Suppose I have 2 resources (models) in my service: Degree, Course.
When a user sends a post request to the rest API https://example.com/degrees/degreeId/courses
with data like [courseId => 2]
It associates Degree with id 1 to a Course with id 1.
What should status code be for representing this update?
Should I return 200 or 201.
Also what do you suggest for response body? The updated Degree along with associated Courses?
I am not sure about the big picture of your API. It somehow seems that a specific course (the ID of which you provide in the request body) should be its own resource.
However given the description, I see two possibilities:
1) Create a new resources, such as
/degrees/1/course/2
This resource could for example be used to issue a later DELETE request.
In this case you can return 201 and include a Link to the newly created resource.
2) If there is no such resource, 200 seems to be the only applicable positive response.
Option 1 seems preferable to me.