# Comparing field's old value to new value

**URL:** <https://www.ruby-forum.com/t/comparing-fields-old-value-to-new-value/72670>\
**Category:** Rails\
**Created:** [November 27, 2005, 11:39am UTC](https://www.ruby-forum.com/t/comparing-fields-old-value-to-new-value/72670 "2005-11-27T11:39:45Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![cool\_screen\_name900](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/c68b51/32.png) [@cool\_screen\_name900](https://www.ruby-forum.com/u/cool_screen_name900)\
**Post date:** [November 27, 2005, 11:39am UTC](https://www.ruby-forum.com/t/comparing-fields-old-value-to-new-value/72670/1 "2005-11-27T11:39:45Z")

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I want to expire caches if a certain field of an item changes. How can I  
compare the item field’s  
old value to its new value (in the create, or after\_save, etc. method)?

thanks  
csn

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**Author:** ![cool\_screen\_name900](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/c68b51/32.png) [@cool\_screen\_name900](https://www.ruby-forum.com/u/cool_screen_name900)\
**Post date:** [November 28, 2005, 4:42pm UTC](https://www.ruby-forum.com/t/comparing-fields-old-value-to-new-value/72670/2 "2005-11-28T16:42:10Z")

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Hi !

There’s after\_find and after\_initialize you can use to copy the  
values. If you have only one or two fields, just override the field’s  
setter methods:

class X \< AR:Base  
def attribute=(new\_value)  
old\_value ||= read\_attribute(:attribute)  
write\_attribute(:attribute, new\_value)  
end  
end

Untested, and I’m not even sure write|read\_attribute are the right  
method names. You get the gist, though.

Bye !  
François

2005/11/27, CSN [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):