CODEWITHSUNDEEP
pythonsqlalchemy
mmitchell
mmitchell

Reputation: 622

SQLAlchemy: How can I add common mapping logic to my custom base class (using declarative style mapping)?

I have a project where every table has some common fields, e.g., status, and I'd like to alias all of them. Is it possible to do this without manually adding the alias to each class? E.g., here's what I have now:

from core import foo_table, bar_table, Status 

Base = declarative_base()

def CustomBase(object):
   @property
   def status(self):
      return Status(self._status)
  ...

def Foo(Base, CustomBase):
   __table__ = foo_table
   _status = foo_table.c.status
   ...


def Bar(Base, CustomBase):
   __table__ = bar_table
   _status = bar_table.c.status
   ...

Ideally, I'd like to be able to set up my _status alias on CustomBase instead of in Foo and Bar, or set up my project so that the alias is added whenever a class extending CustomBase is loaded. Is this possible or am I trying to accomplish this in the wrong way? I know I can make it work if I rename the status field in my db or rename the status property in the CustomBase, but I'd prefer to avoid this if possible since they're both representations of the same thing, and there's no need to directly access in the enum value through the code.

Thanks!

Upvotes: 2

Views: 637

Answers (1)

Julian
Julian

Reputation: 4366

Your best bet is probably to create a custom Column type that adapts Enum to translate to and from your own Status class. See here for a full reference. Below is a draft for your core module, the precise code depends a bit on your situation.

# core module

import sqlalchemy.types as types

class DBStatus (types.TypeDecorator):

    impl = types.Enum

    # what should happen with Status objects on the way into the table
    def process_bind_param(self, value, dialect):
        if value is None:
            return value
        return str(value)  # if Status has a __str__ or __repr__ method

    # what should happen with Enum objects on the way out of the table
    def process_result_value(self, value, dialect):
        if value is None:
            return value
        return Status(value)

foo_table = Table(
    'foo',
    MetaData(),
    Column('status', DBStatus('OK', 'Error')),
    # ...
)

After this you don't have to do anything special anymore in the module with the mappings:

# module with the mappings

Base = declarative_base()

class Foo (Base):
    __table__ = foo_table
    # ...

In fact it's so straightforward you might just as well use full declarative mapping, as far as the Status columns are concerned.

# everything in one module

class DBStatus (types.TypeDecorator):

    # same as above

Base = declarative_base()

class Foo (Base):
    status = Column(DBStatus('OK', 'Error'))
    # ...

Upvotes: 1

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