Make a ComboBox model with a blank item

Sometimes you want to populate a combo box from another table. This is all well and good when you want to pick from a list of items populated from a table, but what if you want to also be able to select "no" item, or otherwise deselect any current selections? When populating a combo box from a model, the combo box will only allow a selection from the list. Therefore, it is necessary to create a model that has an extra blank item.

This is very easy to do with a direct SQL statement.

 97 std::shared_ptr<Wt::WAbstractItemModel> Mrp::Company::getComboBoxModel( int sid, int tid, const std::string & where, Wtx::Dbo::Session & session )
 98 {
 99   auto retVal = std::make_shared< Wtx::Dbo::QueryModel< std::tuple<int,std::string> > >();
100
107   Wt::Dbo::Transaction t(session);
108
109 //  auto w = where;
110 //  if( tid != -1 )
111 //    w = Wt::WString("WHERE id={1}").arg(tid).toUTF8();
112
113   auto sql =
114     Wt::WString
115     (
116      "SELECT id,\"keyField\" || ' ~ ' || \"cfyField\" AS key FROM \"crmCompany\""
117      " {1}"
118      " UNION "
119      "SELECT -1 as id, '' as key"
120      " ORDER BY key"
121     )
122     .arg( where )
123     .toUTF8()
124     ;
125
126
127   auto query =
128    session.query< std::tuple<int,std::string> >( sql )
129    ;
130
131   retVal-> setQuery( query );
132   retVal-> addColumn("id");
133   retVal-> addColumn("key");
134
135   return retVal;
136 }