This blog is part of our Rails 6.1 series.

Rails 6.1 adds *_previously_was attribute methods for dirty tracking the previous attribute value after the model is saved or reset. *_previously_was returns the previous attribute value that was changed before the model was saved

Before Rails 6.1, to retrieve the previous attribute value, we used *_previous_change or previous_changes.

Here is how it can be used.

Rails 6.0.0

>> user = User.new
=> #<User id: nil, name: nil, email: nil, created_at: nil, updated_at: nil>

>> user.name = "Sam"

# *_was returns the original value. In this example, the name was initially nil.
>> user.name_was
=> nil
>> user.save!

# After save, the original value is set to "Sam". To retrieve the
# previous value, we had to use `previous_changes`.
>> user.previous_changes[:name]
=> [nil, "Sam"]

Rails 6.1.0

>> user = User.find_by(name: "Sam")
=> #<User id: 1, name: "Sam", email: nil, created_at: "2019-10-14 17:53:06", updated_at: "2019-10-14 17:53:06">

>> user.name = "Nick"
>> user.name_was
=> "Sam"

>> user.save!

>> user.previous_changes[:name]
=> ["Sam", "Nick"]

# *_previously_was returns the previous value.
>> user.name_previously_was
=> "Sam"

# After reload, all the dirty tracking
# attributes is reset.
>> user.reload
>> user.name_previously_was
=> nil

Check out the pull request for more details on this.