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Reuse an Instance of TVirtualQuery
Posted: Wed 23 Aug 2017 17:03
by Aggie85
Howdy All!
I have for the most part successfully converted my large ~14 year program from TxQuery to TVirtualQuery (over 4500+ instances).
For the most part, my implementation of TVirtualQuery ALWAYS recreates the TVirtualQuery object. I have a few modules in my product where after so many recreations of TVirtualQuery, I get an EAccessError.
I have tried implementing a simple cleanup procedure to reuse an instance of a TVirtualQuery and it never works.
The procedure is as follows:
1) Make sure instance of TVirtualQuery is inactive.
2) Clear the previous Source Data Sets.
3) Add new source data sets.
4) Attempt reuse.
Step #4 always tells me my new source datasets don't exist.
What am I missing?
All the best,
Aggie85
Re: Reuse an Instance of TVirtualQuery
Posted: Mon 28 Aug 2017 08:51
by MaximG
Thank you for the information. Please describe the specified situation in more detail. For this, compose and send us a small sample, which when executed causes the EAccessError error. You can send this sample using the e-support form(
https://www.devart.com - the "Support"\"Request Support" menu)
Re: Reuse an Instance of TVirtualQuery
Posted: Fri 01 Sep 2017 16:56
by Aggie85
Howdy Maxim!
The EAccessError is only caused after a lot of resets and it may/may not be caused by TVirtualQuery.
Back to my original question, could you please tell me how to reuse an instance of a TVirtualQuery to execute a NEW SQL statement with new SourceDataSets? The only way I have found to work is to delete previous instance of TVirtualQuery and recreate it (new).
Have a great weekend,
Aggie85
Re: Reuse an Instance of TVirtualQuery
Posted: Wed 06 Sep 2017 14:06
by MaximG
The sample of deleting and recreating SourceDataSets when using VirtualQuery :
Code: Select all
program VQC;
{$APPTYPE CONSOLE}
{$R *.res}
uses
System.SysUtils, Data.DB, DBAccess, DBClient, VirtualQuery;
var
DataSet1, DataSet2: TClientDataSet;
VirtualQuery: TVirtualQuery;
begin
DataSet1 := TClientDataSet.Create(Nil);
DataSet2 := TClientDataSet.Create(Nil);
try
DataSet1.FieldDefs.Add('DeptNo', ftInteger);
DataSet1.FieldDefs.Add('DName', ftString, 64);
DataSet1.CreateDataSet;
DataSet1.Open;
DataSet1.AppendRecord([10, 'ACCOUNTING']);
DataSet1.AppendRecord([20, 'SALES']);
DataSet2.FieldDefs.Add('EmpNo', ftInteger);
DataSet2.FieldDefs.Add('DeptNo', ftInteger);
DataSet2.FieldDefs.Add('EName', ftString, 64);
DataSet2.CreateDataSet;
DataSet2.Open;
DataSet2.AppendRecord([1, 10, 'JONES']);
DataSet2.AppendRecord([2, 10, 'MARTIN']);
DataSet2.AppendRecord([3, 20, 'SMITH']);
VirtualQuery := TVirtualQuery.Create(Nil);
try
VirtualQuery.SourceDataSets.Add(DataSet1, '', 'Table1');
VirtualQuery.SQL.Text := 'Select * From Table1';
VirtualQuery.Open;
VirtualQuery.SourceDataSets.Clear;
VirtualQuery.SourceDataSets.Add(DataSet2, '', 'Table2');
VirtualQuery.SQL.Text := 'Select * From Table2';
VirtualQuery.Open;
finally
VirtualQuery.Free;
end;
finally
DataSet1.Free;
DataSet2.Free;
end;
end.
Re: Reuse an Instance of TVirtualQuery
Posted: Wed 06 Sep 2017 16:19
by Aggie85
Howdy Maxim!
I will need to put together a small example as I was doing what your small example showed.
The only difference is I was performing an Update on a table in the first SQL statement and then a DELETE on the second table. The second statement is where I get the unknown table message.
Thanks,
Aggie85