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Writing and Reading Variables to/from BLOB fields

Posted on 1998-02-26
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Last Modified: 2010-04-06
I have a record variable I am trying to write to and read from a BLOB field.

How do I go about this?

I have tried using BlobStreams but without much success.
The examples in the Help file will not even compile.

Thanks

Chris
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Question by:ChrisBerry
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6 Comments
 
LVL 2

Expert Comment

by:Waldek
ID: 1359821
Do You create a Stream before save to stream ?
0
 
LVL 5

Expert Comment

by:JimBob091197
ID: 1359822
Hi

Here is my answer from a previous similar question.



Here is code to save an array, record, structure, etc. to a blob field, and read it back in.  In my e.g. I use an array of integer, but it could be Delphi type (i.e. record) etc.

procedure SaveToBlob;
var
  i: Integer;
  tbl: TTable;
  bf: TBlobField;
  arr: array[1..20] of Integer;
  MemStr: TMemoryStream;
begin
  // Fill array with values.
  FillChar(arr, SizeOf(arr), 0);
  for i := 1 to 20 do
    arr[i] := i * 5;

  // Create a memory stream, and copy array into it.
  MemStr := TMemoryStream.Create;
  MemStr.SetSize(SizeOf(arr));
  CopyMemory(MemStr.Memory, @arr, SizeOf(arr));

  // Open table, save contents of memory stream to blob field.
  tbl := TTable.Create(Self);
  try
    tbl.DatabaseName := 'C:\Temp';
    tbl.TableName := 'Test.db';
    tbl.Active := True;
    tbl.Append;
      bf := TBlobField(tbl.FieldByName('MyBlob'));
      bf.LoadFromStream(MemStr);
    tbl.Post;
  finally
    tbl.Free;
  end;

  MemStr.Free;
end;

procedure LoadFromBlob;
var
  i: Integer;
  tbl: TTable;
  bf: TBlobField;
  arr: array[1..20] of Integer;
  MemStr: TMemoryStream;
begin
  // Create memory stream.
  MemStr := TMemoryStream.Create;
  MemStr.SetSize(SizeOf(arr));

  // Open table, move to last record, get content of blob field into memory stream.
  tbl := TTable.Create(Self);
  try
    tbl.DatabaseName := 'C:\Temp';
    tbl.TableName := 'Test.db';
    tbl.Active := True;
    tbl.Last;
    bf := TBlobField(tbl.FieldByName('MyBlob'));
    bf.SaveToStream(MemStr);
  finally
    tbl.Free;
  end;

  // Copy content of memory stream to arr.
  CopyMemory(@arr, MemStr.Memory, SizeOf(arr));
  MemStr.Free;
  // arr now contains the data from blob field...
end;

Regards,
JB
0
 
LVL 5

Accepted Solution

by:
inter earned 50 total points
ID: 1359823
Assume you have the following record:

type
  DullRecord = record
     x : integer;
     y : real;
     z : array[0..200] of char;
  end;

and you have created a database with the field called Blob1. The in you program you should create a field coresponding to it and name it as MyBlob(just double click on the table and add the field, rename it or not);

Assuming that your table is open, and current record is set, the following code writes the record to your blob field:(assume you table name is Table1):

procedure Form1.WriteToBlob(var R : TDullRecord);
var
  S : TMemoryStream; //besure to include the coresponding unit
begin
  // Create the memory stream
  S := TMemoryStream.Create;
  try
    //Put table to edit state
    Table1.Edit;
    //write record to memory
    S.Write(R, Sizeof(R));
    //read it to blob field
    MyBlob.LoadFromStream(S);
    //make changes permenant
    Table1.Post;
  finally
    S.Free;
  end;
end;

Ask anything, I am here,
Sincerely
Igor
0
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Author Comment

by:ChrisBerry
ID: 1359824
Thanks one and all, the fastest replies ever.

The problem I had was that the Borland example uses TBlobStream := CreateBlobStream which returns TStream and there for incompatible.

Chris

0
 
LVL 5

Expert Comment

by:inter
ID: 1359825
JimBob excuse me,

Yyour coment is not there when I enter the page, I just cut and paste, (tell me if I could do anything for you-although you are our superstar-my special subject is image processing, optimization and computer vision)sorry again.

Igor
0
 
LVL 5

Expert Comment

by:JimBob091197
ID: 1359826
Hi Igor

No problem.  It's happened to me before.  ;-)

Cheers,
JB
0

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