drupal not detecting function in custom module's inc file

Question

I have a custom module, in my module call commerce_discount_add_line_item() function, it exists in commerce_discount.rules.inc, this modules is installed correctly and status is enabled but when I call it returns:

( ! ) Fatal error: Call to undefined function commerce_discount_add_line_item().

It is strange, why has this happened?

Answer 1

By default inc files are not included(only module files are included). They should be called explicitly like below before making a call to a function in that file.

module_load_include('inc', 'commerce_discount', 'commerce_discount.rules');

Further Reading on inc files and how they differ from .module files : https://drupal.stackexchange.com/a/54069/4876

Answer 2

I hope you have included commerce_discount.rules.inc file in your module file.

You can use below code to include file, if you have not included files into your module file.

module_load_include('inc', 'commerce_discount', 'commerce_discount.rules');