RE: [sqlalchemy] Inheriting a functionality in SQLA

[Alexey Vihorev]

Thanks for the clarification, it works now.

 

From: sqlalchemy@googlegroups.com [mailto:sqlalchemy@googlegroups.com] On
Behalf Of Michael Bayer
Sent: 21 ноября 2012 г. 3:58
To: sqlalchemy@googlegroups.com
Subject: Re: [sqlalchemy] Inheriting a functionality in SQLA

 

 

On Nov 20, 2012, at 4:31 AM, AlexVhr wrote:





I'm trying to incapsulate some functionality (some columns mainly) into base
classes to inherit my models from them. The setup looks like this:

class EntityTemplate():
    @declared_attr
    def __tablename__(cls):
        return cls.__name__.lower()
    id = Column(Integer(), primary_key=True)
    timestamp = Column(DateTime())
 
class DocumentTemplate(EntityTemplate):
    date = Column(Date())
    number = Column(String(5))
 
Entity = declarative_base(cls=EntityTemplate, name='Entity')
Document = declarative_base(cls=DocumentTemplate, name='Document')

I'm trying to use it like this:

class Customer(Entity):    
    name = Column(String(25))
    address = Column(String(50))
 
class Invoice(Document):
    customer_id = Column(Integer, ForeignKey('customer.id'))
    customer = relationship("Customer")
    total = Column(Numeric(10,2))
 
Entity.metadata.create_all(engine)
Document.metadata.create_all(engine)

But on the last line I get this:

    sqlalchemy.exc.NoReferencedTableError: Foreign key associated with
column
'invoice.customer_id' could not find table 'customer' with which to generate
 a foreign key to target column 'id'

If I inherit Invoice from Entity instead of Document, everything is fine
(except the fact that columns date and number are missing). Why? (I'm using
SQLAlchemy-0.7.9-py3.2). Thanks!

 

 

the use of two different declarative_base() makes this more complicated as
there is no common MetaData collection between the two classes which allows
foreign keys to be resolve based on string names, as well as class names
like "Customer" to be resolved.  the Base + MetaData combination represent a
pair of registries that allow these string lookups to work.

 

So using one declarative base would solve the issue, else you need to forego
the usage of string identifiers and pass object references instead:

 

customer_id = Column(Integer, ForeignKey(Customer.id))

customer = relationship(Customer)

 

but there's really no need to use two different declarative bases, your
EntityTemplate and DocumentTemplate are mixins, which if you'd like them to
be packed into a single base class can be accomplished using a subclass of a
single Base with the __abstract__ = True flag.

 

http://docs.sqlalchemy.org/en/rel_0_8/orm/extensions/declarative.html#abstra
ct

 

 

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