How can I add fields to a clientdataset at runtime?

Question

I have a TClientDataSet, which is provided by a TTable’s dataset. The dataset has two fields: postalcode (string, 5) and street (string, 20)

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Answer 1

Well i found a simpler solution, as i have 24 fields in my sql i didnt wanted to add them all manually so i added a dummy field to the sql statement instead like:

select '      ' as city, the rest of the fields ... 

which i can modify in my program OnAfterOpen event.

Well i had to define in the sql how long that field should be by leaving enough empty spaces, for instance 5 empty spaces for 5 characters, so i must know how long the city name could be.

Answer 2

If you want to add additional fields other than those exist in the underlying data, you need to also add the existing fields manually as well. The dataset needs to be closed when you're adding fields, but you can have the necessary metadata with FieldDefs.Update if you don't want to track all field details manually. Basically something like this:

var
  i: Integer;
  Field: TField;
begin    
  cds.SetProvider(Table1);

  // add existing fields
  cds.FieldDefs.Update;
  for i := 0 to cds.FieldDefs.Count - 1 do 
    cds.FieldDefs[i].CreateField(cds);

  // add calculated field
  Field := TStringField.Create(cds);
  Field.FieldName := 'city';
  Field.Calculated := True;
  Field.DataSet := cds;

  cds.Open;
end;

Also see this excellent article by Cary Jensen.

Answer 3

You should use CreateDataset after add field:

cds.SetProvider(Table1);
cds.FieldDefs.Add('city', ftString, 20);
cds.CreateDataset; 

cds.Open;
cds.Edit;
cds.FieldByName('city').AsString := 'Test';  
cds.Post;

Answer 4

Would like to share more accurate Query for unexisting fields. I bet it's better to use cast, neither spaces!

select E.NAME, E.SURNAME, cast(null as varchar(20)) as CITY
from EMPLOYEE E

e.g. | Marc'O | Polo | <NULL> |

It's more accurate, can definetly see field size, understandable, easy, safe!

Answer 5

if you want to combine already existing "dynamic" data fields (from provider side) with additional client side persistent fields (calculated, lookup, internalcalc, aggregate) you should subclass CDS. just introduce extra boolean property CombineFields and either override BindFields (in newer delphi versions) or the entire InternalOpen (as I did in d2006/2007) with the following line

if DefaultFields or CombineFields then CreateFields; { TODO -ovavan -cSIC : if CombineFields is true then persistent fields will coexist with Default ones }

that will allow you to avoid all that runtime mess with FieldDefs/CreateField