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How to properly define SQLAlchemy backrefs so that they can be reflected?

Let's assume we have the following code in some Models.py file:
class Person(db.Model):
__tablename__ = 'Persons'
ID = db.Column(db.Integer, primary_key=True, nullable=False)
Name = db.Column(db.String(255), nullable=False)
class House(db.Model):
__tablename__ = 'Houses'
ID = db.Column(db.Integer,primary_key=True,nullable=False)
OwnerID = db.Column(db.Integer, nullable=False)
TenantID = db.Column(db.Integer, nullable=False)
__table_args__ = (
db.ForeignKeyConstraint(
['OwnerID'],
['Persons.ID'],
),
db.ForeignKeyConstraint(
['TenantID'],
['Persons.ID'],
),
)
OwnerBackref = db.relationship('Person', backref='OwnerBackref', lazy=True, foreign_keys=[OwnerID])
TenantBackref = db.relationship('Person', backref='TenantBackref', lazy=True, foreign_keys=[TenantID])
And we want to reflect these models using the automap base, so we have this code in another module called Database.py:
Base = automap_base()
engine = create_engine(DB_CONNECTION, pool_size=10, max_overflow=20)
db_session = scoped_session(sessionmaker(autocommit=False, autoflush=False, bind=engine))
Base.prepare(engine, reflect=True)
Person = Base.classes.Persons
House = Base.classes.Houses
Now, when I import House in some other module I want to be able to do this:
h = db_session.query(House).first()
print(h.OwnerBackref.Name)
print(h.TenantBackref.Name)
But instead I get an error saying that those 2 backrefs do not exist and instead a field called 'persons' gets added to my House object but the problem here is that it links only 1 (either the Tenant either the Owner). By this I mean that if I do this:
print(h.persons.Name)
It will only print the Name either for the respective Tenant either for the Owner leaving me with no way of accessing the informations for the other one. (Note here that the names that I set to the backrefs are nowhere to be found)
So, my question is how can I use the backrefs I created to access my desired informations ? Am I doing something wrong here ?

The error in your code is that you are using foreign_keys= to define the relationship between the tables but you are passing the local key name not the foreign key name to the function. For your code you cannot use foreign_keys= to define the relationship within the House model as there is only one possible foreign key Person.ID but two possible local keys House.OwnerID and House.TenantID. The primaryjoin= argument should be used instead to specify this.
class Person(db.Model):
__tablename__ = 'Persons'
ID = db.Column(db.Integer, primary_key=True)
Name = db.Column(db.String(255), nullable=False)
class House(db.Model):
__tablename__ = 'Houses'
ID = db.Column(db.Integer,primary_key=True)
OwnerID = db.Column(db.Integer, db.ForeignKey('Persons.ID'), nullable=False)
TenantID = db.Column(db.Integer, db.ForeignKey('Persons.ID'), nullable=False)
Owner = db.relationship('Person', backref='HousesOwned', primaryjoin='House.OwnerID == Person.ID')
Tenant = db.relationship('Person', backref='HousesOccupied', primaryjoin='House.TenantID == Person.ID')
If you placed the relationship statements in in the Person model rather than the House model then you could use either foreign_keys= or primaryjoin= to define the relationship. The following code will result in exactly the same relationships as in the previous code.
class Person(db.Model):
__tablename__ = 'Persons'
ID = db.Column(db.Integer, primary_key=True)
Name = db.Column(db.String(255), nullable=False)
HousesOwned = db.relationship('House', backref='Owner', foreign_keys='[House.OwnerID]')
HousesOccupied = db.relationship('House', backref='Tenant', foreign_keys='[House.TenantID]')
class House(db.Model):
__tablename__ = 'Houses'
ID = db.Column(db.Integer,primary_key=True)
OwnerID = db.Column(db.Integer, db.ForeignKey('Persons.ID'), nullable=False)
TenantID = db.Column(db.Integer, db.ForeignKey('Persons.ID'), nullable=False)

using flask SQLAlchemy unable to insert foreign key data

In the following code snippet, In the events table event_id is created first and then i need to insert multiple records into even_logins table based on the event_id.
class Events(db.Model):
__tablename__ = 'events'
event_name= db.column(db.String(100))
event_id = db.Column(db.String(10),primary_key=True)
scheduled_date = db.Column(db.DateTime, nullable = False)
event_logins_event_id = db.Column(db.String(10), db.ForeignKey('event_logins.event_id'))
event_registrations_event_id = db.Column(db.String(10), db.ForeignKey('event_registrations.event_id'))
class Event_logins(db.Model):
__tablename__ = 'event_logins'
event_id= db.column(db.String(10),db.ForeignKey('events.event_id'))
username = db.Column(db.String(20),primary_key=True)
password = db.Column(db.String(300))
class Event_registrations(db.Model):
__tablename__ = 'event_registrations'
event_id= db.column(db.String(10), db.ForeignKey('events.event_id'))
registration_id = db.Column(db.String(50),primary_key=True)
cust_name = db.Column(db.String(100))
cust_contactno = db.Column(db.String(50))
cust_email = db.Column(db.String(50))
cust_imageid = db.Column(db.String(200))
Finally with the below query am trying insert a record into event_logins table
event_logins = Event_logins(event_id='jk12',username='testuser', password='aaaa')
db.session.add(event_logins)
db.session.commit()
In the table it is stored as NULL the value is not being stored pls suggest
Thanks
vijay

SqlAlchemy Database Issue

I created a Table a Bmarks which has two foreign keys which have relation with same table Url_hash
class Hashed(Base):
__tablename__ = "url_hash"
hash_id = Column(Unicode(22), primary_key=True)
url = Column(UnicodeText)
clicks = Column(Integer, default=0)
def __init__(self, url):
cleaned_url = str(unidecode(url))
self.hash_id = unicode(generate_hash(cleaned_url))
self.url = url
class Bmark(Base):
__tablename__ = "bmarks"
bid = Column(Integer, autoincrement=True, primary_key=True)
hash_id = Column(Unicode(22), ForeignKey('url_hash.hash_id'))
clean_hash_id = Column(Unicode(22), ForeignKey('url_hash.hash_id'))
description = Column(UnicodeText())
extended = Column(UnicodeText())
stored = Column(DateTime, default=datetime.utcnow)
updated = Column(DateTime, onupdate=datetime.utcnow)
clicks = Column(Integer, default=0)
inserted_by = Column(Unicode(255))
username = Column(Unicode(255), ForeignKey('users.username'),
nullable=False,)
tag_str = Column(UnicodeText())
hashed = relation(Hashed,
foreign_keys="Bmark.hash_id",
backref="bmark",
uselist=False
)
clean_hashed = relation(Hashed,
foreign_keys="Bmark.clean_hash_id",
backref="bmark",
uselist=False
)
I am trying to store url after cleaning it a little bit like removing headers,utm parameters etc for indexing purposes
Error is occurring while creating the database
sqlalchemy.exc.ArgumentError: Error creating backref 'bmark' on relationship 'Bmark.clean_hashed': property of that name exists on mapper 'Mapper|Hashed|url_hash'

Actually the error message is very informative.
Just rename one of your backref="bmark" to something else like backref="my_clean_bmark".

Trouble mapping multiple foreign keys to parent table

I have two tables, users and contacts. I query the contacts table and get a list of a user's contacts. I would then like to be able to write Contact.first_name (where first_name is a row from the users table) and print out that contact's first name.
Currently, my Contact object does not recognize any attributes of the user table.
Here is some code:
class User(Base):
""" Basic User definition """
__tablename__ = 'users'
id = Column(Integer, primary_key=True)
first_name = Column(Unicode(255))
last_name = Column(Unicode(255))
contacts = relationship('Contact', backref='users')
class Contact(Base):
__tablename__ = 'contacts'
id = Column(Integer, primary_key=True)
user_id = Column(Integer)
contact_id = Column(Integer)
__table_args__ = (ForeignKeyConstraint([id], [User.id]), {})
Here is my query:
Contact.query.filter(Contact.user_id == self.user_id).filter(Contact.state == True).all()
To be honest, I'm unsure of how to properly map my two foreign keys Contact.user_id and Contact.contact_id to the User.id row. Maybe this is the source of my problem?
I'm very new to using SQLAlchemy, so this is a learning experience here. Thanks for your help.

What you have here is class User which essentially refers to itself. In other words, it's a self-referential many-to-many relationship. Your model definitions should look like this:
# This is so called association table, which links two tables in many-to-many
# relationship. In this case it links same table's ('users') different rows.
user_contacts = Table(
'user_contacts', Base.metadata,
Column('user_id', Integer, ForeignKey('users.id'), primary_key=True),
Column('contact_id', Integer, ForeignKey('users.id'), primary_key=True),
)
class User(Base):
__tablename__ = 'users'
id = Column(Integer, primary_key=True)
first_name = Column(String)
last_name = Column(String)
contacts = relationship(
'User',
secondary=user_contacts,
primaryjoin=id==user_contacts.c.user_id,
secondaryjoin=id==user_contacts.c.contact_id
)
Then you can do things like the following:
u1 = User(first_name='Foo', last_name='Foo')
u2 = User(first_name='Bar', last_name='Bar')
u3 = User(first_name='Baz', last_name='Baz')
u1.contacts = [u2, u3]
session.add(u1)
session.commit()
# ... and in some other place in your code ...
u = User.query.get(1)
print u.contacts[0].first_name

Sqlalchemy many to many mapping with extra fields

I created a many to many relationship with sqlalchemy like this:
subject_books = Table('subject_books', Base.metadata,
Column('subject_id', Integer, ForeignKey('subjects.id')),
Column('book_id', Integer, ForeignKey('books.id')),
Column('group', Integer)
)
class Subject(Base):
__tablename__ = 'subjects'
id = Column(Integer, primary_key=True)
value = Column(Unicode(255), unique=True)
class Book(Base):
__tablename__ = 'books'
id = Column(Integer, primary_key=True)
title = Column(Unicode(255))
isbn = Column(Unicode(24))
subjects = relationship('Subject', secondary=subject_books, collection_class=attribute_mapped_collection('group'), backref='books')
after that I created a test like following:
book = Book(title='first book',isbn='test')
book.subjects[0] = Subject(value='first subject')
book.subjects[1] = Subject(value='second subject')
session.add(book)
transaction.commit()
and it works fine. But what I really want is to store more than one subject with the same group value, so I tried the following test:
book = Book(title='first book',isbn='test')
book.subjects[0] = [Subject(value='first subject'),Subject(value='second subject')]
book.subjects[1] = [Subject(value='third subject'),Subject(value='forth subject')]
session.add(book)
transaction.commit()
but it does not work.
Can this be done using sqlalchemy?
Thanks in Advance

Razi
I think you are constructing wrong relation ship.
Your relation ship must be
book M2M subject
subject M2M group
So you have to create one more model for group and that must be assign as m2m in Subject
Your models will be like.
subject_books = Table('subject_books', Base.metadata,
Column('subject_id', Integer, ForeignKey('subjects.id')),
Column('book_id', Integer, ForeignKey('books.id')),
)
subject_group = Table('subject_groups', Base.metadata,
Column('group_id', Integer, ForeignKey('groups.id')),
Column('subject_id', Integer, ForeignKey('subjects.id')),
)
class Subject(Base):
__tablename__ = 'subjects'
id = Column(Integer, primary_key=True)
value = Column(Unicode(255), unique=True)
groups = relationship('Groups', secondary=subject_groups, backref='subjects')
class Groups(Base):
__tablename__ = 'groups'
id = Column(Integer, primary_key=True)
name = Column(Unicode(255), unique=True)
class Book(Base):
__tablename__ = 'books'
id = Column(Integer, primary_key=True)
title = Column(Unicode(255))
isbn = Column(Unicode(24))
subjects = relationship('Subject', secondary=subject_books, backref='books')
I also check the docs for attribute_mapped_collection. But each time I found that each key is associated with only one object not more then one. If you read anywhere then please provide the link so I can check that how it will be fit in your code.
I think this will be help you.