Implementation of One-to-One relationship in Flask SQLAlchemy - sqlalchemy

Question

I am inexperienced programmer. I'd like to add one-to-one relationship CurrencyDefault (that value will be assigned to the field in FlaskForm) between User and Currency:
class User(UserMixin, db.Model):
id = db.Column(db.Integer, db.Sequence('user_id_seq'), primary_key=True)
username = db.Column(db.String(64), index=True, unique=True)
email = db.Column(db.String(64), index=True, unique=True)
pswd_hash = db.Column(db.String(128))
expenses = db.relationship('Expense', backref='user')
currency = db.relationship('Currency', backref='user')
curr_default = ?
# ...
class Currency(db.Model):
id = db.Column(db.Integer, db.Sequence('expense_id_seq'), primary_key=True)
abbr = db.Column(db.String(10))
name = db.Column(db.String(64))
user_id = db.Column(db.Integer, db.ForeignKey('user.id'))
default = ?
# ...
What I want to achieve is to assign to each user.id one currency.id (one-to-one)
I'd like to ask for some advice what is the best way to make it.
After considering the problem I have some ideas like:
Association Table with uselist=False relationship,
Create a new class CurrencyDefault(id, user_id, currency_id),
Or maybe there is other, better way to achieve it?
I'm very curious of your point of view on this problem.
Implementing solution:
This is how my classes look like now:
class User(UserMixin, db.Model):
id = db.Column(db.Integer, db.Sequence('user_id_seq'), primary_key=True)
username = db.Column(db.String(64), index=True, unique=True)
email = db.Column(db.String(64), index=True, unique=True)
pswd_hash = db.Column(db.String(128))
currency_default_choice = db.Column(db.Integer, b.ForeignKey('currency.id'))
expenses = db.relationship('Expense', backref='user')
currency = db.relationship('Currency', backref='user', foreign_keys="Currency.user_id")
# currency_default = db.relationship(
# 'Currency',
# foreign_keys='User.currency_default_choice',
# backref='currency_default',
# uselist=False,
# )
# ...
class Currency(db.Model):
id = db.Column(db.Integer, db.Sequence('expense_id_seq'), primary_key=True)
abbr = db.Column(db.String(10), b.ForeignKey('currency_official_abbr.abbr'))
name = db.Column(db.String(64))
user_id = db.Column(db.Integer, db.ForeignKey('user.id'))
currency_default = db.relationship(
'User',
foreign_keys='User.currency_default_choice',
backref='currency_default',
uselist=False,
)
The first problem I find is that I can set Currency.id object created by other_user as currency_default_choice. How to restrict currency_default_choice only to the <Currency> that was created by this user?
What is the difference between setting relationship having foreign key in User class (currency_default_choice = db.Column(db.Integer, b.ForeignKey('currency.id'))) with:
class Currency(db.Model):
# ...
currency_default = db.relationship(
'User',
foreign_keys='User.currency_default_choice',
backref='currency_default',
uselist=False,
)
and setting this relationship on User side with:
class User(db.Model):
# ...
currency_default = db.relationship(
'Currency',
foreign_keys='User.currency_default_choice',
backref='currency_default',
uselist=False,
)
Ad.2. What seems to me is that there is no difference between these two ways because the backref parameter implicates bidirectional behavior so it doesn't matter if I placed db.relationship() in User or Currency class. Is it correct?
Using Python shell I added value to the User.currency_default
>>> app = create_app()
>>> app.app_context().push()
>>> admin = User.query.filter_by(username='admin').first()
<User(id= 1, username = admin, email = admin#admin.com)
>>> currency = Currency.query.filter_by(user=admin)
>>> currency
<flask_sqlalchemy.BaseQuery object at 0x03EA05D0>
>>> currency[0].id
1
>>> admin.currency_default = currency[0]
>>> db.session.commit()
>>> currency[0].currency_default
<User(id= 1, username = admin, email = admin#admin.com)
>>> admin.currency_default_choice
1
and then using Admin Panel after running flask run I wanted to remove introduced value but I got error that I don't understand. Why there is circular dependency between (Currency.currency_default),(User.currency_default) and (User.currency)? I don't understand what is happening. How to fix it?
sqlalchemy.exc.CircularDependencyError: Circular dependency detected.
(ProcessState(OneToManyDP(Currency.currency_default),
<Currency at 0x46542b0>, delete=False),
ProcessState(ManyToOneDP(User.currency_default),
<User at 0x4669b10>, delete=False),
SaveUpdateState(<Currency at 0x46542b0>),
ProcessState(OneToManyDP(User.currency), <User at 0x4669b10>, delete=False),
SaveUpdateState(<User at 0x4669b10>))