SQLAlchemy: tables have more than one foreign key constraint relationship

Question

i work on a new database for school-administration. i work with mysql for a long time now but i'm quite new to python and sqlalchemy. first i designed the (giant) database, then generated the classes using flask-sqlacodegen... The problem: i've two tables: person and staff. in person, all persons are stored, each dataset has a mod_user. in staff only company-staff are stored. a mod_user has to be staff...

This are my staff/person classes:

#connect_test.py
from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False
app.config['SQLALCHEMY_DATABASE_URI'] = 'mysql://XXX:XXX@localhost/schooldb_model'
db = SQLAlchemy(app)


class TPerson(db.Model):
    __tablename__ = 't_person'

    idt_person = db.Column(db.Integer, primary_key=True, info='ID der Person')
    salutation = db.Column(db.String(10), nullable=False, server_default=db.FetchedValue(),
                           info='Anrede - Geschlecht der Person ist daraus ersichtlich')
    firstname = db.Column(db.String(45), nullable=False, server_default=db.FetchedValue(), info='Vorname')
    lastname = db.Column(db.String(45), nullable=False, server_default=db.FetchedValue(), info='Nachname')
    birthdate = db.Column(db.Date, info='Geburtsdatum - wird 1900-01-01 gesetzt wenn nicht bekannt')
    birthplace = db.Column(db.String(45), info='Geburtsort')
    cityzenship = db.Column(db.String(45), nullable=False, server_default=db.FetchedValue(), info='Staatsbürgerschaft')
    familystatus = db.Column(db.String(30), nullable=False, server_default=db.FetchedValue())
    dmtx_idt = db.Column(db.LargeBinary)
    qr_idt = db.Column(db.LargeBinary)
    mod_timestamp = db.Column(db.DateTime, nullable=False, index=True, server_default=db.FetchedValue(),
                              info='Timestamp of modification')
    mod_user = db.Column(db.ForeignKey('t_staff.t_person_idt_person'), nullable=False, index=True)

    t_staff = db.relationship('TStaff', primaryjoin='TPerson.mod_user == TStaff.t_person_idt_person',
                              backref='t_people')
    #mod_user = db.Column(db.Integer, nullable=False, index=True, server_default=db.FetchedValue())


class TStaff(TPerson):
    __tablename__ = 't_staff'

    t_person_idt_person = db.Column(db.ForeignKey('t_person.idt_person'), primary_key=True, info='Person id')
    begin = db.Column(db.Date, nullable=False, info='Ist Mitarbeitr seit.')
    end = db.Column(db.Date, info='War Mitarbeiter bis.')

running the code from the python console with >>> from connect_test import TPerson results in: sqlalchemy.exc.AmbiguousForeignKeysError: Can't determine join between 't_person' and 't_staff'; tables have more than one foreign key constraint relationship between them. Please specify the 'onclause' of this join explicitly.

I know, in this special case i dont really need the mod_users foreign key constraint - deleting it and doing mod_user = db.Column(db.Integer, nullable=False, index=True, server_default=db.FetchedValue()) works fine. But i've hundrets of other constraints in the rest of the database that i can not delete - so i hoped solving this easy? case will help me to understand the rest... greetings, c.

Answer 1

@above_c_level: Thanks a lot! - it works. Just had to change the conditions. Here's the complete code:

class TPerson(db.Model):
    __tablename__ = 't_person'

    idt_person = db.Column(db.Integer, primary_key=True, info='ID der Person')
    salutation = db.Column(db.String(10), nullable=False, server_default=db.FetchedValue(),
                           info='Anrede - Geschlecht der Person ist daraus ersichtlich')
    firstname = db.Column(db.String(45), nullable=False, server_default=db.FetchedValue(), info='Vorname')
    lastname = db.Column(db.String(45), nullable=False, server_default=db.FetchedValue(), info='Nachname')
    birthdate = db.Column(db.Date, info='Geburtsdatum - wird 1900-01-01 gesetzt wenn nicht bekannt')
    birthplace = db.Column(db.String(45), info='Geburtsort')
    cityzenship = db.Column(db.String(45), nullable=False, server_default=db.FetchedValue(), info='Staatsbürgerschaft')
    familystatus = db.Column(db.String(30), nullable=False, server_default=db.FetchedValue())
    dmtx_idt = db.Column(db.LargeBinary)
    qr_idt = db.Column(db.LargeBinary)
    mod_timestamp = db.Column(db.DateTime, nullable=False, index=True, server_default=db.FetchedValue(),
                              info='Timestamp of modification')
    mod_user = db.Column(db.ForeignKey('t_staff.t_person_idt_person'), nullable=False, index=True)

    t_staff = db.relationship('TStaff', primaryjoin='TPerson.mod_user == TStaff.t_person_idt_person',
                              backref='people')


class TStaff(TPerson):
    __tablename__ = 't_staff'

    t_person_idt_person = db.Column(db.ForeignKey('t_person.idt_person'), primary_key=True, info='Person id')
    begin = db.Column(db.Date, nullable=False, info='Ist Mitarbeitr seit.')
    end = db.Column(db.Date, info='War Mitarbeiter bis.')
    # person = db.relationship('TPerson', primaryjoin='TPerson.idt_person == TStaff.t_person_idt_person',
    #                         backref='staff')
    __mapper_args__ = {'polymorphic_identity': 'staff',
                       'inherit_condition': TPerson.idt_person == t_person_idt_person}