In rails, how to destroy an 'join table item' with out delete the real record?

Question

Hi guys:
I get confuse now, I don't know how to delete/destroy a record in a join table:

---

class Task < ActiveRecord::Base
  belongs_to :schema
  belongs_to :to_do
end

class Todo < ActiveRecord::Base
  belongs_to :schema
  has_many :tasks
end

class Shcema < AcitveRecord::Base
  has_many :todos
  has_many :tasks, :through => :todos
end

---

>> sc = Schema.new
>> sc.tasks << Task.new
>> sc.tasks << Task.new
>> sc.tasks << Task.new
...
>> sc.tasks.delete(Task.first) # I just want to delete/destroy the join item here.
# But that deleted/destroyed the Task.first.

---

What can I do if I just want to destroy the relation item?