On May 25, 2012, at 4:53 PM, Stephan Hügel wrote:
> I'd like to represent a Correspondent object, which can refer to two separate
> objects
>
> Corr_1
> - id
> - email_address
>
> Corr_2
> - id
> - email_address
>
> Correspondent
> - id
> - corr_1.id
> - corr_2.id
>
> But I have two problems:
> I'm using MySQL, so I can't use a check constraint to ensure that only one of
> the corr_*.id fields can be populated.
Assuming corr_1 and corr_2 are two separate tables, if you have control over
the schema of this application and you wish to have "email_address" be unique
across the union of these two tables, the best practice here would be to have
only one table of email addresses. Otherwise you have to make due with what
you're given.
> Is it it a bad idea to enforce the constraint at the SQLAlchemy level, by
> doing
> if all(corr1.id, corr2.id) or not any(corr_1.id, corr2.id):
> raise …
> in my __init__?
the general idea is fine though I think you mean something more like this (note
I'm using the exclusive or operator here though the all/any() approach would
work too):
class Correspondent(Base):
def __init__(self, corr_1, corr2):
assert (corr_1 is None) ^ (coor_2 is None)
the important part is that when you create objects, the primary key/foreign key
attributes like "id" and such aren't populated until a flush occurs, so we deal
with references to the objects themselves.
>
> My second problem relates to then being able to retrieve the related
> .email_address attribute. My naïve relationship above obviously won't work
> for that. Any pointers would be greatly appreciated.
usually @property achieves this:
class Correspondent(Base):
@property
def email_address(self):
return self.corr_1.email_address if self.corr_1 else
self.corr_2.email_address
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