Question

Trying to get previous values from the object on save. Think of this scenario:

@object = {:name => 'Dan', :occupation => 'student'}
@object[:occupation] = 'Full time employee'
@object.value_was[:occupation] # => 'student'

I hope it is understandable enough that there is no method value_was. More over I would like to do the same on model objects:

@student = Student.find(1)
@student.occupation = 'Full time employee'
@student.save
@student.value_was(:occupation) # => 'student'

Any help will be appreciated.

That would be really helpful

Answer 1

ActiveModel includes support for “dirty field marking“, which preserves before and after states for changed fields.

You can use @student.occupation_was to get the previous value of occupation, and @student.occupation_changed? to get whether the value has changed or not.

This only works BEFORE the save, as saving resets the changed states of the values. However, you could capture this data in a before_save callback if you need to use it after a record has been saved. You can preserve all of the changes by duplicating #changed_attributes in a before_save, for example, then query on them.