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 (eg, using before-insert triggers?)
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.
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