I'm new to SQLAlchemy and Flask. I'm trying to create an object (book) with m2m relation and append an existed object (tag_2) to the relation:
book = Book(title='title')
tag_1 = Tag(name='tag')
book.tags.append(tag_1) # New tag works well
tag_2 = Tag.query.get(123) # Get existed tag by id
print(tag_2) # >>> Tag #123
book.tags.append(tag_2) # ERROR: Object '<Tag at ...>' is already attached to session '1'
self.session.add(book)
self.session.commit()
Have no problems creating new related objects, but can't point the existing object.
My models:
book_tags = db.Table('book_tags', db.metadata,
db.Column('tag_id', db.Integer, db.ForeignKey('tag.id')),
db.Column('book_id', db.Integer, db.ForeignKey('book.id'))
)
class Tag(db.Model):
id = db.Column(db.Integer, primary_key=True)
name = db.Column(db.String(50), unique=True)
class Book(db.Model):
id = db.Column(db.Integer, primary_key=True)
title = db.Column(db.String(300), nullable=False)
tags = db.relationship('Tag', secondary=book_tags)
I use sqlite as DB.
It seems that you use different self.session for getting Tag and for getting Book you are then trying to append Tag to. In order to be able to append object A to object B they must both exist in same session.
It's hard to tell how you create your session because your post only shows code that operates with it but make sure you have only one session.
tags = db.relationship(
'Tag',
secondary=book_tags,
backref=db.backref('book', lazy='dynamic')
)
You need to add backref in m2m relationship because
Using backref just automates the creation of a relationship property at the other end. backref='book' is somewhat akin to having book = db.relationship('book') explicitly in the Tag class (+ back population). Using the backref() object you can pass arguments to that relationship.
Why this code returns an error?
Error: When initializing mapper Mapper|Pessoa|pessoa, expression 'Imovel' failed to locate a name ("name 'Imovel' is not defined").
from flask_sqlalchemy import SQLAlchemy
from flask_marshmallow import Marshmallow
db=SQLAlchemy()
ma=Marshmallow()
class Pessoa(db.Model):
__tablename__ = 'pessoa'
idLocal= db.Column(db.Integer, primary_key=True)
Nome=db.Column(db.String(100), default=u'')
imovelList = db.relationship("Imovel", back_populates="PessoaList")
def get_id(self):
return self.idLocal
class PessoaSchema(ma.ModelSchema):
class Meta: model = Pessoa
class Imovel(db.Model):
__tablename__ = 'imovel'
idLocal= db.Column(db.Integer, primary_key=True)
CodigoImovel=db.Column(db.String(30), default=u'')
idPessoa = db.Column(db.Integer, db.ForeignKey('pessoa.idLocal'))
PessoaList = db.relationship("Pessoa", back_populates="imovelList")
def get_id(self):
return self.idLocal
class ImovelSchema(ma.ModelSchema):
class Meta: model = Imovel
You have an 'order of declaration' issue. Relationships are initialized immediately when their Mappers are constructed when the relationship is defined with a String. But when you define the relationship on "Imovel" you have yet to declare a Mapper called "Imovel". The Imovel Mapper or class is defined a couple lines after that.
So you could move the Imovel Mapper above the Pessoa Mapper, except then you would get the exact same error as you are also building a relationship from Imovel to Pessoa.
So instead you want to declare your relationship using a callable function that will return the "Imovel" Mapper. This function will typically only be called after all Mappers are constructed. Thus by using a lambda function we can ensure the relationship is not invoked until you have had a chance to setup the Imovel class.
In practice, to fix this error, replace this line
imovelList = db.relationship("Imovel", back_populates="PessoaList")
with this
imovelList = db.relationship(lambda: Imovel, back_populates="PessoaList")
I've got two models, Company and Employee, in a many-to-one relationship. They are defined in different Flask blueprints. I'm trying to add a cascade, which makes me need to define a relationship on Company (instead of just a relationship on Employee with backref set).
company_blueprint/models.py:
class Company(Base):
id = Column(Integer, primary_key=True)
employees = relationship("Employee", back_populates="company", cascade="all")
employee_blueprint/models.py:
from app.company_blueprint.models import Company
class Employee(Base):
name = Column(String)
company_id = Column(ForeignKey(Company.id))
company = relationship("company", back_populates="employees")
The problem is, when I try to delete a company in company_blueprint/views.py, the Employee model is not loaded. I get:
sqlalchemy.exc.InvalidRequestError: When initializing mapper Mapper|Company|company, expression 'Employee' failed to locate a name ("name 'Employee' is not defined"). If this is a class name, consider adding this relationship() to the <class 'app.company_model.models.Company'> class after both dependent classes have been defined.
I could try to import Employee in company_blueprint.models, but then I'm running into a circular import problem.
How do I fix this?
Edit: Thanks to Paradoxis, for now I've settled on the following:
Using strings to refer to foreign key columns, e.g. company_id = Column(ForeignKey("company.id"))
In my app.py, first import all models before anything else, i.e.
-
import flask
import app.employee_blueprint.models
import app.company_blueprint.models
# import other views, modules, etc.
This still feels a bit awkward.
I am using such relationships in most of the model classes. According to my understanding, you don't need any special thing or relationship in both classes. Simply add a relationship in parent model class and add a backref, it will work.
class Company(Base):
id = Column(Integer, primary_key=True)
employees = relationship("Employee", backref="company", cascade="all, delete-orphan")
Is it possible to get the name of our custom attributes of a class in Python ? For instance, here's my class :
class User(db.Model):
__tablename__ = 'users'
id = db.Column(db.Integer, primary_key = True)
login = db.Column(db.String(100))
password = db.Column(db.String(100))
first_name = db.Column(db.String(100))
last_name = db.Column(db.String(100))
email = db.Column(db.String(100))
age = db.Column(db.Integer)
sex = db.Column(db.String(10))
What I want is to get the list of my class attributes (and only those that I defined !). I was thinking about using dir(self) and filtering on those not starting with __ but it's not really revelant because there are other fields who are built-in such as metadata, query and so on.
I saw a function getattr (or getattribute) but it's only for a given field.
I don't want to use a dict of keys because it have to stay generic and I don't want to modify the dict everytime I add a field.
As I'm using SqlAlchemy ORM, I got this when trying self.__dict__ :
{'_sa_instance_state': <sqlalchemy.orm.state.InstanceState object at 0x7ffbcf252050>}
I also tried a lot of things such as those described here :
Python dictionary from an object's fields but nothing worked.
Does anyone have a solution ?
Thanks !
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')