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    <p>Hi,</p> <p>I imagine a pretty easy SQL-approach with just one caveat: add an additional column called something like <code>list_position</code> and add a unique index to the classroom-student-relation-table consisting of <code>classroom_id</code>, <code>student_id</code> and <code>list_position</code>.</p> <p>Inserting than happens by setting <code>list_position</code> to the number of students in class plus one. Ruby code would probably be somewhere along</p> <pre class="lang-ruby prettyprint-override"><code>classroomStudent = ClassroomStudent.new classroomStudent.list_position = classroom.size + 1 until classroomStudent.save classroomStudent.list_position += 1 do_stuff_about_not_enough_room if classroomStudent.list_position &gt; MAX_SIZE end </code></pre> <p>Result: If two inserts try to insert a student into the same class at the same time the unique index will keep one student out (<code>INSERT</code> fails). This means you may have to retry until <code>list_position=MAX_SIZE</code>, but are guaranteed you will never have too many students in class. You could also use this approach to build up a waiting-queue.</p> <p>If you already have data in your relation-table you will have to add a value for <code>list_position</code> first. I would guess something like</p> <pre class="lang-sql prettyprint-override"><code>UPDATE ClassroomStudents AS c1 SET c1.list_position = COALESCE((SELECT MAX(c2.list_position) FROM ClassroomStudents AS c2 WHERE c2.classroom_id = c1.classroom_id), 0) + 1; </code></pre> <p>would do the job here although it might be a bit on the slow side.</p> <p>I know the solution still has some rough edges, but maybe it helps.</p> <p>Regards</p> <p>TC</p>
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