Hi Pierre,
On Aug 17, 2007, at 5:38 AM, Pierre Bernard wrote:
Hi Chuck!
You might want to simulate this using an attribute with write
format that contains SQL to do this. E.g. something along the lines
of "COLUMN + %V"
In your code you would set the special attribute's value to the
increment you want. On the next commit it would do the stuff.
The problem is that your real attribute would go out of sync and
that your fake attribute would apply the increment on each commit.
I am not so worried about that. It will go out of sync any way as
other instances are also updating the database.
You could also try dynamically adding the attribute in an editor to
the editing context and remove it once saveChanges went trough.
I considered this, but I see a potential problem. In the update
would be both set foo = 367, foo = foo + 1. I am not sure if that is
legal SQL (same column twice) and even if it is, the order of
execution is probably undefined. I guess I could just not update the
foo attribute so that it would not appear in the update and just set
fooOffset. Still the idea makes me uncomfortable and then there is
the problem of adding / removing this attribute. Hmmm, there is
maybe another problem here too. After the first update, EOF would
have a value of 1 cached for fooOffset in the snapshot. That would
make it not generate that attribute in the update if I again tried to
increment by 1. I would need to invalidate the object after saving.
I think what I really want is to get the offset into the row diffs
that are in EODatabaseOperation. These diffs are used by
EOSQLExpression when adding the attributes to the update statement.
I could easily do that with an EODatabaseOperation subclass, but
there is no easy way to get EODatabaseContext to use my sub-class. I
could create an EODatabaseContext.Delegate and implement
databaseContextWillOrderAdaptorOperations(EODatabaseContext dbCtxt,
NSArray databaseOps)
And substitute my own EODatabaseOperation subclass. I would then
need to get EOF to use my EOSQLExpression sub-class as well. That
will also be an annoyance as it is DB specific so I would need one
for each database. This path is looking less promising.
I think what I will end up doing is creating a method like
public static void applyOffsetToAttribute(EOEditingContext ec, String
entityName, String attributeName, Number offset)
or perhaps finding one in Wonder. I wonder....
When foo is updated, I would set an instance variable to snapshot
value - current value. Then, using the CooperatingEditingContext,
implement the didSave() method to check this instance variable and,
if not null, call applyOffsetToAttribute and set it to null.
Much fun.
Chuck
On Aug 16, 2007, at 11:53 PM, Chuck Hill wrote:
I recall someone making something that did this, but now I can't
find it. I don't even recall how it worked. What it did was
allow WO to update an attribute of a entity using bar = bar + 1
instead of bar = 7 so that updates from multiple sources would
not step on each other.
Does this sound familiar to anyone?
Thanks!
Chuck
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