Update Invoice template to include new field

I understand that I can introduce a new variable to an invoice template by adding a custom field.
However, I need to add a new field to be filled by a specific query.
( I’m trying to add Total_Balance).
How can I do so? Where is the model to introduce a new field to the invoice template?

By convention, models in Codeigniter are stored in the ‘models’ directory. ie

However, since Invoiceplane uses modules, you can also find then in

For invoices, the file will be

You would probably want to look at the following functions to get and save the new field.

  • default_select()
  • create()
  • db_array()