added some error checks

workflows/mysql/context.py

@@ -77,6 +77,8 @@ class DBContext:
         self.target_att = postdata.get('target_att%s' % widget_id)[0]
         self.target_att_val = postdata.get('target_att_val%s' % widget_id)[0]
         self.tables = postdata.get('tables%s' % widget_id, [])
+        if self.target_table not in self.tables:
+            raise Exception('The selected target table "%s" is not among the selected tables.' % self.target_table)
         # Propagate the selected tables
         for table in self.cols.keys():
             if table not in self.tables:
@@ -87,6 +89,8 @@ class DBContext:
             del self.connected[pair]
         for table in self.tables:
             self.cols[table] = postdata.get('%s_columns%s' % (table, widget_id), [])
+            if table == self.target_table and self.target_att not in self.cols[table]:
+                raise Exception('The selected target attribute ("%s") is not among the columns selected for the target table ("%s").' % (self.target_att, self.target_table))
 
     def __repr__(self):
         return str((self.target_table, self.target_att, self.tables, self.cols, self.connected))