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sqlalchemy.exc.ArgumentError: Error creating backref ‘role’ on relationship ‘Role.users’: property of that name exists on mapper ‘mapped class User->users’

The error indicates that there is a conflict between the role attribute and the backref relationship in your SQLAlchemy models.

Issue Explanation

The key error message is:

sqlalchemy.exc.ArgumentError: Error creating backref 'role' on relationship 'Role.users': property of that name exists on mapper 'mapped class User->users'

This means that in your User and Role models, there is a conflict in how role is defined. The problem is likely due to a backref in the relationship definition.

Solution

Check your User and Role models and ensure the relationship does not redefine role as both an attribute and a backref. Here’s the correct way to structure them:

Correct Model Definition

from sqlalchemy import Column, Integer, String, ForeignKey
from sqlalchemy.orm import relationship
from core.database import Base

class Role(Base):
__tablename__ = 'roles'

id = Column(Integer, primary_key=True)
name = Column(String(50), unique=True, nullable=False)

# Define a one-to-many relationship with the User model
users = relationship("User", back_populates="role")

class User(Base):
__tablename__ = 'users'

id = Column(Integer, primary_key=True)
username = Column(String(50), unique=True, nullable=False)
role_id = Column(Integer, ForeignKey('roles.id'))

# Reference the Role model without conflict
role = relationship("Role", back_populates="users")

What Changed?

  1. Fixed the backref issue:
    • Instead of backref="role", we used back_populates="users" in both models.
    • This ensures a clear bidirectional relationship.
  2. Avoided Conflicting Property Names:
    • If you have role = relationship(...) in the User model, you must not use backref="role" in Role.

Alternative Fix

If you must use backref, rename the attribute in User:

from sqlalchemy.orm import backref

class Role(Base):
__tablename__ = 'roles'
id = Column(Integer, primary_key=True)
name = Column(String(50), unique=True, nullable=False)

users = relationship("User", backref=backref("user_role", lazy=True))

Now, User will refer to Role using user_role instead of role, avoiding conflicts.

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