo.employee
- there are no foreign keys linking these tables.
Ensure that referencing columns are associated with a ForeignKey or ForeignKeyConstraint, or specify a 'primaryjoin' expression.
I clearly specify the foreign key right here employee_id = db.Column(db.Integer, db.ForeignKey('employees.id')) . I'm not sure what I'm doing wrong. Additionally, I was reading the documentation and it doesn't help that uselist, backref, and back_populates are so similar.
Can someone assist me with this? Help would be greatly appreciated.
One to One relationship stack overflow question

backref automatically adds the reverse relationship to the related model. You can pass a db.backref object to it to specify options to the relationship. back_populates tells SQLAlchemy to populate an existing reverse relationship, rather than creating it. uselist tells SQLAlchemy whether the relationship is a list or not, for cases where it can't determine that automatically.
In your example, you need one relationship, with one backref that is a single item.
You have two typos in your code. First, back_populates='employees' should refer to 'employee', which is what you called the property on the related model. Second, employee = relationship('Photo' is pointing at the wrong model, it should relate to Employee.
from flask import Flask
from flask_sqlalchemy import SQLAlchemy
app = Flask(__name__)
db = SQLAlchemy(app)
db.engine.echo = True
class Photo(db.Model):
id = db.Column(db.Integer, primary_key=True)
class Employee(db.Model):
id = db.Column(db.Integer, primary_key=True)
photo_id = db.Column(db.ForeignKey(Photo.id))
photo = db.relationship(Photo, backref=db.backref('employee', uselist=False))
db.create_all()
db.session.add(Employee(photo=Photo()))
db.session.commit()
print(Employee.query.get(1).photo)

SQLAlchemy cannot find a class name

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')