fix Field.Virtual use in multi-table queries

gluon/sqlhtml.py

@@ -2625,6 +2625,9 @@ class SQLFORM(FORM):
                         continue
                     if field.type == 'blob':
                         continue
+                    if isinstance(field, Field.Virtual) and field.tablename in row:
+                        value = dbset.db[field.tablename][row[field.tablename][field_id]][field.name]
+                    else:
                         value = row[str(field)]
                     maxlength = maxtextlengths.get(str(field), maxtextlength)
                     if field.represent: