Associating an SQLAlchemy object with an existing record (Python/Pyramid)

Question

When attempting to add a Widget to the (SQLite) DB, and associate it with an already existing User as defined below:

class Widget(Base):
    __tablename__ = 'widgets'
    id = Column(Integer, primary_key=True)
    kind = Column(Text)
    user = Column(Text, ForeignKey('users.name'))

class User(Base):
    __tablename__ = 'users'
    id = Column(Integer, primary_key=True)
    name = Column(Text, unique=True)

@view_config(route_name='name', renderer='json')
def add_widget(request):
    user_name = 'foo'
    user = request.dbsession.query(User).filter_by(name=user_name).one()
    new_widget = Widget(kind='bar', user=user)

I receive the error sqlalchemy.exc.InterfaceError: (sqlite3.InterfaceError) Error binding parameter 1 - probably unsupported type. [SQL: 'INSERT INTO widgets (kind, user) VALUES (?, ?)'] [parameters: ('bar', <example.models.user.User object at 0x1050bdghd>)] sqlalchemy.exc.InterfaceError: (sqlite3.InterfaceError) Error binding parameter 1 - probably unsupported type. [SQL: 'INSERT INTO widgets (kind, user) VALUES (?, ?)'] [parameters: ('bar', <example.models.user.User object at 0x1050bdghd>)]

I can add the value for user.name directly, but this breaks the relationship. For example, widget.user.name would give an error like 'str' object has no attribute 'repository'

Answer 1

The backref attribute will do what you need. Add a relationship to user class as follows:

class User(Base):
    ...
    widget = sqlalchemy.orm.relationship('Widget', backref='user')

With backref attribute sqlalchemy will add a reference to your Widget class which will point User object:

user = request.dbsession.query(User).filter_by(name=user_name).one()
new_widget = Widget(kind='bar', user=user)

Answer 2

Your Widget class has a user attribute which is of type Text , which is basically a string attribute on the Python side of things and is something like VARCHAR column in the database. You can not assign a User object to an attribute which expects to be a string.

It also appears that you're trying to create a foreign key relationship using the users.name column - this is certainly possible but is quite unusual considering you also have a surrogate primary key on your User model.

You need to set up a relationship between your two ORM classes:

class Widget(Base):
    __tablename__ = 'widgets'
    id = Column(Integer, primary_key=True)
    kind = Column(Text)
    user_id = Column(Integer, ForeignKey('users.id'))
    user = sqlalchemy.orm.relationship('User', backref='widgets')

then you'll be able to do any of the following:

new_widget = Widget(kind='bar', user=user)
new_widget = Widget(kind='bar', user_id=user.id)
user.widgets.append(Widget(kind='bar'))
new_widget.user_id = user.id
new_widget.user = user

You can also set up things from the other end as @metmirr suggested, but the magic ingredient here is sqlalchemy.orm.relationship , not the backref which you may or may not choose to have.