Context: I'm making an auctioning website for which I am using Flask-SQLAlchemy. My tables will need to have a many-to-many relationship (as one artpiece can have many user bids and a user can bid on many artpieces)
My question is: it is possible to add another column to my joining table to contain the id of the user bidding, the id of artpiece that they are bidding on and also how much they bid? Also if yes, how would I include this bid in the table when I add a record to said table?
bid_table = db.Table("bid_table",
db.Column("user_id", db.Integer, db.ForeignKey("user.user_id")),
db.Column("item_id", db.Integer, db.ForeignKey("artpiece.item_id"))
)
class User(db.Model):
user_id = db.Column(db.Integer, unique=True, primary_key=True, nullable=False)
username = db.Column(db.Integer, unique=True, nullable=False)
email = db.Column(db.String(50), unique =True, nullable=False)
password = db.Column(db.String(60), nullable=False)
creation_date = db.Column(db.DateTime, default=str(datetime.datetime.now()))
bids = db.relationship("Artpiece", secondary=bid_table, backref=db.backref("bids", lazy="dynamic"))
class Artpiece(db.Model):
item_id = db.Column(db.Integer, unique=True, primary_key=True, nullable=False)
artist = db.Column(db.String(40), nullable=False)
buyer = db.Column(db.String(40), nullable=False)
end_date = db.Column(db.String(40))
highest_bid = db.Column(db.String(40))
It is possible to do this with SQL Alchemy, but it's very cumbersome in my opinion.
SQLAlchemy uses a concept called an Association Proxy to turn a normal table into an association table. This table can have whatever data fields you want on it, but you have to manually tell SQLAlchemy which columns are foreign keys to the other two tables in question.
This is a good example from the documentation.
In your case, the UserKeyword table is the association proxy table that you want to build for your user/bid scenario.
The special_key column is the arbitrary data you would store like the bid amount.
from sqlalchemy import Column, Integer, String, ForeignKey
from sqlalchemy.ext.associationproxy import association_proxy
from sqlalchemy.orm import backref, declarative_base, relationship
Base = declarative_base()
class User(Base):
__tablename__ = 'user'
id = Column(Integer, primary_key=True)
name = Column(String(64))
# association proxy of "user_keywords" collection
# to "keyword" attribute
keywords = association_proxy('user_keywords', 'keyword')
def __init__(self, name):
self.name = name
class UserKeyword(Base):
__tablename__ = 'user_keyword'
user_id = Column(Integer, ForeignKey('user.id'), primary_key=True)
keyword_id = Column(Integer, ForeignKey('keyword.id'), primary_key=True)
special_key = Column(String(50))
# bidirectional attribute/collection of "user"/"user_keywords"
user = relationship(User,
backref=backref("user_keywords",
cascade="all, delete-orphan")
)
# reference to the "Keyword" object
keyword = relationship("Keyword")
def __init__(self, keyword=None, user=None, special_key=None):
self.user = user
self.keyword = keyword
self.special_key = special_key
class Keyword(Base):
__tablename__ = 'keyword'
id = Column(Integer, primary_key=True)
keyword = Column('keyword', String(64))
def __init__(self, keyword):
self.keyword = keyword
def __repr__(self):
return 'Keyword(%s)' % repr(self.keyword)
Check out the full documentation for instructions on how to access and create this kind of model.
Having used this in a real project, it's not particularly fun and if you can avoid it, I would recommend it.
https://docs.sqlalchemy.org/en/14/orm/extensions/associationproxy.html
Related
I have this error:
sqlalchemy.exc.NoForeignKeysError: Could not determine join condition between parent/child tables on relationship User.car - there are no foreign keys linking these tables via secondary table 'user'. Ensure that referencing columns are associated with a ForeignKey or ForeignKeyConstraint, or specify 'primaryjoin' and 'secondaryjoin' expressions.
127.0.0.1 - - [26/Jul/2019 23:05:40] "GET /user HTTP/1.1" 500 -
The following is my program
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String, ForeignKey
from sqlalchemy.orm import relationship
Base = declarative_base()
class User(Base):
__tablename__ = 'user'
user_id = Column(Integer)
passport_number = Column(String(8), nullable=False, primary_key=True)
user_email = Column(String(10), nullable=False)
user_name = Column(String(10), nullable=False)
car = relationship('Car', secondary='user')
class Car(Base):
__tablename__ = 'car'
car_number = Column(Integer, primary_key=True, nullable=False)
car_model = Column(String(10), nullable=False)
user_passport_number = Column(String(8), ForeignKey('user.passport_number'), primary_key=True)
part = relationship('Part', secondary='car')
class Part(Base):
__tablename__ = 'part'
part_name = Column(String(10), primary_key=True, nullable=False)
part_price = Column(Integer, nullable=False)
car_number = Column(Integer, ForeignKey('car.car_number'), primary_key=True)
From the error message below:
...there are no foreign keys linking these tables via secondary table 'user'...
we understand that the error might be happening in the setting of the secondary table. Per the docs :
The default behavior of relationship() when constructing a join is that it equates the value of primary key columns on one side to that of foreign-key-referring columns on the other. We can change this criterion to be anything we’d like using the primaryjoin argument, as well as the secondaryjoin argument in the case when a “secondary” table is used.
Therefore, and assuming that you a have a one-to-many or one-to-one relationship between User and Car, you do not seem to need the secondary parameter:
class User(Base):
__tablename__ = 'user'
user_id = Column(Integer)
passport_number = Column(String(8), nullable=False, primary_key=True)
user_email = Column(String(10), nullable=False)
user_name = Column(String(10), nullable=False)
car = relationship('Car', backref='User') #removed secondary relationship
Also, note that you are passing a primary_key constraint to the foreign key field, which is used only in one-to-one relationships, in case it was not intentional. In case it was, then the docs show a simple example for the one-to-one tables:
class User(Base):
__tablename__ = 'user'
passport_number = Column(String(8), nullable=False, primary_key=True)
car = relationship("Car", uselist=False, back_populates="car")
class Car(Base):
__tablename__ = 'car'
id = Column(Integer, primary_key=True)
user_passport_number = Column(String(8), ForeignKey('user.passport_number'))
user = relationship("User", back_populates="car")
I'm trying to create one-to-one and one-to-many relationship at the same time in Flask-SQLAlchemy. I want to achieve this:
"A group has many members and one administrator."
Here is what I did:
class Group(db.Model):
id = db.Column(db.Integer, primary_key=True)
name = db.Column(db.String(140), index=True, unique=True)
description = db.Column(db.Text)
created_at = db.Column(db.DateTime, server_default=db.func.now())
members = db.relationship('User', backref='group')
admin = db.relationship('User', backref='admin_group', uselist=False)
def __repr__(self):
return '<Group %r>' % (self.name)
class User(db.Model):
id = db.Column(db.Integer, primary_key=True)
group_id = db.Column(db.Integer, db.ForeignKey('group.id'))
admin_group_id = db.Column(db.Integer, db.ForeignKey('group.id'))
created_at = db.Column(db.DateTime, server_default=db.func.now())
However I got an error:
sqlalchemy.exc.AmbiguousForeignKeysError: Could not determine join
condition between parent/child tables on relationship Group.members -
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.
Does anyone know how to do that properly?
The solution is to specify the foreign_keys argument on all relationships:
class User(Base):
__tablename__ = 'users'
id = Column(Integer, primary_key=True)
group_id = Column(Integer, ForeignKey('groups.id'))
admin_group_id = Column(Integer, ForeignKey('groups.id'))
class Group(Base):
__tablename__ = 'groups'
id = Column(Integer, primary_key=True)
members = relationship('User', backref='group', foreign_keys=[User.group_id])
admin = relationship('User', backref='admin_group', uselist=False, foreign_keys=[User.admin_group_id])
Perhaps consider the admin relation in the other direction to implement "a group has many members and one admin":
class User(Base):
__tablename__ = 'users'
id = Column(Integer, primary_key=True)
group_id = Column(Integer, ForeignKey('groups.id'))
group = relationship('Group', foreign_keys=[group_id], back_populates='members')
class Group(Base):
__tablename__ = 'groups'
id = Column(Integer, primary_key=True)
members = relationship('User', foreign_keys=[User.group_id], back_populates='group')
admin_user_id = Column(Integer, ForeignKey('users.id'))
admin = relationship('User', foreign_keys=[admin_user_id], post_update=True)
See note on post_update in the documentation. It is necessary when two models are mutually dependent, referencing each other.
The problem you're getting comes from the fact that you've defined two links between your classes - a User has a group_id (which is a Foreign Key), and a Group has an admin (which is also defined by a Foreign Key). If you remove the Foreign Key from the admin field the connection is no longer ambiguous and the relationship works. This is my solution to your problem (making the link one-to-one):
from app import db,app
class Group(db.Model):
id = db.Column(db.Integer, primary_key=True)
name = db.Column(db.String(140), index=True, unique=True)
description = db.Column(db.Text)
created_at = db.Column(db.DateTime, server_default=db.func.now())
admin_id = db.Column(db.Integer) #, db.ForeignKey('user.id'))
members = db.relationship('User', backref='group')
def admin(self):
return User.query.filter_by(id=self.admin_id).first()
class User(db.Model):
id = db.Column(db.Integer, primary_key=True)
name = db.Column(db.String(80), unique=True)
group_id = db.Column(db.Integer, db.ForeignKey('group.id'))
created_at = db.Column(db.DateTime, server_default=db.func.now())
The one drawback to this is that the group object doesn't have a neat admin member object you can just use - you have to call the function group.admin() to retrieve the administrator. However, the group can have many members, but only one of them can be the administrator. Obviously there is no DB-level checking to ensure that the administrator is actually a member of the group, but you could add that check into a setter function - perhaps something like:
# setter method
def admin(self, user):
if user.group_id == self.id:
self.admin_id = user.id
# getter method
def admin(self):
return User.query.filter_by(id=self.admin_id).first()
Ok, I found a workaround for this problem finally. The many-to-many relationship can coexist with one-to-many relationship between the same two tables at the same time.
Here is the code:
groups_admins = db.Table('groups_admins',
db.Column('user_id', db.Integer, db.ForeignKey('user.id')),
db.Column('group_id', db.Integer, db.ForeignKey('group.id'))
)
class Group(db.Model):
id = db.Column(db.Integer, primary_key=True)
name = db.Column(db.String(140), index=True, unique=True)
description = db.Column(db.Text)
created_at = db.Column(db.DateTime, server_default=db.func.now())
members = db.relationship('User', backref='group')
admins = db.relationship('User',
secondary=groups_admins,
backref=db.backref('mod_groups', lazy='dynamic'),
lazy='dynamic')
def __repr__(self):
return '<Group %r>' % (self.name)
class User(db.Model):
id = db.Column(db.Integer, primary_key=True)
group_id = db.Column(db.Integer, db.ForeignKey('group.id'))
created_at = db.Column(db.DateTime, server_default=db.func.now())
I still want someone to tell me how to set one-to-many and one-to-one relationship at the same time, so I leave my answer here and won't accept it forever.
This link solved it for me
most important thing is to specify foreign_keys value in the relation as well as the primary join
I have rather simple models like these:
TableA2TableB = Table('TableA2TableB', Base.metadata,
Column('tablea_id', BigInteger, ForeignKey('TableA.id')),
Column('tableb_id', Integer, ForeignKey('TableB.id')))
class TableA(Base):
__tablename__ = 'TableA'
id = Column(BigInteger, primary_key=True)
infohash = Column(String, unique=True)
url = Column(String)
tablebs = relationship('TableB', secondary=TableA2TableB, backref='tableas')
class TableB(Base):
__tablename__ = 'TableB'
id = Column(Integer, primary_key=True)
url = Column(String, unique=True)
However, sqla generates queries like
SELECT "TableB".id, "TableB".url AS "TableB_url" FROM "TableB", "TableA2TableB"
WHERE "TableA2TableB".tableb_id = "TableB".id AND "TableA2TableB".tablea_id = 408997;
But why is there a cartesian product in the query when the attributes selected are those in TableB? TableA2TableB shouldn't be needed.
Thanks
As it is right now, there is a backref relationship in TableB (tableas) and it's loaded because the default loading mode is set to select.
You may want to change the TableA.tablebs to
tablebs = relationship('TableB', secondary=TableA2TableB, backref='tableas', lazy="dynamic")
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
Im using sqlalchemy to design a forum style website. I started knocking out the design but everytime I try to test it with a few inserts, it dumps a brick;
NoForeignKeysError: Could not determine join condition between parent/child
tables on relationship Thread.replies - there are no foreign keys linking
these tables. Ensure that referencing columns are associated with a
ForeignKey or ForeignKeyConstraint, or specify a 'primaryjoin' expression.
Here are my "models"
from sqlalchemy import Integer, Column, String, create_engine, ForeignKey
from sqlalchemy.orm import relationship, sessionmaker, backref
from .database import Base # declarative base instance
class User(Base):
__tablename__ = "user"
id = Column(Integer, primary_key=True)
username = Column(String, unique=True)
email = Column(String, unique=True)
threads = relationship("Thread", backref="user")
posts = relationship("Post", backref="user")
class Post(Base):
__tablename__ = "post"
id = Column(Integer, primary_key=True)
name = Column(String)
body = Column(String)
author = Column(Integer, ForeignKey("user.id"))
class Thread(Base):
__tablename__ = "thread"
id = Column(Integer, primary_key=True)
name = Column(String)
desc = Column(String)
replies = relationship("Post", backref="thread")
author_id = Column(Integer, ForeignKey("user.id"))
board_id = Column(Integer, ForeignKey("board.id"))
class Board(Base):
__tablename__ = "board"
id = Column(Integer, primary_key=True)
name = Column(String)
desc = Column(String)
threads = relationship("Thread", backref="board")
category_id = Column(Integer, ForeignKey("category.id"))
class Category(Base):
__tablename__ = "category"
id = Column(Integer, primary_key=True)
name = Column(String)
desc = Column(String)
threads = relationship("Board", backref="category")
engine = create_engine('sqlite:///:memory:', echo=True)
Base.metadata.create_all(engine)
session_factory = sessionmaker(bind=engine)
session = session_factory()
Your Post model has no thread reference. Add a column to Post referencing the Thread a post belongs to:
class Post(Base):
__tablename__ = "post"
id = Column(Integer, primary_key=True)
name = Column(String)
body = Column(String)
author = Column(Integer, ForeignKey("user.id"))
thread_id = Column(Integer, ForeignKey('thread.id'))
We can't use the name thread because that's what the Post.replies relationship will add to retrieved Thread instances.
This is a One to Many relationship as documented in the SQLAlchemy Relationship Configuration documentation.
You should add a field in the Post model that reads:
thread_id = Column(Integer, ForeignKey("thread.id"), nullable=True, default=None)
How is SQLAlchemy supposed to know how this relationship you defined supposed to link a thhread to a post? That's why you should have a foreign key from a post to its thread. You can allow it to be null, if it does not belong to a thread, it depends on your use case.