SQLAlchemy - Query id from lookup table via link

I am trying to create a flexible database model that will allow me to store firewall rules. My (slightly simplified by removing irrelevant parts for clarity) database schema looks like this:

My SQLAlchemy database (for a Flask project) looks like this:

class FirewallRule(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    src_ip = db.Column(db.String(32), nullable=True)
    dst_ip = db.Column(db.String(32), nullable=True)
    port_expression = db.relationship('PortExpression', backref='firewall_rules',
            lazy='dynamic')
    firewall_rule_protocol = db.relationship('Protocol', secondary=firewall_rule_protocol,
            lazy='dynamic', backref=db.backref('firewall_rules', lazy='dynamic'))
    def add_protocol(self, protocol_number):
        protocol = Protocol.query.filter_by(protocol_number=protocol_number).first()
        if protocol and protocol in self.firewall_rule_protocol:
            return
        if not protocol:
            return
        self.firewall_rule_protocol.append(protocol)

class PortExpression(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    direction = db.Column(db.String(3), nullable=True)
    port_expression = db.Column(db.String(32), nullable=False)
    firewall_rule_id = db.Column(db.Integer, db.ForeignKey('firewall_rule.id'), nullable=False)
    firewall_rule_protocol_id = db.Column(db.Integer, db.ForeignKey('firewall_rule_protocol.id'), nullable=False)

    def add_port_expr(self, direction, expr, firewall_rule_id, firewall_rule_protocol_id):
        firewall_rule=FirewallRule.query.filter_by(id=firewall_rule_id).first()
        self.direction = direction
        self.port_expression = expr
        self.firewall_rule_id = firewall_rule_id
        self.firewall_rule_protocol_id = firewall_rule_protocol_id

class Protocol(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    protocol_name = db.Column(db.String(8), nullable=False)
    protocol_number = db.Column(db.Integer, nullable=False)

firewall_rule_protocol = db.Table('firewall_rule_protocol',
        db.Column('id', db.Integer, primary_key=True),
        db.Column('firewall_rule_id', db.Integer, db.ForeignKey('firewall_rule.id'), nullable=False),
        db.Column('protocol_id', db.Integer, db.ForeignKey('protocol.id'), nullable=False))

      

        
        
        
      

    

Now my question is, how can I query for id

records in a table firewall_rule_protocol

related to a record certain firewall_rule

?

In my views.py, I do the following:

    if form.validate_on_submit(): 
        firewall_rule = FirewallRule()
        form.populate_obj(firewall_rule)
        firewall_rule.add_protocol(form.protocol.data)
        db.session.add(firewall_rule)
        db.session.flush()
        if form.port.data:
            port = PortExpression()
            direction = None
            port.add_port_expr(None, form.port.data, firewall_rule.id, firewall_rule.firewall_rule_protocol.id)

      

        
        
        
      

    

The following message appears above with an error:

port.add_port_expr(None, form.port.data, firewall_rule.id, firewall_rule.firewall_rule_protocol.id)

      

        
        
        
      

    

AttributeError: 'AppenderBaseQuery' object has no attribute 'id'

It looks like I am doing something stupid, but I cannot figure out what it is. Thanks in advance for any help or pointers in the right direction.

Regards, Sergiusz