I have a problem that I can't solve((( Sample code below...
from sqlalchemy import Column, ForeignKey
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship
from sqlalchemy.types import Integer, String, Text
Base = declarative_base()
class Model(Base):
__abstract__ = True
#classmethod
def need_run(cls):
pass
class Profile(Model):
__tablename__ = 'profile'
id = Column('id', Integer, primary_key=True)
email = Column('email', String(length=128), unique=True, index=True)
password = Column('password', String(length=255), index=True)
messages = relationship('Message', lazy='select')
class Message(Model):
__tablename__ = 'message'
id = Column('id', Integer, primary_key=True)
profile_id = Column('profile_id', Integer, ForeignKey('profile.id', ondelete='CASCADE'))
title = Column('title', String(length=128), unique=True, index=True)
body = Column('text', Text, index=True)
Through the message attribute, i want to access the Message class to run the need_run method
Through the profile_id attribute, i want to access the Profile class to run the need_run method
Is it possible?
It should be possible but you probably want to change your relationship to include a backref:
class Profile(Model):
#...
messages = relationship('Message', lazy='select', backref="profile")
Then you can access messages from a profile like:
for msg in some_profile.messages:
msg.need_run()
And you can access a profile from a message like:
some_message.profile.need_run()
Related
I am trying to use ORM with SQLAlchemy in Python. My current solution fails and throws an exception right in the moment the ORM is first used. I receive the following exception:
sqlalchemy.exc.InvalidRequestError: When initializing mapper mapped class Event->event, expression 'Tag' failed to locate a name ('Tag'). If this is a class name, consider adding this relationship() to the <class 'backend.source.database.event.Event'> class after both dependent classes have been defined.
My classes are defined like in the offical SQLAlchemy-Documentation (https://docs.sqlalchemy.org/en/14/orm/basic_relationships.html#many-to-many), which is why I am kinda confused about that error.
association = Table('event_to_tag', declarative_base().metadata,
Column('event_id', Integer, ForeignKey('event.id'), primary_key=True),
Column('tag_id', Integer, ForeignKey('tag.id'), primary_key=True))
class Event(declarative_base()):
__tablename__ = "event"
id = Column(Integer, primary_key=True)
title = Column(String(255))
location = Column(String(255))
organizer_id = Column(Integer, ForeignKey(Organizer.id))
start = Column(DateTime)
end = Column(DateTime)
lang = Column(String(255))
costs = Column(DECIMAL)
registration = Column(TINYINT)
url = Column(String(255))
description = Column(Text)
tags = relationship("Tag", secondary=association, back_populates="events")
class Tag(declarative_base()):
__tablename__ = "tag"
id = Column(Integer, primary_key=True)
name = Column(String(255))
events = relationship("Event", secondary=association, back_populates="tags")
Thank you, greetings
I think you need to define a Base = declarative_base(), need to use in your models and associations.
from sqlalchemy import Column, Integer, ForeignKey, String, DECIMAL, Text, DateTime, Table, create_engine
from sqlalchemy.dialects.mssql import TINYINT
from sqlalchemy.dialects.postgresql import UUID
from sqlalchemy.orm import declarative_base, relationship, scoped_session, sessionmaker
Base = declarative_base()
association = Table('event_to_tag',
Base.metadata,
Column('event_id', Integer, ForeignKey('events.id'), primary_key=True),
Column('tag_id', Integer, ForeignKey('tags.id'), primary_key=True))
class Event(Base):
__tablename__ = "events"
id = Column(Integer, primary_key=True)
title = Column(String(255))
location = Column(String(255))
# organizer_id = Column(Integer, ForeignKey(Organizer.id))
start = Column(DateTime)
end = Column(DateTime)
lang = Column(String(255))
costs = Column(DECIMAL)
registration = Column(UUID)
url = Column(String(255))
description = Column(Text)
tags = relationship("Tag", secondary=association, back_populates="events")
class Tag(Base):
__tablename__ = "tags"
id = Column(Integer, primary_key=True)
name = Column(String(255))
events = relationship("Event", secondary=association, back_populates="tags")
class CreateEngine:
def __init__(self):
self.connection_string = "postgresql+psycopg2://<user_name>:<password>#127.0.0.1/<table_name>"
self.engine = create_engine(self.connection_string)
def create_table(self):
return Base.metadata.create_all(self.engine)
def create_session(self):
session_factory = sessionmaker(bind=self.engine)
Session = scoped_session(session_factory)
with Session() as session:
pass
if __name__ == "__main__":
CreateEngine().create_table()
On this question I learned how to set the schema on an ORM definition:
from sqlalchemy import Column, Integer, String
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class Notification(Base):
__tablename__ = "dog"
__table_args__ = {"schema": "animal"}
id = Column(Integer, primary_key=True)
name = Column(String)
But I now need to make the schema configurable. I have tried passing the table_args parameter at object creation, but it's still trying the schema I put on the class definition.
The better solution I have found so far is to create a function that returns the class:
function get_notification_class(schema: str):
class Notification(Base):
__tablename__ = "dog"
__table_args__ = {"schema": schema}
id = Column(Integer, primary_key=True)
name = Column(String)
return Notification
And then, to use it
Notification = get_notification_class('animal')
obj = Notification('1', 'doggy')
I have two very simple models. In my Post model there are supposed to be two relationships into the User table. One is for the owner of the post and one is for the last editor of the post. They can be different values, but both refer to the same User table.
My models are set up like this
class Post(Base):
last_editor_id = Column(BigInteger, ForeignKey('users.id'), nullable=True)
last_editor = relationship('User', backref='posts', foreign_keys=[last_editor_id])
owner_id = Column(BigInteger, ForeignKey('users.id'), nullable=False, index=True)
owner = relationship('User', backref='posts', foreign_keys=[owner_id])
class User(Base):
'''This represents a user on the site'''
__tablename__ = 'users'
id = Column(BigInteger, primary_key=True, unique=True)
name = Column(BigInteger, nullable=False)
When I attempt to create these models though, I get the following error
sqlalchemy.exc.ArgumentError: Error creating backref 'posts' on relationship 'Post.owner': property of that name exists on mapper 'Mapper|User|users'
How do I correct this so that I can maintain both forgeign keys in the Post model?
The error is telling you that you've used post as a name more then once for your backrefs, all you need to do is give the backref's unique names. Here's a complete example-- I've added a id primary key to the Post class, and also some __repr__s so we get some readable output.
from sqlalchemy import create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, BigInteger, ForeignKey, Integer
from sqlalchemy.orm import relationship, sessionmaker
Base = declarative_base()
engine = create_engine('sqlite://') ## In Memory.
Session = sessionmaker()
Session.configure(bind=engine)
session = Session()
class Post(Base):
__tablename__ = 'post'
id = Column(Integer, primary_key=True)
last_editor_id = Column(BigInteger, ForeignKey('users.id'), nullable=True)
last_editor = relationship('User', backref='editor_posts', foreign_keys=[last_editor_id])
owner_id = Column(BigInteger, ForeignKey('users.id'), nullable=False, index=True)
owner = relationship('User', backref='owner_posts', foreign_keys=[owner_id])
def __repr__(self):
return '<Post: {}>'.format(self.id)
class User(Base):
'''This represents a user on the site'''
__tablename__ = 'users'
id = Column(BigInteger, primary_key=True, unique=True)
name = Column(BigInteger, nullable=False)
def __repr__(self):
return '<User: {}>'.format(self.name)
Base.metadata.create_all(engine)
bob = User(name='Bob', id=1)
alice = User(name='Alice', id=2)
post = Post(owner=alice, last_editor=bob, id=1)
session.add(post)
session.commit()
bob = session.query(User).get(1)
print bob
# <User: Bob>
print bob.editor_posts
# [<Post: 1>]
print bob.owner_posts
# []
post = session.query(Post).get(1)
print post.owner
# <User: Alice>
print post.last_editor
# <User: Bob>
Now when you query a user, you can ask that object user.owner_posts or user.editor_posts.
In general it's a naming Problem of the backref.
Since 1:n relationships are sometimes a bit confusing, I set the relationship attribute
always on the singular site, to avoid confusion.
then the backref name is always singular. and the relationship attribute is always in the Class where the foreignkey is referencing to.
Now to my suggestion for the fixed code:
class Post(Base):
last_editor_id = Column(BigInteger, ForeignKey('users.id'), nullable=True)
owner_id = Column(BigInteger, ForeignKey('users.id'), nullable=False, index=True)
class User(Base):
'''This represents a user on the site'''
__tablename__ = 'users'
id = Column(BigInteger, primary_key=True, unique=True)
name = Column(BigInteger, nullable=False)
owned_posts = relationship('Post', backref='owner')
edited_posts = relationship('Post', backref='last_editor')
Now you can get all the owned posts of a User with User.owned_posts and all owners of a post with Post.owner. Same with the last_edited attribute.
For additional info you could read the docs how to set up relationships
How can I get the required validator in SQLAlchemy? Actually I just wanna be confident the user filled all required field in a form. I use PostgreSQL, but it doesn't make sense, since the tables created from Objects in my models.py file:
from sqlalchemy import (
Column,
Integer,
Text,
DateTime,
)
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import (
scoped_session,
sessionmaker,
)
from zope.sqlalchemy import ZopeTransactionExtension
from pyramid.security import (
Allow,
Everyone,
)
Base = declarative_base()
class Article(Base):
""" The SQLAlchemy declarative model class for a Article object. """
__tablename__ = 'article'
id = Column(Integer, primary_key=True)
name = Column(Text, nullable=False, unique=True)
url = Column(Text, nullable=False, unique=True)
title = Column(Text)
preview = Column(Text)
content = Column(Text)
cat_id = Column(Integer, nullable=False)
views = Column(Integer)
popular = Column(Integer)
created = Column(DateTime)
def __unicode__(self):
return unicode(self.name)
So this nullable=False doesn't work, because the records added in any case with empty fields. I can of course set the restrictions at the database level by set name to NOT NULL for example. But there must be something about validation in SQLAlchemy isn't it? I came from yii php framework, there it's not the problem at all.
By empty fields I guess you mean an empty string rather than a NULL. A simple method is to add validation, e.g.:
class Article(Base):
...
name = Column(Text, unique=True)
...
#validates('name')
def validate_name(self, key, value):
assert value != ''
return value
To implement it at a database level you could also use a check constraint, as long as the database supports it:
class Article(Base):
...
name = Column(Text, CheckConstraint('name!=""')
...
Edit: I would like to model a 1 to 0:1 relationship between User and Comment (a User can have zero or one Comment). Instead of accessing the object Comment I would rather directly access the comment itself. Using SQLAlchemys association_proxy works perfect for that scenario except for one thing: accessing User.comment before having a Comment associated. But in this case I would rather expect None instead of AttributeError as result.
Look at the following example:
import sqlalchemy as sa
import sqlalchemy.orm as orm
from sqlalchemy import Column, Integer, Text, ForeignKey, Table
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.ext.associationproxy import association_proxy
Base = declarative_base()
class User(Base):
__tablename__ = 'users'
id = Column(Integer, primary_key=True)
name = Column(Text)
def __init__(self, name):
self.name = name
# proxy the 'comment' attribute from the 'comment_object' relationship
comment = association_proxy('comment_object', 'comment')
class Comment(Base):
__tablename__ = 'comments'
id = Column(Integer, primary_key=True)
comment = Column('comment', Text, nullable=False, default="")
user_id = Column(ForeignKey('users.id'), nullable=False, unique=True)
# user_id has to be unique to ensure that a User can not have more than one comments
def __init__(self, comment):
self.comment = comment
user_object = orm.relationship(
"User",
uselist=False, # added after edditing the question
backref=orm.backref('comment_object', uselist=False)
)
if __name__ == "__main__":
engine = sa.create_engine('sqlite:///:memory:', echo=True)
Session = orm.sessionmaker(bind=engine)
Base.metadata.create_all(engine)
session = Session()
Now, the following code throws an AttributeError:
u = User(name="Max Mueller")
print u.comment
What would be the best way to catch that exception and provide a default value instead (like an empty string)?
You don't really need association_proxy for this. You could really get by just fine with a regular property. The AttributeError is (probably) caused because the comment_object is itself None, since there is no dependent row, and None has no comment attribute.
class User(Base):
__tablename__ = 'users'
id = Column(Integer, primary_key=True)
name = Column(Text)
def __init__(self, name):
self.name = name
# proxy the 'comment' attribute from the 'comment_object' relationship
#property
def comment(self):
if self.comment_object is None:
return ""
else:
return self.comment_object.comment
#comment.setter
def comment(self, value):
if self.comment_object is None:
self.comment_object = Comment()
self.comment_object.comment = value
Try this
import sqlalchemy as sa
import sqlalchemy.orm as orm
from sqlalchemy import Column, Integer, Text, ForeignKey, Table
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.ext.associationproxy import association_proxy
Base = declarative_base()
class User(Base):
__tablename__ = 'users'
id = Column(Integer, primary_key=True)
name = Column(Text)
def __init__(self, name):
self.name = name
# proxy the 'comment' attribute from the 'comment_object' relationship
comment = association_proxy('comment_object', 'comment')
class Comment(Base):
__tablename__ = 'comments'
id = Column(Integer, primary_key=True)
comment = Column('comment', Text, nullable=False, default="")
user_id = Column(ForeignKey('users.id'), nullable=False)
def __init__(self, comment):
self.comment = comment
user_object = orm.relationship(
"User",
backref=orm.backref('comment_object'),
uselist=False
)
if __name__ == "__main__":
engine = sa.create_engine('sqlite:///:memory:', echo=True)
Session = orm.sessionmaker(bind=engine)
Base.metadata.create_all(engine)
session = Session()
u = User(name="Max Mueller")
# comment = Comment("")
# comment.user_object = u
# session.add(u)
# session.commit()
print "SS :", u
print u.comment
You gave uselist in backref which must be in relationship.
I do not see any answer that would solve the issue and also would work with "sort_by" for example.
Maybe it is just better to use 'column_property", see Order by association proxy: invalid sql.