Permit checking whether a belongs_to association has been pointed to a new target record in the previous save and whether it will point to a new target record in the next save.

    post.category # => #<Category id: 1, name: "Ruby">

    post.category = Category.second   # => #<Category id: 2, name: "Programming">
    post.category_changed?            # => true
    post.category_previously_changed? # => false

    post.save!

    post.category_changed?            # => false
    post.category_previously_changed? # => true