Implemented add_user in courses controller

[cynthia-lixinyi]

General Info

• Pivotal Tracker Story
• Breaking?

Changes

Explain your changes here (in such a way that you would understand why you made them a year from now).
Implemented add_user method in class CoursesController. This method accepts course_id and user_id in the request body and adds an entry to the user_to_courses table and UserToCourse model.

Testing

Explain how you tested your changes. If testing is not necessary, explain why.
Tests cover these cases:
(1) If either the provided course_id does not exist in the courses table or the provided user_id does not exists in the users tables, the add_user method should return not_found status and an error message in the JSON response.
(2) If the user is already added to the course, in the JSON response return an error message saying "The user is already added to the course".
(3) Happy path: course_id and user_id exist in their corresponding tables, and an entry with the course_id, user_id, and role is successfully added to the user_to_courses table and UserToCourse model.

Documentation

Does this PR require documentation. If so, explain where it can be found.

Checklist

• Name of branch corresponds to story

[Zzz212zzZ]

lgtm

[Sep26]

Good to go.