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')
Related
Basing this question off this similar post, but about SORT ORDER
I understand that you can change the lazy to dynamic in the relatonship, and then that will allow you to query against the relationship before loading, but is there a way to LIMIT the return results directly from a selectin or on of the other loading techniques?
Use case is, Im trying to pass the record into Marshmallow and limit the number of nested records returned. dynamic at that point doesnt work, as Marshmallow includes it as an all() and selectin appears to just included it unqueryable at time of load, and again Marshmallow get the entire record set.
from sqlalchemy import Column, Integer, ForeignKey
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship
Base = declarative_base()
class Example(Base):
__tablename__ = 'examples'
id = Column(Integer, primary_key=True)
related_items = relationship('RelatedItem', back_populates='example', order_by='RelatedItem.id')
class RelatedItem(Base):
__tablename__ = 'related_items'
id = Column(Integer, primary_key=True)
example_id = Column(Integer, ForeignKey('examples.id'), nullable=False)
example = relationship('Example', back_populates='related_items')
I'm trying to set up a database with a few specific fields (and I can't move away from the specification). One of the fields would be a column called metadata, but sqlalchemy prevents that:
sqlalchemy.exc.InvalidRequestError: Attribute name 'metadata' is reserved for the MetaData instance when using a declarative base class.
Is there a decent workaround for this? Do I need to monkeypatch the declarative_base function to rename the metadata attribute? I couldn't find an option to rename that attribute in the api docs.
Here's some example code that will fail with the above error:
#!/usr/bin/env python3.7
from sqlalchemy.ext.declarative import declarative_base, declared_attr
from sqlalchemy import Column, Integer
class CustomBase(object):
#declared_attr
def __tablename__(cls):
return cls.__name__.lower()
DBBase = declarative_base(cls=CustomBase)
class Data(DBBase):
id = Column(Integer, primary_key=True)
metadata = Column(Integer)
if __name__ == "__main__":
print(dir(Data()))
You can use like:
class Data(DBBase):
id = Column(Integer, primary_key=True)
# metadata = Column(Integer)
metadata_ = Column("metadata", Integer)
The constructor of Column class has a name parameter. You can find it from https://docs.sqlalchemy.org/en/13/core/metadata.html#sqlalchemy.schema.Column
The name field may be omitted at construction time and applied later
In other words, you could write a name as you want originally.
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.
I have some standard SQLAlchemy models that I reuse across projects. Something like this:
from sqlalchemy import Column, Integer, String, Unicode
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class Category(Base):
__tablename__ = 'category'
id = Column(Integer, primary_key=True)
slug = Column(String(250), nullable=False, unique=True)
title = Column(Unicode(250), nullable=False)
def __call__(self):
return self.title
I'd like to put this in a shared library and import it into each new project instead of cutting and pasting it, but I can't, because the declarative_base instance is defined separately in the project. If there's more than one, they won't share sessions. How do I work around this?
Here's another question that suggests using mixin classes. Could that work? Will SQLAlchemy accurately import foreign keys from mixin classes?
When you call
Base = declarative_base()
SA create new metadata for this Base.
To reuse your models you must bind metadata of main models to reusable models, but before any import of your reusable models by:
Base.metadata = my_main_app.db.metadata
MixIn classes useful for repeating column declarations, and extending class methods.
For connecting reusable apps based on MixIns you must define concrete class in code manualy for each model.
Will SQLAlchemy accurately import
foreign keys from mixin classes?
MixIn class with foreign key and constraint
from sqlalchemy.schema import UniqueConstraint
from sqlalchemy.ext.declarative import declared_attr
class MessageMixIn(object):
ttime = Column(DateTime)
#declared_attr
def sometable_id(cls):
return Column(Integer, ForeignKey('sometable.id'))
#declared_attr
def __table_args__(cls):
return (UniqueConstraint('sometable_id', 'ttime'), {})
I want to dynamically get the SQLAlchemy mapper class that instances of InstrumentedList use.
I have a 1-M ParentClass-ChildClass (let's call the column myRelation), and the parentInstance.myRelation are InstrumentedList instances. I could hack it and grab the class of the first instance in InstrumentedList, but this doesn't work if there are no objects in the InstrumentedList.
The reason: I need to append a Python dictionary containing the properties of mapper class X to the InstrumentedList, but I don't know the mapper class at runtime. Since I can't append a dict, I need to get the mapper class.
Thanks.
the relationship is associated with the instrumented list via property. so start with this plain vanilla mapping:
from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class A(Base):
__tablename__ = "a"
id = Column(Integer, primary_key=True)
bs = relationship("B")
class B(Base):
__tablename__ = "b"
id = Column(Integer, primary_key=True)
a_id = Column(Integer, ForeignKey('a.id'))
in any recent version of SQLAlchemy you can see it like this:
print A.bs.property.mapper.class_
in 0.8 there's a little more API available and you can do this:
from sqlalchemy import inspect
print inspect(A.bs).mapper.class_
docs for inspect-> http://docs.sqlalchemy.org/en/rel_0_8/core/inspection.html
docs for "mapper->class_" -> http://docs.sqlalchemy.org/en/rel_0_8/orm/mapper_config.html#sqlalchemy.orm.mapper.Mapper.class_