#18 Initial values of part objects are lost

[Nando Dessena]

[Extracted from a newsgroup message by Mark Smith]

I just finished about 1 day of struggle to find why
when I initialized attributes of a Part() the
initialization did not hold. Then I found the answer
which I post here.

To many this is obvious, but since it was a "gotcha"
for me I thought maybe this would help others.

When writing an attribute of a part to a stream, IO
(as designed) checks IsRequired and IsMandatory to see
if it needs to write the attribute's data.
I assume this is in order to save space in the BLOB
where needed.

however this causes the following scenario (I will
use a string value for the example):

1. The object is created, and the string value is
   initialized in
   AfterCreate() (or Initialize, or an overloaded
   constructor, or wherever) with "1111".
2. I edit the object on an edit screen and change the
   string to '' (empty).
   --> this is also the "default" value for a string.
3. On calling Exposer.PostChanges the object is stored,
   and in iterating over the members the writer checks the
   string against the default value.
4. Since my string ('') IS the default value, it passes
   this test and is not written to the stream.
5. Next time I load my object, it is initialized with
   '1111' rather than the '' I wanted it stored as.

In order not to have to open every part object and mark
every attribute "required" I changed the following code
in InstantPersistence.pas:

function TInstantAttribute.GetIsRequired: Boolean;
begin
// Commented out the IsRequired check
Result := Assigned(Metadata){ and Metadata.IsRequired};
end;

While this kills the IsRequired functionality, it it
more important IMHO that I can successfully initialize
my Part(s).

[Nando Dessena]

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The "required" functionality (which needs to be fixed since
it appears to be broken anyway) cannot be sacrificed, so I
don't think the fix is acceptable. I'd prefer to not check
IsRequired & such and always store all attributes regardless
of their default values.