TDataset.CopyFields

Description

Often when manipulating datasets with similar structures, you need to copy the records from one dataset to another.  E.g. you may have fetched some records in a query or clientdaset and have located the matching records in another dataset.  You might then want to ensure the record values match without refetching the target dataset.
This looks like it would be easy to implement - i have rewritten it to take care of nested datasets as well as ordinary dataset fields.

Steps to Reproduce:

This is a requested new public method in the base TDataset class.

function TDataSet.CopyFields(Source: TDataSet): Integer;
// copies matching fields in current records- returns number of fields copied
var
  FieldCtr: Integer;
  DestField, SourceField: TField;
begin
  Result := 0;
  for FieldCtr := 0 to Source.FieldCount - 1 do begin
    SourceField := Source.Fields[FieldCtr];
    Field := FindField(SourceField.FieldName);
    if not Assigned(Field) then Continue;
    if Field.ClassType = TDataSetField then begin  // nested datasets
      while TDataSetField(Field).NestedDataSet.RecordCount > 0 do
        TDataSetField(Field).NestedDataSet.Delete;
      TDataSetField(SourceField).NestedDataSet.First;
      while not TDataSetField(SourceField).NestedDataSet.Eof do begin
        TDataSetField(Field).NestedDataSet.Append;
        CopyFields(TDataSetField(Field).NestedDataSet, TDataSetField(SourceField).NestedDataSet);
        TDataSetField(Field).NestedDataSet.Post;
        TDataSetField(SourceField).NestedDataSet.Next;
      end; 
    end else
      Field.Value := SourceField.Value;
    Inc(Result);
  end;
end;


This would typically be used as follows:

SourceDS.First;
while not SourceDS.EOF do begin
  if DestDS.Locate({info required to find matching record}) then begin
    DestDS.Edit;
    DestDS.CopyFields(SourceDS);
    DestDS.Post;
  end;
  SourceDS.Next;
end;