SQLAlchemy Foreign Key Error: "Could not find table 'user' for announcement.creator_id"

Problem Description:

I'm encountering an error when running my Flask application. The error occurs when I try to log in, and it seems related to the Announcement model's foreign key referencing the User model. Here's the error traceback:

sqlalchemy.exc.NoReferencedTableError: Foreign key associated with column 'announcement.creator_id' could not find table 'user' with which to generate a foreign key to target column 'id'

Relevant Code:

Here are the models involved:

User Model:

class User(db.Model, UserMixin):
    __bind_key__ = 'main'  # Bind to 'main' database
    __tablename__ = 'user'
    metadata = metadata_main  # Explicit metadata
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(80), unique=True, nullable=False)
    email = db.Column(db.String(120), unique=True, nullable=False)
    password_hash = db.Column(db.String(128), nullable=False)
    role = db.Column(db.String(20), nullable=False)
    is_admin_field = db.Column(db.Boolean, default=False)

    def set_password(self, password):
        self.password_hash = generate_password_hash(password)

    def check_password(self, password):
        return check_password_hash(self.password_hash, password)

    @property
    def is_admin(self):
        """Return True if the user is an admin."""
        return self.role == 'admin'

    def get_role(self):
        """Return the role of the user."""
        return self.role

    def __repr__(self):
        return f"User('{self.username}', '{self.email}', '{self.role}')"

Announcement Model:

class Announcement(db.Model):
    __bind_key__ = 'main'
    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String(150), nullable=False)
    content = db.Column(db.Text, nullable=False)
    created_at = db.Column(db.DateTime, default=datetime.utcnow)
    created_by = db.Column(db.String(50), nullable=False)

    # ForeignKeyConstraint ensures the reference to user.id in 'main' database
    creator_id = db.Column(db.Integer, nullable=False)
    __table_args__ = (
        ForeignKeyConstraint(
            ['creator_id'],
            ['user.id'],
            name='fk_creator_user_id',
            ondelete='CASCADE'
        ),
    )

    def __repr__(self):
        return f"<Announcement {self.title}>"

Where the Module Was Declared:

# school_hub/__init__.py
from flask import Flask
from flask_sqlalchemy import SQLAlchemy
from flask_login import LoginManager
from flask_migrate import Migrate

# Initialize extensions
db = SQLAlchemy()
login_manager = LoginManager()
migrate = Migrate()

def create_app():
    app = Flask(__name__)

    # Configurations
    app.config['SQLALCHEMY_DATABASE_URI'] = 'mysql://root:Root1234!@localhost/school_hub'
    app.config['SECRET_KEY'] = '8d8a72493996de3050b75e0737fecacf'
    app.config['SQLALCHEMY_BINDS'] = {
        'main': 'mysql+pymysql://root:Root1234!@localhost/main_db',
        'teacher_db': 'mysql+pymysql://root:Root1234!@localhost/teacher_database',
        'student_db': 'mysql+pymysql://root:Root1234!@localhost/student_database',
    }
    app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False

    # Initialize extensions with the app
    db.init_app(app)
    login_manager.init_app(app)
    migrate.init_app(app, db)

    # Set up Flask-Login user loader
    from .models import User  # Import User model here to ensure it's loaded

    @login_manager.user_loader
    def load_user(user_id):
        return User.query.get(int(user_id))

    # Register Blueprint
    from .routes import main
    app.register_blueprint(main)

    # Ensure app context is pushed before calling db.create_all()
    with app.app_context():
        # Create all tables for the 'main' database
        db.create_all()  # This will create tables for the default 'main' database

        # Explicitly create tables for the 'teacher_db' and 'student_db'
        from .models import Teacher, Student, User  # Ensure models are imported

        # Create tables for 'teacher_db'
        Teacher.metadata.create_all(bind=db.get_engine(app, bind='teacher_db'))

        # Create tables for 'student_db'
        Student.metadata.create_all(bind=db.get_engine(app, bind='student_db'))

    return app

My Environment:

• Flask: Latest version
• Flask-SQLAlchemy: Latest version
• SQLAlchemy: Latest version
• Python: Latest version

My Question:

Why is SQLAlchemy unable to find the user table, even though the table name matches the foreign key reference? How can I resolve this error?

Additional Context:

I'm using Flask-Migrate for database migrations. The User model is bound to the main database, and the Announcement model references this table. The error occurs when SQLAlchemy tries to create the foreign key constraint, and it cannot find the user table.

What Did I Try?

1. Ensuring Correct Database Binding:

   • I’ve ensured both models explicitly set __bind_key__ = 'main' to associate them with the same database.

2. Ensuring Correct Foreign Key Reference:

   • The Announcement model has a foreign key referencing the id column of the User model:

    creator_id = db.Column(db.Integer, db.ForeignKey('user.id'), nullable=False)
   

• I verified that the User model is correctly bound to 'main' and the user table exists.

1. Database Initialization:

   • I’ve ensured that tables are created in the correct order, with the User table being created before the Announcement table due to the foreign key constraint.

2. Error Handling and Suggestions:

   • I’ve checked that both the User and Announcement models are correctly imported and initialized.
   • I’ve confirmed that the foreign key reference should work as both models are bound to the same database.

3. Repeated Checks on Database Bind:

   • I double-checked the bind for the User and Announcement models, ensuring both are using 'main' as the bind key.

4. Potential Missing Table Issue:

   • The error could happen if the User table isn’t visible to SQLAlchemy at the time of the foreign key creation, so I ensured that the User table exists and is properly created before running the Announcement model migration.