CourseFavourite           -- which user accessed which course when, only displayed to user for convenience;
    user         UserId
    course       CourseId OnDeleteCascade OnUpdateCascade
    reason       FavouriteReason
    lastVisit    UTCTime
    UniqueCourseFavourite user course
    deriving Generic
CourseNoFavourite
    user         UserId
    course       CourseId OnDeleteCascade OnUpdateCascade
    UniqueCourseNoFavourite user course
    deriving Generic