According to the Rails documentation:
collection.delete(object, …)
Removes one or more objects from the
collection by removing their associations from the join table. This
does not destroy the objects.
Brilliant reference here for you
You can use:
product = Product.find(x)
special = product.specials.find(y)
product.specials.delete(special)
This creates ActiveRecord objects for both the object you’re trying to remove, which gives clear definition to the function
collection.clear
Removes all objects from the
collection by removing their associations from the join table. This
does not destroy the objects.
In this example:
product = Product.find(x)
product.specials.clear