Issue in the update table

Dear all,

I have a code in the modified method table level of the ProjExtCatMap . Here if a user selected the category id Then automatically a enum field mapping has to be updated as true for the respective categoryid and projid.in the PrjExtCat Table.

public void modifiedField(fieldId _fieldId)
{

    Object          formDataSource;
    CaeProjExtCat caeprojextcat;
    ;

    super(_fieldId);

if(this.ExtCategoryId)
      {
      ttsbegin;
        select forupdate projextcat where projextcat.CategoryID == this.CategoryID && projextcat.ProjId == this.ProjId && projextcat.Mapping == NoYes::No;
        {
            print projextcat.Mapping;
            projextcat.Mapping = NoYes::Yes;
            projextcat.update();
             print projextcat.Mapping;
        }
        ttscommit;
      }
}

when I run this code I get an error saying that projextcat cannot be updated. Please let me know if there is any other solution