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Tim · Guest

I have a master/detail form showing a list of detail items displayed in a
DBGrid and need to show a balance column, depending upon the value of an
'in' and an 'out' field, also shown in the grid. Because of the variable
sort order of the grid, I cannot maintain a balance field in the database,
calculated when an 'in' or an 'out' is initially entered, so it must be done
when the grid is displayed.

I have added a global variable, which is set to 0 when the master
ClientDataSet is scrolled, added a new field in the detail ClientDataSet and
then used the OnCalcFields method to generate the balance field value. (I
could do it all with SQL, but there are a couple of checks that need to be
done that make it easier to do programmatically).

In theory this should be straightforward, but it depends upon OnCalcFields
being fired once for each row of data, but it is actually fired 3 or 4 times
for each row, making the balance nonsense.

Could anyone suggest a better way of doing this?

Thanks.

Craig Stuntz [TeamB] · Guest

Your requirements aren't clear from your message, but have you
investigated the aggregate field features of TClientDataSet?

--
Craig Stuntz [TeamB] · Vertex Systems Corp. · Columbus, OH
Delphi/InterBase Weblog : http://blogs.teamb.com/craigstuntz
IB 6 versions prior to 6.0.1.6 are pre-release and may corrupt
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Tim · Guest

Craig,

I suppose that my question comes down to this: when is the OnCalcFields
method fired? I assumed that when a DBGrid was being populated, for each row
the method is fired once for any one particular calculated field. However,
if in populating a DBGrid it might be fired more than once, then I need to
look elsewhere for solving my problem.

To answer your question, no I have never looked at Aggregates, so I will
check it out and if it helps, then I appreciate the idea.

Thanks.

"Craig Stuntz [TeamB]" <craig_stuntz (AT) nospam (DOT) please [a.k.a. acm.org]> wrote
in message news:44fd7e89$1 (AT) newsgroups (DOT) borland.com...

Craig Stuntz [TeamB] · Guest

Tim wrote:

Tim · Guest

Thanks Craig. I am not sure that Aggregates will quite do what I have in
mind, but I think I can get round the firing problem, although I will bear
in mind your comment about 'For this reason, any code in an OnCalcFields
handler needs to be very
lightweight and in general designed to run multiple times.'

Tim

"Craig Stuntz [TeamB]" <craig_stuntz (AT) nospam (DOT) please [a.k.a. acm.org]> wrote
in message news:44fee737$1 (AT) newsgroups (DOT) borland.com...

Craig Stuntz [TeamB] · Guest

You could also try an InternalCalc field, which is re-evaluated for a
row only when the row is posted.

--
Craig Stuntz [TeamB] · Vertex Systems Corp. · Columbus, OH
Delphi/InterBase Weblog : http://blogs.teamb.com/craigstuntz
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Tim · Guest

Craig,

I have been playing around a bit and getting nowhere, so if you have the
patience, I realise that my initial post was misleading.

The balance field I was referring to should have been described as a
'running balance' field. An example would be:

Invoice Credit Debit Running Balance
1 1 1
2 1 0
3 1 1
4 1 2

Obviously a 'balance' field as I initially described it would be simple, but
a 'running balance' needs to know what the previous record's running balance
was and if one record changes, then all the records after it will be
effected.

I don't think that it can easily be done, if at all, using calculated
fields. However, adding a new field to the database called running_balance,
iterating through the dataset, calculating the running balance as I go and
then displaying this field as a column in the grid does work. Not as quick
or as neat as I was hoping, but it works.

Tim

Craig Stuntz [TeamB] · Guest

Tim wrote: