Flask-SQLalchemy one-to-one relationship: associate child to already existing parent? - python-3.x

Question

first time I use sqlalchemy and flask and I miss something
I'm trying to do a one-to-one relationship between two tables with Flask-SQLalchemy.
This is my first table (the parent)
class User(db.Model):
__tablename__ = 'users'
user_id = Column(Integer, primary_key=True, autoincrement=True)
user_email = Column(String(150), nullable=False)
user_password = Column(String(60), nullable=False)
child_employee = relationship('Employee', uselist=False, backref='users')
def __init__(self, user_email, user_password):
self.user_email = user_email
self.user_password = user_password
And the child
class Employee(User):
__tablename__ = 'employees'
employee_id = Column(Integer, primary_key=True, autoincrement=True)
user_id = Column(Integer, ForeignKey('users.user_id'), nullable=False)
employee_lastname = Column(String(30), nullable=False)
employee_firstname = Column(String(30), nullable=False)
employee_comment = Column(String)
def __init__(self, user_email, user_password, employee_lastname, employee_firstname, employee_comment=None):
super().__init__(user_email, user_password)
self.employee_lastname = employee_lastname
self.employee_firstname = employee_firstname
self.employee_comment = employee_comment
Know when I create an user, I have a user in my table "users" and nothing in my table "employees" : That make sense, perfect
Then if I create an employee, I have a new entry in my table "employees" and a new entry in my table "users". They are linked to each other : that make also sense, perfect
Now the question :
I created an user so there is no employee linked to it.
Now I want to create an employee and I want to link it to the already existing user, how can I do it ?
I tried
# creation of the new employee
new_employee = Employee(exisiting_user.user_email,
exisiting_user.user_password, "email", "lastname", "firstname")
#trying to associate the new employee to the already existing user
exisiting_user.child_employee = new_employee
# save the employee to the database
db.session.add(new_employee)
db.session.commit()
but I get this error
sqlalchemy.exc.IntegrityError: (psycopg2.IntegrityError) duplicate key value violates unique constraint "users_pkey"
DETAIL: Key (user_id)=(2) already exists.
[SQL: 'INSERT INTO users (user_id, user_email, user_password) VALUES (%(user_id)s, %(user_email)s, %(user_password)s)'] [parameters: {'user_id': 2, 'user_email': 'email', 'user_password': 'password'}]
Thanks
----------------First try to resolve it-------------------------
So as if I understand correctly, when I create an instance of "Employee" I also have an instance of the super class "User". Then whend I db.session.add(new_employee), SQLalchemy create an INSERT for the employee and then for the user, but the user already exist so I have the error.
I change my Employee table :
class Employee(User):
__tablename__ = 'employees'
employee_id = Column(Integer, ForeignKey(User.user_id), primary_key=True)
employee_lastname = Column(String(30), nullable=False)
employee_firstname = Column(String(30), nullable=False)
employee_comment = Column(String)
parent = relationship("User", back_populates="child_employee")
def __init__(self, employee_id, employee_lastname, employee_firstname, employee_comment=None):
self.employee_id = employee_id
self.employee_lastname = employee_lastname
self.employee_firstname = employee_firstname
self.employee_comment = employee_comment
and to add only an employee i'm doing
emp = Employee(1, "lastname", "firstname", "comment")
db.session.execute("INSERT INTO employees VALUES("emp.employee_id",'"emp.employee_lastname+"', '"+emp.employee_firstname+"', '"+emp.employee_comment+"')")
Because of the Id who is also a foreign key, the employee and the user are automatically linked
It works but I would prefer to use something more simple like db.session.add(emp), I still miss something