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Saving to BLOB field in dbExpress with a join

Posted on 2003-11-13
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Last Modified: 2008-01-16
Writing to and from Interbase BLOB fields in Delphi7 like this….

Memo.Text:=ClientDataset.FieldByName(‘NOTES’).AsString;
and
ClientDataset.FieldByName(‘NOTES’).AsString:=Memo.Text;

...works fine until the underlying dataset is a join and the fields need to be written explicitly to the joined tables by the providers BeforeUpdateRecord event.
Building SQL statements in this event from the fields to be updated works for non-blob field but doesn’t work for BLOB fields as there is no INSERT syntax allowable for BLOB fields.
The question is therefore what should the providers BeforeUpdateRecord event look like to handle this kind of field? Is it possible to do it this way or is there another way?
A solution involving the datasets provider is preferable as this nicely encapsulates updates to all affected tables.
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Question by:RogerFroud
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Accepted Solution

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RogerFroud earned 0 total points
ID: 9782120
This solution seems to work ok but is a bit clumsy. Any better solution gets the points....

Dataset Provider BeforeUpdateRecord event
var
    SQL: String;
    myParams: TParams;
    FileStream : TFileStream; //debug only
    StringStream : TStringStream;
......
code to save all other fields using INSERT statements in SQL then....

      if (DeltaDS.FieldByName('NOTES').NewValue<>Null) then
        begin
          myParams := TParams.Create;
          myParams.Clear;
          myParams.CreateParam(ftBlob, 'BlobParam', ptInput);
          StringStream:=TStringStream.Create(DeltaDS.FieldByname('NOTES').NewValue);
          myParams.ParamByName('BlobParam').LoadFromStream(StringStream, ftBlob);
          StringStream.Free;
          SQL:=Format('UPDATE STOCKSUPPLIERS SET NOTES = :BlobParam WHERE STOCKSUPPLIERID = %d', [ID]);
          Connection.Execute(SQL, myParams);
          myParams.Free;
        end;
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