On 7/2/14, 10:05 PM, Paul Molodowitch wrote:
> Suppose I have a super simple table like this:
>
> class Dinosaur(Base):
> __tablename__ = 'dinosaurs'
> id = Column(Integer, primary_key=True)
> name = Column(String(255))
>
>
> We assume that the id is set up in such a way that by default it
> always gets a unique value - ie, it uses autoincrement in MySQL, or a
> sequence in postgres, etc.
>
> Now, suppose I get an instance of this class, and then delete it:
>
> steggy = session.query(Dinosaur).filter_by(name='Steggy').one()
> print steggy.id
> session.delete(steggy)
> session.commit()
> print steggy.id
>
>
> What I'd ideally like to see is that it first print the id of the row
> that it pulled from the database, and then print 'None':
>
> 30
> None
>
>
> Is there any way that I can configure the id column / property so that
> it is automatically "cleared" on delete like this?
the "steggy" object is a proxy for a database row. when you delete that
row, then commit the transaction, the object is detached from the
session, and everything on it is expired. there is no row. check
inspect(steggy).deleted, it will say True - that means in your system,
the object is meaningless. ideally no part of your program would be
looking at that proxy any more, you should throw it away. it means nothing.
as far as setting everything to None, you could try a handler like this:
@event.listens_for(Session, 'after_flush')
def after_flush(sess, ctx):
for obj in sess.deleted:
mapper = inspect(obj)
for key in mapper.attrs.keys():
obj.__dict__[key] = None
If not, as a consolation prize, I'd also be interested in the easiest
way to query if a given instance exists in the database - ie, I could do
something like:
>
> session.exists(steggy)
>
>
> OR
>
> steggy.exists()
>
from sqlalchemy import inspect
def exists(session, obj):
state = inspect(obj)
return session.query(state.mapper).get(state.identity) is None
print exists(sess, a1)
>
> ...which, in this case, would simply run a query to see if any
> dinosaurs exist with the name "Steggy".
that's totally different. That's a WHERE criterion on the "name" field,
which is not the primary key. that's something specific to your class
there.
> Needing to set up some extra parameters to make this possible - such
> as adding a unique constraint on the name column -
OK, so you want a function that a. receives an object b. looks for
UNIQUE constraints on it c. queries by those unique constraints (I guess
you want the first one present? not clear. a table can have a lot of
unique constraints on it....) that would be:
from sqlalchemy import inspect, UniqueConstraint
def exists(session, obj):
state = inspect(obj)
table = state.mapper.local_table
for const in table.constraints:
if isinstance(const, UniqueConstraint):
crit = and_(*[col == getattr(obj, col.key) for col in const])
return session.query(state.mapper).filter(crit).count() > 0
else:
return False
the unique constraints are a set though. not necessarily deterministic
which one it would locate first. I'd use more of some kind of declared
system on the class:
class X(Base):
lookup_class_via = ('name',)
id = Column(Integer, primary_key=True)
name = Column(String)
from sqlalchemy import inspect, UniqueConstraint
def exists(session, obj):
crit = and_(*[col == getattr(obj, col.key) for col in
obj.__class__.lookup_class_via])
return session.query(state.mapper).filter(crit).count() > 0
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