Flask: how to add related One-to-Many models from forms - flask

Question

I have two models with One-to-Many relations
first_model
class Datacenter(db.Model):
id = db.Column(db.Integer, primary_key=True)
number = db.Column(db.Integer)
name = db.Column(db.String(64), index=True, unique=True)
place = db.Column(db.String(64))
capacity = db.Column(db.Integer, index=True)
server = db.relationship('Server', backref='datacenter', lazy=True)
tier = db.Column(db.Integer)
def __repr__(self):
return '<Datacenter {}>'.format(self.name)
second_models
class Server(db.Model):
id = db.Column(db.Integer, primary_key=True)
number = db.Column(db.Integer)
name = db.Column(db.String(64), index=True, unique=True)
factory = db.Column(db.String(64), index=True)
model = db.Column(db.String(64))
serial = db.Column(db.Integer)
os = db.Column(db.String(64))
datacener_id=db.Column(db.Integer, db.ForeignKey('datacenter.id'), nullable=False)
def __repr__(self):
return '<Server {}>'.format(self.name)
However, I'm not sure that I correctly described the relationship between them in the fields of the model.
All I need is to create a new Server model with a connection to the Datacenter model.
I think that this can be done using the select field in the form. so I created a form
class ServerForm(FlaskForm):
number = IntegerField('Number')
name = StringField('Name')
factory = StringField('Factory')
model = StringField('Model')
serial = IntegerField('Model')
os = StringField('OS')
datacener_id = SelectField('Datacener', choices=[ ???? ])
and router
#app.route('/add_server', methods = ['POST', 'GET'])
def add_server():
form = ServerForm(request.form)
if request.method=='POST' and form.validate():
data = Server(
number=form.number.data,
name=form.name.data,
factory=form.factory.data,
model=form.model.data,
serial=form.serial.data,
os=form.os.data,
datacener_id=form.datacener_id.data
)
db.session.add(data)
db.session.commit()
flash('Server created successfully!')
return redirect('/')
return render_template('new_server.html', form=form)
But when I go to the page for adding a new model, the drop-down list is empty. Tell me what I need to change in my code