dirty_objects.rb

# Note: as you will come to see from looking at these notes, you can only use these properties correctly BEFORE SAVE

# If you want to keep track of changes in your model Rails provides "Dirty Objects". E.g. Your model has a 'name' attribute:

my_model = MyModel.last
my_model.changed?  # it returns false

# You can Track changes to attributes with my_model.name_changed? accessor
my_model.name  # returns  "Name"
my_model.name = "New Name"                                        
my_model.name_changed? # returns true                              
                                                   
# Access previous value with 'name_was' accessor                   
my_model.name_was  # "Name"                        

# You can also see both the previous and the current values, using name_change
my_model.name_change  #=> ["Name", "New Name"]

changed? #=> Boolean
attribute_changed? #=> Boolean
attribute_was #=> access previous value of attribute
attribute_change #=> array of the previous and current values of the attribute

dirty_objects_table.rb

method       |       changed?        |  attribute_changed?  |        attribute_was       |         attribute_change
             -------------------------------------------------------------------------------------------------------------------------
# return val |       Boolean         |       Booleaan       |      prev attr val         |     ary of attr's prev & current val
             -------------------------------------------------------------------------------------------------------------------------
example      | m = MyModel.first     | m.name `[#=> "Name"]`| m.name = "Name3"           | m.name = "Name4"
             | m.changed?            | m.name = "Name2"     | m.name_was `[#=> "Name"]`  | m.name_change `[#=> ["Name3", "Name4"]]`
             |  `[#=> false]`        | m.name_changed?      | m.save!                    | m.name = "Name5"
             |                       |  `[#=> true]`        | m.name_was `[#=> "Name3"]` | m.name_change `[#=> ["Name4", "Name5"]]`                     
             |                       | m.save!              | m.name `[#=> "Name3"]`     | m.save!
             |                       | m.name_changed?      |                            | m.name_change `[#=> nil]`
             |                       |  `[#=> false]`       |                            |# `name_change` updates w/out save unlike
# `name_was` returns the 1st name SINCE THE LAST SAVE no matter how many x overrwrite it | `name_was`. But, becomes nil after save 
# until u save the instance again. Then it becomes last val set it to b4 the save.       |  while `name_was` stays last val b4 save