Tracking Data Changes (entities)

So for simple entities (intrinsic property types int, long,
DateTime, etc)
within the Set I'd mark it as modified (should I check
whether the value
being set is the same as the original?). using a bitArray to
indicate what
fields have changed (I presume implementing IPropertyChanged
would also be a
requirement - using this interface I could even move the
code that tracks
changes outside the entity good/bad?)

Versions is something that I think is a must -> in the
application that I
have I have a view screen which then becomes a modify if the
modify fails I
have to restore the original values. With a RejectChanges
type method on the
base class.

Would you think it wise to have a TrackChanges property on
the base so that
I can programmatically monitor for changes or not -> bulk
updates?

I like the idea of the aggregated member for versions I as
thinking
hashtable with corresponding type converters.

How do you track changes to collections? i.e.,
adding/removing members,
knowing that an entity is dirty based on an item within the
collection being
Modified - I'm not to sure whether I'll have to worry
about collections but
some pointers would be appreciated.

Thanks,
Rob.


-----Original Message-----
From: Discussion of development on the .NET platform using
any managed
language [mailtoOTNET-CL
RDISCUSS.DEVELOP.COM] On Behalf Of Frans Bouma
Sent: Thursday, August 31, 2006 4:30 PM
To: DOTNET-CLRDISCUSS.DEVELOP.COM
Subject: Re: [DOTNET-CLR] Tracking Data Changes (entities)

> I'm thinking of adding support to my entity classes to
track
> when values change similar to the dataset - i.e. when a
> property changes I store the original value. I'm
thinking
> that my base class will have a accept/reject changes
etc.
> Does anybody have any suggestions or things not to do
up front.
> Any papers blogs greatly appreciated.

        There are two ways to do this which are common among
the O/R mapper
world:
1) store the original data in a session/context and when you
have to do
something with the entity again (save it for example), you
compare values and thus know which values were changed.
(n)Hibernate uses
this, DLinq does too.
2) have per field a flag which signals if the field has been
changed. Also
use a flag for the whole entity to signal if it's dirty,
so you don't have to traverse the flags. Some O/R mappers
store these flags
also in the session/context and implement an interface
on the entities, others (like my LLBLGen Pro) store these
flags in an object
in the central base class of the entities.

        Method 2 is far superior over method 1, because
it's way faster. If
you don't care if you're using POCO or non POCO classes,
you can store the flags in a base class. This has great
advantages, that you
can track changes across tiers/appdomains for example
without messy code for attaching/detaching entities with a
central session,
as most O/R mappers out there need to use who don't use
a central base class.

        I'm not sure how far you want to go. If you also
want to have
versioning, you can store the values in a separate object,
aggregated inside the entity, and clone these for versioning
purposes so you
can rollback fields to a given version if you want to
(I do that too). This has the advantage that you can simply
save the field
values under a name internally in the entity when a
transaction starts, and when the transaction mid-way rolls
back for example,
you don't have to mess with fk fields which have to
roll back to previous values, identity fields which have to
be reset etc.,
you just rollback your fields object, done 

                Frans

------------------------------------------------------------
------------
Lead developer of LLBLGen Pro, the productive O/R mapper for
.NET
LLBLGen Pro website: http://www.llblgen.com
My .NET blog: http://weblogs.asp.net/
fbouma
Microsoft MVP (C#)
------------------------------------------------------------
------------

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