Hello

I am trying to sum a column in the invoice line module in a field on the invoice module.  I have tried the following formula 

for record in self:

  record['x_studio_field_8kUMr']=(record.x_studio_field_zJdWj).sum() 

I am getting an error saying the x_studio_field_zjdWj is not found.   I assume that the error is due to the field I want to sum is not in the same module as the computed field.  Is there a way to compute the sum of the column in question?