Question

I got the following models. The join table references the user_id and the course_id and the combination is set unique. However, after testing my page on the server, e.g. leaving and joining courses many times,also with another current_user my join table created records where the course_id was empty. This got me errors in fetching data afterwards. So now I tried adding :false => null. Could this help? thx for your time

def self.up
  create_table :course_enrollments do |t|
  t.references :user, :null => false
  t.references :course, :null => false
  t.timestamps
end
  add_index :course_enrollments, [:user_id, :course_id], :unique => true
end

Answer 1

Adding :null => false will cause an exception when you try to save a CourseEnrollment with a null ID. That’s fine if you write the controller action to handle the exception. You could (and should) add validates_presence_of :user_id, :course_id in the CourseEnrollment model, so that instances with null IDs will not be valid, and can be handled with the normal <model>.save method.

However the bigger question is why does your app save rows with null IDs in the first place? For example, you might be creating orphaned rows (by deleting the associated Course or User). Generally it’s a good idea to add :dependent => :destroy on has_many associations to prevent this.