Flask-SQLAlchemy set relationship default value

Question

I have an app making by Flask and for database management I using Flask-admin and Flask-SQLAlchemy. And in my app there has three role, which is:

1. admin,
2. school
3. parent

here is the snipet of code on my table:

roles_users = db.Table(
    'roles_users',
    db.Column('user_id', db.Integer(), db.ForeignKey('user.id')),
    db.Column('role_id', db.Integer(), db.ForeignKey('role.id'))
)

class Role(db.Model, RoleMixin):
    id = db.Column(db.Integer(), primary_key=True)
    name = db.Column(db.String(80), unique=True)

class User(db.Model, UserMixin):
        id = db.Column(db.Integer, primary_key=True)
        name = db.Column(db.String(255))
        roles = db.relationship('Role', secondary=roles_users,
                                backref=db.backref('users', lazy='dynamic'))

class School(User):
    id = db.Column(db.Integer, primary_key=True)
    user_id = db.Column(db.Integer, db.ForeignKey('user.id'))

class Parent(User):
    id = db.Column(db.Integer, primary_key=True)
    user_id = db.Column(db.Integer, db.ForeignKey('user.id'))
    school_id = db.Column(db.Integer, db.ForeignKey('school.id'))

The code already works fine.

• admin can make the school account and set the role for the user to school.
• and a user that has a school role can make parents account.

I can set the role for all of the users manually with Flask-admin interface, I can make it with a user that has admin role.

But, for the user that has school role, I don't give access to set the role like admin does.

What I want is when the user that has school role make an account, it automatically set the role to parent.

In my ModelView, I try to insert it with this code:

class ParentModelView(sqla.ModelView):
    def on_model_change(self, form, model, is_created):
        if is_created:
            model.school_id = current_user.id
            model.roles = 'parent'

and then I got this error:

TypeError: Incompatible collection type: str is not list-like

and then I try to wraping it with list:

   def on_model_change(self, form, model, is_created):
        if is_created:
            model.school_id = current_user.id
            model.roles = ['parent']

then the error be like this:

AttributeError: 'str' object has no attribute '_sa_instance_state'

And when I try to set the default value on my relationship column :

class User(db.Model, UserMixin):
            id = db.Column(db.Integer, primary_key=True)
            name = db.Column(db.String(255))
            roles = db.relationship('Role', secondary=roles_users,
                     backref=db.backref('users', lazy='dynamic'),
                     default='parent') # or default=3 (which is '3' is the parent id in roles_user association table.

and then I got this error:

TypeError: relationship() got an unexpected keyword argument 'default'

So, the point what I want is to set a default value on my relationship column that defines roles users.

Any help will be much appreciated.

Answer 1

In your code you are trying to assign the string 'parent' when you should be assigning a list of Role instances.

In your on_model_change method you need to fetch the role that has name 'Parent' from the database and then assign it to the roles relationship as a list.

Example (untested)

def on_model_change(self, form, model, is_created):
    if is_created:
        #  find the role with name 'Parent'
        _parent_role = Role.query.filter(Role.name == 'Parent').first()
        if _parent_role:
            model.school_id = current_user.id
            model.roles = [_parent_role]