Re: [sqlalchemy] Automatically set primary key to None when deleted?

[Paul Molodowitch]

>
> Particularly since sqlalchemy has already established that it's willing to
> expire dict members when they may not be valid anymore - ie, what it does
> to clear any "cached" values from a row proxy after the session is
> committed.
>
> well it doesn't expire the deleted object right now because it's been
> evicted from the Session by the time the commit goes to expire things.
> Changing that behavior now would definitely bite a lot of people who depend
> on how it is right now (other people who are also looking at their deleted
> objects against my recommendations... :) )
>

Makes sense - maybe it could be a configurable option? Dunno how many
people (besides me!) would be interested in such behavior, though... =P

> 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)
>>
>
>  Hmm... very interesting.  I'll have to read up what what exactly this is
> doing (ie, what is state.identity?)...
>
> it's documented here:
> http://docs.sqlalchemy.org/en/rel_0_9/orm/internals.html?highlight=instancestate#sqlalchemy.orm.state.InstanceState.identity
>
>
Thanks for the link!

 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:

>
>  Not clear on why this matters - if we're iterating through all the
> constraints, and returning True if any of them is matched, what difference
> does it make which one is evaluated first?  Except potentially from a
> performance standpoint, I suppose...
>
> what if there are two constraints, and only one is satisfied for a given
> object's values (e.g. the constraint is now satisfied by some other row),
> the other one is not present.  Is the answer True or False?
>

In the scenario I was envisioning, True (ie, it exists).  Basically, "Would
it violate ANY unique constraints if I tried to insert it? Yes".

Of course, I see your point: that in some situations, this might not fit
conceptually with the answer to the question, "Does THIS object exist in
the database?"  But I guess that's likely your point... that there isn't
really a good "universal" way to answer that question.

-- 
You received this message because you are subscribed to the Google Groups 
"sqlalchemy" group.
To unsubscribe from this group and stop receiving emails from it, send an email 
to sqlalchemy+unsubscr...@googlegroups.com.
To post to this group, send email to sqlalchemy@googlegroups.com.
Visit this group at http://groups.google.com/group/sqlalchemy.
For more options, visit https://groups.google.com/d/optout.

Re: [sqlalchemy] Automatically set primary key to None when deleted?

[Mike Bayer]

On 7/3/14, 1:01 PM, Paul Molodowitch wrote:
>
>
> That makes sense... but if if it really means nothing, and we
> shouldn't be looking at it, then why keep it's attributes around at all? 
because it is an additional step to actually erase the attributes and
just hadn't been considered.


> Particularly since sqlalchemy has already established that it's
> willing to expire dict members when they may not be valid anymore -
> ie, what it does to clear any "cached" values from a row proxy after
> the session is committed.
well it doesn't expire the deleted object right now because it's been
evicted from the Session by the time the commit goes to expire
things. Changing that behavior now would definitely bite a lot of
people who depend on how it is right now (other people who are also
looking at their deleted objects against my recommendations... :) )




>
> Of course, you could make the case that other pieces of the program
> may want to inspect the data that was on there, after the fact...
> maybe you're going to print out something that says, "RIP Steggy", or
> something - but in that case, the one field that really DOESN'T make
> any sense in this case (and it seems like it would be a common
> pattern!) is the one that exists solely as a means to look it up in
> the database, it's auto-incremented id column.  Which is what prompted
> this question...

well all the cols don't exist anymore, not just the primary key. the
inspect(obj).deleted call does allow this information (that the object
was deleted) to be known, though not very transparently.

> 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)
>
>
> Hmm... very interesting.  I'll have to read up what what exactly this
> is doing (ie, what is state.identity?)...
it's documented here:
http://docs.sqlalchemy.org/en/rel_0_9/orm/internals.html?highlight=instancestate#sqlalchemy.orm.state.InstanceState.identity

 
>
> 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
>
>
> Yep, it looks like that's doing basically what I was thinking of.  Thanks!
>
> 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:
>
>
> Not clear on why this matters - if we're iterating through all the
> constraints, and returning True if any of them is matched, what
> difference does it make which one is evaluated first?  Except
> potentially from a performance standpoint, I suppose...
what if there are two constraints, and only one is satisfied for a given
object's values (e.g. the constraint is now satisfied by some other
row), the other one is not present.  Is the answer True or False?  


-- 
You received this message because you are subscribed to the Google Groups 
"sqlalchemy" group.
To unsubscribe from this group and stop receiving emails from it, send an email 
to sqlalchemy+unsubscr...@googlegroups.com.
To post to this group, send email to sqlalchemy@googlegroups.com.
Visit this group at http://groups.google.com/group/sqlalchemy.
For more options, visit https://groups.google.com/d/optout.

Re: [sqlalchemy] Automatically set primary key to None when deleted?

[Paul Molodowitch]

On Wed, Jul 2, 2014 at 9:22 PM, Mike Bayer  wrote:

>
> 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.
>

That makes sense... but if if it really means nothing, and we shouldn't be
looking at it, then why keep it's attributes around at all?  Particularly
since sqlalchemy has already established that it's willing to expire dict
members when they may not be valid anymore - ie, what it does to clear any
"cached" values from a row proxy after the session is committed.

Of course, you could make the case that other pieces of the program may
want to inspect the data that was on there, after the fact... maybe you're
going to print out something that says, "RIP Steggy", or something - but in
that case, the one field that really DOESN'T make any sense in this case
(and it seems like it would be a common pattern!) is the one that exists
solely as a means to look it up in the database, it's auto-incremented id
column.  Which is what prompted this question...


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)
>

Hmm... very interesting.  I'll have to read up what what exactly this is
doing (ie, what is state.identity?)... It's possibly that
inspect(steggy).deleted may just give me what I need though. Thanks for
both those tips! (In case you couldn't tell, I'm still new to / exploring
sqlalchemy...)


> ...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.
>

True.  There's really no way for a generic "exists" function to know what
conditions you want to query a generic class on to determine "existence."
 Which is why I was suggesting the uniqueness constraint...

 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:
>

Sort of - the thinking here was that you could just ask, "If I tried to
insert this object into the table, would it violate any uniqueness
constraints?", and get back a boolean result... and you could use that as a
reasonable heuristic for determining existence, in a fairly generic way.


> 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
>

Yep, it looks like that's doing basically what I was thinking of.  Thanks!

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:
>

Not clear on why this matters - if we're iterating through all the
constraints, and returning True if any of them is matched, what difference
does it make which one is evaluated first?  Ex

Re: [sqlalchemy] Automatically set primary key to None when deleted?

[Mike Bayer]

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


-- 
You received this message because you are subscribed to the Google Groups 
"sqlalchemy" group.
To unsubscribe from this group and stop receiving emails from it, send an email 
to sqlalchemy+unsubscr...@googlegroups.com.
To post to this group, send email to sqlalchemy@googlegroups.com.
Visit this group at http://groups.google.com/group/sqlalchemy.
For more options, visit https://groups.google.com/d/optout.

[sqlalchemy] Automatically set primary key to None when deleted?

[Paul Molodowitch]

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?


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()


...which, in this case, would simply run a query to see if any dinosaurs 
exist with the name "Steggy".  Needing to set up some extra parameters to 
make this possible - such as adding a unique constraint on the name column 
- would be potentially possible.  And yes, I know I can always fall back on 
just manually constructing a query against the name field myself...

-- 
You received this message because you are subscribed to the Google Groups 
"sqlalchemy" group.
To unsubscribe from this group and stop receiving emails from it, send an email 
to sqlalchemy+unsubscr...@googlegroups.com.
To post to this group, send email to sqlalchemy@googlegroups.com.
Visit this group at http://groups.google.com/group/sqlalchemy.
For more options, visit https://groups.google.com/d/optout.