Ruby on Rail - Format for fetching and displaying Dymanic drop down

I have 3 tables: Student, Subject and Score.

Every student can add 3 Subjects (Physics, Mathematics and Chemistry) marks.

The combination of (student_id + subject_id) is added to Score table. I.e., capturing that sudent '1' has added 'Mathematics' marks with the actual score (say 0-100 range)

开发者_高级运维student id :    subjectid     Score
    1           Mathematics    95 

The Add page of Score has a "subject" drop down. which is displayed from "subject" table.

When the student wants to add the 2nd subject marks, in the add page, he should not be displayed the previoys added subject in the drop down.

Can anyone tell me how to do this?

Yes, you can do it in the following way (Rails 3.0):

class Subject
  scope :not_enrolled_in, lambda{ |user| where("NOT EXISTS(SELECT id FROM scores WHERE user_id = ? AND subject_id = subjects.id)", user.id) }
end

Or this way in Rails 2.x:

class Subject
  named_scope :not_enrolled_in, lambda{ |user| { :conditions => ["NOT EXISTS(SELECT id FROM scores WHERE user_id = ? AND subject_id = subjects.id)", user.id] } }
end

You can then get a list of subjects the current user has not enrolled in this way:

Subject.not_enrolled_in(current_user).all

You can then use this list in your select HTML code.