How to create a foreignkey reference with sqlalchemy

Question

Hi I am not able to understand how to make a foreignkey reference using sqlalchemy. I have created a new table client in my database:

class Client(DeclarativeBase):

    __tablename__ = 'client'

    id = Column(Integer, primary_key=True)

    user_id = Column(
        Integer,
        ForeignKey('user.id', ondelete='CASCADE'),
        nullable=False,
        index=True,
    )

    orgname = Column(Unicode, nullable=False)

    def __init__(self, **kwargs):
        super(Client, self).__init__(**kwargs)

Not I am trying to do something like this

 u = User(user_name=u'dusual')
 session.add(u)
 c = Client(user=u, orgname="dummy_org")
 session.add(c)

But sqlalchemy shouts back saying :

(k, cls_.name))
TypeError: ‘user’ is an invalid keyword argument for Client

Now shouldn’t this be obvious that user should be allowed as a keyword argument how can I make sure my table is able to take user keyword argument.

Asked By: dusual

||

Source

Answer 1

You need to define a relationship between User and Client models:

from sqlalchemy.orm import relationship

class Client(DeclarativeBase):

    __tablename__ = 'client'

    id = Column(Integer, primary_key=True)

    user_id = Column(
        Integer,
        ForeignKey('user.id', ondelete='CASCADE'),
        nullable=False,
        # no need to add index=True, all FKs have indexes
    )
    user = relationship('User', backref='clients')

    orgname = Column(Unicode, nullable=False)

    # no need to add a constructor

Then you can associate instances of User and Client models in two ways – either by assigning an integer to Client.user_id:

u = User(user_name=u'dusual')
session.add(u)
session.flush()  # to make sure the id is fetched from the database
c = Client(user_id=u.id, orgname="dummy_org")
session.add(c)

or by assinging a User instance to Client.user.

u = User(user_name=u'dusual')
# no need to flush, no need to add `u` to the session because sqlalchemy becomes aware of the object once we assign it to c.user
c = Client(user=u, orgname="dummy_org")
session.add(c)

Actually, there’s a third way – since we’ve configured a backref on Client.user, SQLAlchemy added a list-like clients attribute to our User model:

u = User(user_name=u'dusual')
u.clients.append(Client(orgname="dummy_org"))
session.add(u)

Answered By: Sergey

Answer 2

For everyone that came here like me with need of solving that problem dynamically, here it is:

from sqlalchemy.orm import declarative_base
from sqlalchemy import Column, Integer

Base = declarative_base()


class User(Base):
    __tablename__ = "user"

    id = Column(Integer, primary_key=True)


class Client(Base):
    __tablename__ = "client"

    id = Column(Integer, primary_key=True)
    # No relationship, not foreign key
    user_id = Column(Integer)

from sqlalchemy.orm import relationship
from sqlalchemy.sql.ddl import AddConstraint
from sqlalchemy.sql.schema import ForeignKeyConstraint, MetaData, Table

name = "client_user_id_fkey"
fk = ForeignKeyConstraint((Client.user_id,), (User.id,), name=name)
# We need to assign that foreign key to proper table
Table(Client.__table__.name, MetaData(), fk)
# Adding a contraint
session.execute(AddConstraint(fk))
session.commit()
# With that FK existed we can add relationship through the mapper 
Client.__mapper__.add_property("user", relationship(User, backref="clients"))

Now it works:

u = User()
t = Client(user=u)
session.add(t)
session.commit()

Answered By: kosciej16

How to create a foreignkey reference with sqlalchemy

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