Hi,
I would like to perform some action if the value of a specific attribute is changed during a save/update operation. Unfortuently, I ve got no idea how to detect this. Maybe somebody can explain the rails way to do it? Thanks a lto in advance.
Ok, I’m not 100% certain this is the best way to do this, but it’s the first thing that came to mind:
In your model, say the field ‘title’ is the one you want to monitor… then you can do this:
def title=(arg) @old_title = self.title self[:title] = arg end
And also this: def after_save if not @old_title.nil? and @old_title != self.title #do some stuff because the value changed here
@old_title = nil #make sure, if we re-use this same object, that this stuff doesn't happen again unless the value is re-changed
end end
Hi Volker,
I would like to perform some action if the value of a specific attribute is changed during a save/update operation. Unfortuently, I ve got no idea how to detect this. Maybe somebody can explain the rails way to do it?
Have a look at acts_as_modified plugin :
http://agilewebdevelopment.com/plugins/acts_as_modified http://svn.viney.net.nz/things/rails/plugins/acts_as_modified/
-- Jean-François.
Jean-François Trân wrote:
Hi Volker,
I would like to perform some action if the value of a specific attribute is changed during a save/update operation. Unfortuently, I ve got no idea how to detect this. Maybe somebody can explain the rails way to do it?
Have a look at acts_as_modified plugin :
http://agilewebdevelopment.com/plugins/acts_as_modified http://svn.viney.net.nz/things/rails/plugins/acts_as_modified/
Before resorting to a plugin, you might do some research on rails Observers [1] and/or the ActiveRecord lifecycle methods [2] (after_save, after_update, etc.). Both are covered in the Agile book and in the rails docs.
b
if all you want is to check the value of a specific attribute prior to saving, then a before_save callback should suffice.
if you need to monitor the state of the model as a whole, the acts_as_modified plugin is probably what you want.
Well, no we are getting to the interesting part.. I m in a before_save callback. how to access the new and the old value in before_save... ? Is that possible?
Sorry for these basic questions, I just want to make sure that I m using the easiest way to implement it... I ve searched already some news groups but so far it seems that before_save has already stored the new values and no way to see the old values by just accessing some array... So, I ve nly seen the two possibilities: a) store the old value in a self implemented accessor b) request the record from the database again in before_save..
I was hoping for a c) like... well just do self[:name].old_value 
you'd have to query the db to get the old data
i've never tried this, so i may be off, but i imagine it would be something like:
def before_save old_obj = Model.find(self.id) # compare self to old_obj ... end
I think you would be better off using an accessor like I showed in my earlier response… just def title=(blah) @old_title = self.title self[:title] = blah end
then in your before_save you can compare vs that and set it to nil aftewards… if you do what Chris is suggesting, it’s another trip to the DB every time you try to save one of these models.
true.
the only issue i see, and it would probably never happen, is if you changed the attribute more than once before saving. you would lose the original value of the attribute. a fix for that might be:
def title=(blah) # who cares how many times it is changed # just record the original @old_title = self.title unless @old_title self[:title] = blah end
def title_changed? (@old_title && @old_title != self.title) end