Save/Load objects as blob in a database - delphi

Question

Whats wrong with my code? I try to save and then load again an object to a blob field in a database, but get nothing back.
Records are saved, but I can't say if the data was written correctly because I cant read the data back.
Here is the object type:
TMyObject = class
Name: string;
end;
And here I try to save:
procedure TForm1.btnSaveObjectClick(Sender: TObject);
var
myObject: TmyObject;
aMemoryStream: TMemoryStream;
begin
myObject:= TMyObject.Create;
myObject.Name:=edtName.Text;
aMemoryStream:= TMemoryStream.Create;
aMemoryStream.Write(myObject, myObject.InstanceSize);
aMemoryStream.Position:=0;
with TSQLQuery.Create(nil) do
begin
DataBase:=Conn;
SQL.Text:='INSERT INTO testtable (data) VALUES (:data)';
try
ParamByName('data').LoadFromStream(aMemoryStream, ftBlob);
ExecSQL;
TX.CommitRetaining;
finally
aMemoryStream.Free;
myObject.Free;
Free;
end;
end;
end;
Trying to read the data back up again:
procedure TForm1.btnLoadObjectClick(Sender: TObject);
var
myObject: TMyObject;
BlobStream : TStream;
begin
with TSQLQuery.Create(nil) do
begin
DataBase:=Conn;
SQL.Text:='SELECT data FROM testtable';
myObject:= TmyObject.Create;
try
Open;
Last;
BlobStream:= CreateBlobStream(FieldByName('data'), bmread);
BlobStream.Position:=0;
BlobStream.Read(myObject, BlobStream.Size);
ShowMessage('Stored Name: ' +myObject.Name);
finally
myObject.Free;
Free;
end;
end;
end;
Also, should BlobStream be free'd?

Answer 1

The correct way for storing your objects into files, streams or blob fields is to first extend your object with additional methods for loading and saving data from your objects fields (objects internal variables) into single memory block.
You do this by saving one field after another.
If your objects are dynamically sized (containing dynamic arrays or strings) don't forget to store the size of these separately so you will know how much data belongs to them when loading your objects later on.
Also if your objects contain some other objects you also need them to have similar methods for storing and loading their data.
The implementation depends heavily on your object's class design. Here a code example for a string field:
type
TMyObject = class
public
Name: string;
procedure SaveToStream(AStream: TStream);
procedure LoadFromStream(AStream: TStream);
end;
procedure TMyObject.SaveToStream(AStream: TStream);
var
Len: Integer;
begin
Len := Length(Name);
AStream.Write(Len, SizeOf(Len));
AStream.Write(PChar(Name)^, Len);
end;
procedure TMyObject.LoadFromStream(AStream: TStream);
var
Len: Integer;
begin
AStream.Read(Len, SizeOf(Len));
SetString(Name, PChar(nil), Len);
AStream.Read(PChar(Name)^, Len);
end;
With this, it is possible to use the stream that CreateBlobStream returns and just save myObject to the blobfield:
BlobField := FieldByName('data') as TBlobField;
Stream := CreateBlobStream(BlobField, bmWrite);
myObject.SaveToStream(Stream);
..or load it from the stream:
Stream:= CreateBlobStream(FieldByName('data'), bmread);
myObject.LoadFromStream(Stream);