My database structure is something like this (I'm using declarative style):
class Character(Base):
__tablename__="characters"
id = Column(Integer, primary_key=True)
name = Column(String)
player = Column(String)
inventory = relation(Inventory)
class Item(Base):
__tablename__="items"
id = Column(Integer, primary_key=True)
name = Column(String)
class Inventory(Base):
__tablename__="inventory"
id = Column(Integer, primary_key=True)
char_id = Column(Integer, ForeignKey("characters.id"))
item_id = Column(Integer, ForeignKey("characters.id"))
quantity = Column(Integer)
item = relation(Item)
My problem is that when I remove an "Inventory" object from "Character.inventory" this isn't updated until the session get committed. For example:
>>> torch_inv=character.inventory[0] # Inventory object referred to a torch
>>> torch_inv.item, torch_inv.quantity
(<Item object, torch>, 3)
>>> session.delete(torch_inv)
>>> character.inventory[0]
<Inventory object, torch>
I've seen that there is a relation option "cascade" but I can't find a way to make it working in this case.
Session.delete() method just marks an instance as "to be deleted", so your relation won't change untill you flush changes to database independent on cascade rules. On other hand you can just remove Inventory instance from character.inventory collection, then having 'delete-orphan' cascade rule will mark removed Inventory instance for deletion.
Related
I am setting up a Sqlalchemy mapper for a sqlite database. My User class has a non-nullable relationship with my Team class. The code I already have is as follows:
class Team(Base):
__tablename__ = 'teams'
team_id = Column(Integer, primary_key=True)
# Using Integer as holder for boolean
is_local = Column(Integer, default=0)
class User(Base):
__tablename__ = 'users'
user_id = Column(Integer, primary_key=True)
team_id = Column(Integer, ForeignKey(Team.team_id), default=1, nullable=False)
team = relationship('Team')
is_local = Column(Integer, default=0)
I would like to establish that the value of User.is_local is by default the value of Team.is_local for the User's linked team.
However, after the creation of the User, I would still like the ability to modify the user's is_local value without changing the values of the team or any other user on the team.
So if I were to execute
faraway = Team(is_local=1)
session.add(faraway)
session.commit()
u = User(team=faraway)
session.add(u)
session.commit()
print(bool(u.is_local))
The result should be True
So far, I have tried context-sensitive default functions as suggested by https://stackoverflow.com/a/36579924, but I have not been able to find the syntax allowing me to reference Team.is_local
Is there a simple way to do this?
The first suggestion from SuperShoot, using a sql expression as the default appears to work. Specifically,
is_local = Column(Integer, default=select([Team.is_local]).where(Team.team_id==team_id))
gives me the logic I require.
I'm trying to accomplish the following joined inheritance with one-to-one relationship structure with SQLAlchemy: Box and Item are both Elements, and Box has an Item.
The class definitions are as follows:
class Element(Base):
__tablename__ = 'element'
id = Column(Integer, primary_key=True)
el_type = Column(String)
__mapper_args__ = dict(
polymorphic_identity='element',
polymorphic_on=el_type,
)
class Box(Element):
# Joined table inheritance: Element.
__tablename__ = 'box'
id = Column(Integer, ForeignKey('element.id'), primary_key=True)
__mapper_args__ = dict(polymorphic_identity='box')
# One-to-one relationship: Item.
item = relationship('Item',
back_populates='box', uselist=False)
def __init__(self, item_name=None):
self.item = Item(item_name)
class Item(Element):
# Joined table inheritance: Element.
__tablename__ = 'item'
id = Column(Integer, ForeignKey('element.id'), primary_key=True)
__mapper_args__ = dict(polymorphic_identity='item')
# One-to-one relationship: Box.
box_id = Column(Integer, ForeignKey('box.id'))
box = relationship('Box',
back_populates='item')
name = Column(String)
def __init__(self, name):
self.name = name
When I try to instantiate b = Box('rock'), I get:
sqlalchemy.exc.AmbiguousForeignKeysError: Could not determine join condition between parent/child tables on relationship Box.item - there are multiple foreign key paths linking the tables. Specify the 'foreign_keys' argument, providing a list of those columns which should be counted as containing a foreign key reference to the parent table.
Now I have gone to SQLAlchemy's page on this issue (great docs, I might add), but I'm not sure how to bridge the gap between their example and my situation. I thought this might fix it:
class Item(Element):
# ...
box = relationship('Box',
back_populates='item', foreign_keys=[id, box_id])
# ...
...but I get the same error.
What am I doing wrong?
UPDATE:
I have now tried using ForeignKeyConstraint constructs in an effort to more explicitly describe the desired behavior to SQLAlchemy; to no avail:
class Box(Element):
# ...
id = Column(Integer, primary_key=True)
# ...
ForeignKeyConstraint([id], ['element.id'])
# ...
class Item(Element):
# ...
id = Column(Integer, primary_key=True)
# ...
# One-to-one relationship: Box.
box_id = Column(Integer)
box = relationship('Box',
back_populates='item')
ForeignKeyConstraint([id, box_id], ['element.id', 'box.id'])
# ...
A simple AssertionError is thrown back at me (no description). Given that SQLAlchemy has a good track record of producing meaningful error messages, could this suggest that this is an unanticipated situation?
I'm trying to implement a user-facing PreviewList of Articles, which will keep its size even if an Article is deleted. So if the list has four objects [1, 2, 3, 4] and one is deleted, I want it to contain [1, 2, None, 4].
I'm using a relationship with a secondary table. Currently, deleting either Article or PreviewList will delete the row in that table. I've experimented with cascade options, but they seem to affect the related items directly, not the contents of the secondary table.
The snippet below tests for the desired behaviour: deleting an Article should preserve the row in ArticlePreviewListAssociation, but deleting a PreviewList should delete it (and not the Article).
In the code below, deleting the Article will preserve the ArticlePreviewListAssociation, but pl.articles does not treat that as a list entry.
from db import DbSession, Base, init_db
from sqlalchemy import Column, String, Integer, ForeignKey
from sqlalchemy.orm import relationship
session = DbSession()
class Article(Base):
__tablename__ = 'articles'
id = Column(Integer, primary_key=True)
title = Column(String)
class PreviewList(Base):
__tablename__ = 'preview_lists'
id = Column(Integer, primary_key=True)
articles = relationship('Article', secondary='associations')
class ArticlePreviewListAssociation(Base):
__tablename__ = 'associations'
article_id = Column(Integer, ForeignKey('articles.id'), nullable=True)
previewlist_id = Column(Integer, ForeignKey('preview_lists.id'), primary_key=True)
article = relationship('Article')
preview_list = relationship('PreviewList')
init_db()
print(f"Creating test data")
a = Article(title="StackOverflow: 'Foo' not setting 'Bar'?")
pl = PreviewList(articles=[a])
session.add(a)
session.add(pl)
session.commit()
print(f"ArticlePreviewListAssociations: {session.query(ArticlePreviewListAssociation).all()}")
print(f"Deleting PreviewList")
session.delete(pl)
associations = session.query(ArticlePreviewListAssociation).all()
print(f"ArticlePreviewListAssociations: should be empty: {associations}")
if len(associations) > 0:
print("FAIL")
print("Reverting transaction")
session.rollback()
print("Deleting article")
session.delete(a)
articles_in_list = pl.articles
associations = session.query(ArticlePreviewListAssociation).all()
print(f"ArticlePreviewListAssociations: should not be empty: {associations}")
if len(associations) == 0:
print("FAIL")
print(f"Articles in PreviewList: should not be empty: {articles_in_list}")
if len(articles_in_list) == 0:
print("FAIL")
# desired outcome: pl.articles should be [None], not []
print("Reverting transaction")
session.rollback()
This may come down to "How can you make a many-to-many relationship where pk_A == 1 and pk_B == NULL include the None in A's list?"
The given examples would seem to assume that the order of related articles is preserved, even upon deletion. There are multiple approaches to that, for example the Ordering List extension, but it is easier to first solve the problem of preserving associations to deleted articles. This seems like a use case for an association object and proxy.
The Article class gets a new relationship so that deletions cascade in a session. The default ORM-level cascading behavior is to set the foreign key to NULL, but if the related association object is not loaded, we want to let the DB do it, so passive_deletes=True is used:
class Article(Base):
__tablename__ = 'articles'
id = Column(Integer, primary_key=True)
title = Column(String)
previewlist_associations = relationship(
'ArticlePreviewListAssociation', back_populates='article',
passive_deletes=True)
Instead of a many to many relationship PreviewList uses the association object pattern, along with an association proxy that replaces the many to many relationship. This time the cascades are a bit different, since the association object should be deleted, if the parent PreviewList is deleted:
class PreviewList(Base):
__tablename__ = 'preview_lists'
id = Column(Integer, primary_key=True)
article_associations = relationship(
'ArticlePreviewListAssociation', back_populates='preview_list',
cascade='all, delete-orphan', passive_deletes=True)
articles = association_proxy(
'article_associations', 'article',
creator=lambda a: ArticlePreviewListAssociation(article=a))
Originally the association object used previewlist_id as the primary key, but then a PreviewList could contain a single Article only. A surrogate key solves that. The foreign key configurations include the DB level cascades. These are the reason for using passive deletes:
class ArticlePreviewListAssociation(Base):
__tablename__ = 'associations'
id = Column(Integer, primary_key=True)
article_id = Column(
Integer, ForeignKey('articles.id', ondelete='SET NULL'))
previewlist_id = Column(
Integer, ForeignKey('preview_lists.id', ondelete='CASCADE'),
nullable=False)
# Using a unique constraint on a nullable column is a bit ugly, but
# at least this prevents inserting an Article multiple times to a
# PreviewList.
__table_args__ = (UniqueConstraint(article_id, previewlist_id), )
article = relationship(
'Article', back_populates='previewlist_associations')
preview_list = relationship(
'PreviewList', back_populates='article_associations')
With these changes in place no "FAIL" is printed.
Situation
I have the Self-Referential Many-to-Many Relationship (almost identical to the sqlalchemy manual entry of the same heading). This relationship is governed by the table entity_weights. This code works!
Question
How do I include an attribute in class Entity representing the table column entity_weights.weight. Lets say the attribute would be called Entity.child_weights. It is important that the rank of Entity.child_entities and Entity.child_weights are identical.
entity_weights = Table('entity_weights', Base.metadata,
Column('id',Integer, primary_key=True),
Column('parent_entity_id',Integer, ForeignKey('entity.id')),
Column('entity_id',Integer, ForeignKey('entity.id')),
Column('weight',Float))
class Entity(Base):
__tablename__ = 'entity'
id = Column(Integer, primary_key=True)
name = Column(String)
domicile_id = Column(Integer, ForeignKey('domicile.id'))
entity_type = Column('type',Enum('asset','institution','model'))
source_table_id = Column(Integer)
child_entities = relationship('Entity',
secondary=entity_weights,
primaryjoin=id==entity_weights.c.parent_entity_id,
secondaryjoin=id==entity_weights.c.entity_id,
backref='parent_entity_id'
)
The cleanest solution I've found for this scenario is to break up the child_entities relationship by adding entity_weights as a one-to-many relationship on Entity and use an association proxy to proxy the weight value as well as the remote side of the many-to-many relationship:
class EntityWeight(Base):
__tablename__ = "entity_weights"
id = Column(Integer, primary_key=True)
parent_entity_id = Column(Integer, ForeignKey('entity.id'))
entity_id = Column(Integer, ForeignKey('entity.id'))
weight = Column(Float)
entity = relationship("Entity", primaryjoin=lambda: EntityWeight.entity_id == Entity.id)
class Entity(Base):
...
_child_weights = relationship(EntityWeight, primaryjoin=id == EntityWeight.parent_entity_id)
child_weights = association_proxy("_child_weights", "weight")
child_entities = association_proxy("_child_weights", "entity")
I have two models, related with many-to-many, one of them is hierarchical model:
#hierarchical model
class Tag(Base):
__tablename__ = "tags"
id = Column(Integer, primary_key=True)
name = Column(String)
Tag.parent_id = Column(Integer, ForeignKey(Tag.id, ondelete='CASCADE'))
Tag.childs = relationship(Tag, backref=backref('parent', remote_side=[Tag.id]),
cascade="all, delete")
class Subject(Base):
__tablename__ = "subjects"
id = Column(Integer, primary_key=True, doc="ID")
name = Column(String)
tags = relationship(Tag, secondary="tags_subjects", backref="subjects")
#many-to-many relations model
class TagsSubjects(Base):
__tablename__ = "tags_subjects"
id = Column(Integer, primary_key=True)
tag_id = Column(Integer, ForeignKey("tags.id"))
subject_id = Column(Integer, ForeignKey("subjects.id"))
So, I'll try to explain what I want to do... I want to make one (or several) query, for search all Subject's objects,
that have 'name' field value like 'foo' OR that has related tags having names with values like 'foo'
OR that has related tags, that has one or more parents (or above by hierarchy) tag with 'name' value like 'foo'
I've tried to do somethis like this:
>>> subjects = session.query(Subject).filter(or_(
Subject.name.ilike('%{0}%'.format('foo')),
Subject.tags.any(
Tag.name.ilike('%{0}%'.format('foo')))
)).order_by(Subject.name).all()
But it isn't correct and "flat" query, without hierarchical feature :(
How to do this by SQLAlchemy's API?
Thanks!
P.S. I'm using SQLite backend