Write-once read-only field

Question

We need to implement a write-once (on object create) read-only field with SQLAlchemy/Elixir.

A quick-and-dirty solution:

class User(Entity):
    # some fields ...    
    _created    = Field(DateTime, default=datetime.utcnow)
    @property
    def created(self):
        return self._created

Is there a better approach (e.g., using before-insert triggers?)

Answer 1

Probably a lot of ways but one is to use a @validates:

from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class A(Base):
    __tablename__ = 'a'

    id = Column(Integer, primary_key=True)

    readonly1 = Column(String)
    readonly2 = Column(String)


    @validates('readonly1', 'readonly2')
    def _write_once(self, key, value):
        existing = getattr(self, key)
        if existing is not None:
            raise ValueError("Field '%s' is write-once" % key)
        return value


a1 = A()
a1.readonly1 = 'foo'
assert a1.readonly1 == 'foo'

try:
    a1.readonly1 = 'bar'
    assert False
except ValueError, e:
    print e

e = create_engine("sqlite://")
Base.metadata.create_all(e)
s = Session(e)
s.add(A(readonly1='foo', readonly2='bar'))
s.commit()

a2 = s.query(A).first()

try:
    a2.readonly2 = 'bar2'
    assert False
except ValueError, e:
    print e

@validates is just shorthand for using attribute events which you could use that to build out other ways of setting it up.