Rails - Feb 2007 - Using before_save to update a record only once.

I''d like to use before_save to perform an action (I won''t get
into the
details), but only the first time the record is changed from 0 to 1.
Here''s the code I have so far in my model:

def before_save
   if completed == 1
      # perform the action
   end
end

What the above does is perform the action when "completed" is set to
one, but the problem is I don''t want this to happen every single time
there is a save. So what I''d like to be able to do is check the
previous
value of "completed" and only perform the action if it used to be 0
and
is now 1. For example (psuedo code):

def before_save
   if old_completed == 0 and completed == 1
      # perform the action
   end
end

How would I go about this? Obviously, I could add code outside the
model, but before_save seems like the nicest, cleanest place to for this
code to be. I searched this forum and found a post that kind of helps
answer my question, but it''s not super clear and I''m still not
sure what
the best solution is. It sounds like I have to do an external find just
to do this check, and that just doesn''t seem right to me. Is that
really
the only way?

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> I''d like to use before_save to perform an action (I won''t
get into the
> details), but only the first time the record is changed from 0 to 1.
> Here''s the code I have so far in my model:
>
> def before_save
>   if completed == 1
>      # perform the action
>   end
> end
>
> What the above does is perform the action when "completed" is set
to
> one, but the problem is I don''t want this to happen every single
time
> there is a save. So what I''d like to be able to do is check the
previous
> value of "completed" and only perform the action if it used to be
0 and
> is now 1. For example (psuedo code):
>
> def before_save
>   if old_completed == 0 and completed == 1
>      # perform the action
>   end
> end
>
> How would I go about this? Obviously, I could add code outside the
> model, but before_save seems like the nicest, cleanest place to for this
> code to be. I searched this forum and found a post that kind of helps
> answer my question, but it''s not super clear and I''m
still not sure what
> the best solution is. It sounds like I have to do an external find just
> to do this check, and that just doesn''t seem right to me. Is that
really
> the only way?
Don''t have a specific answer for you, but the acts_as_changed plugin 
might come in handy for you...

http://agilewebdevelopment.com/plugins/acts_as_changed

-philip

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Philip Hallstrom wrote:
>> What the above does is perform the action when "completed" is
set to
>>
>> How would I go about this? Obviously, I could add code outside the
>> model, but before_save seems like the nicest, cleanest place to for
this
>> code to be. I searched this forum and found a post that kind of helps
>> answer my question, but it''s not super clear and I''m
still not sure what
>> the best solution is. It sounds like I have to do an external find just
>> to do this check, and that just doesn''t seem right to me. Is
that really
>> the only way?
> 
> Don''t have a specific answer for you, but the acts_as_changed
plugin
> might come in handy for you...
> 
> http://agilewebdevelopment.com/plugins/acts_as_changed
> 
> -philip
Thanks, Philip. I''m having a difficult time understanding exactly what 
it does. The one line explanation isn''t 100% clear to me. It sort of 
looks like a speed related plug-in that simply updates fields that been 
changed, rather than going through each field in the entire table, but I 
could be wrong about that.

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> Philip Hallstrom wrote:
>>> What the above does is perform the action when
"completed" is set to
>>>
>>> How would I go about this? Obviously, I could add code outside the
>>> model, but before_save seems like the nicest, cleanest place to for
this
>>> code to be. I searched this forum and found a post that kind of
helps
>>> answer my question, but it''s not super clear and
I''m still not sure what
>>> the best solution is. It sounds like I have to do an external find
just
>>> to do this check, and that just doesn''t seem right to me.
Is that really
>>> the only way?
>>
>> Don''t have a specific answer for you, but the acts_as_changed
plugin
>> might come in handy for you...
>>
>> http://agilewebdevelopment.com/plugins/acts_as_changed
>>
>> -philip
>
> Thanks, Philip. I''m having a difficult time understanding exactly
what
> it does. The one line explanation isn''t 100% clear to me. It sort
of
> looks like a speed related plug-in that simply updates fields that been
> changed, rather than going through each field in the entire table, but I
> could be wrong about that.
I''ve never used it myself :)  ANd I think you''re right about
that.
There''s a "changed_attributes" method that returns
what''s changed though
that you could use perhaps...

Guess I thought it might provide more inspiration than an outright answer 
:)

Thinking more about it, you could override the completed method to record 
that it was changed and then check that in your before_save...

-philip


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Of course there is another way, you could write your own plugin
acts_as_update_once to do this across all your controllers. Make sure
that the logic you use short-circuits quickly for the condition that
happens most of the time. This way performance will be good.

On Feb 28, 1:31 pm, MenDAKE
<rails-mailing-l...-ARtvInVfO7ksV2N9l4h3zg@public.gmane.org>
wrote:
> I''d like to use before_save to perform an action (I won''t
get into the
> details), but only the first time the record is changed from 0 to 1.
> Here''s the code I have so far in my model:
>

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Philip Hallstrom wrote:
>>>
>> changed, rather than going through each field in the entire table, but
I
>> could be wrong about that.
> 
> I''ve never used it myself :)  ANd I think you''re right
about that.
> There''s a "changed_attributes" method that returns
what''s changed though
> that you could use perhaps...
> 
> Guess I thought it might provide more inspiration than an outright 
> answer
> :)
> 
> Thinking more about it, you could override the completed method to 
> record
> that it was changed and then check that in your before_save...
> 
> -philip
Sorry I''m a little late here -- been away for a couple days. That
sounds
interesting. Can you explain in more detail how I''d do something like 
that?

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> Philip Hallstrom wrote:
>>>>
>>> changed, rather than going through each field in the entire table,
but I
>>> could be wrong about that.
>>
>> I''ve never used it myself :)  ANd I think you''re
right about that.
>> There''s a "changed_attributes" method that returns
what''s changed though
>> that you could use perhaps...
>>
>> Guess I thought it might provide more inspiration than an outright
>> answer
>> :)
>>
>> Thinking more about it, you could override the completed method to
>> record
>> that it was changed and then check that in your before_save...
>>
>> -philip
>
> Sorry I''m a little late here -- been away for a couple days. That
sounds
> interesting. Can you explain in more detail how I''d do something
like
> that?
Hrm... something like this assuming your model has 
''completed''
attribute.

class MyModel < ...

 	attr :is_complete

 	def completed=(val)
 		if completed == 0 && val == 1 then
 			is_complete = true
 		else
 			is_complete = false
 		end
 		completed = val
 	end

 	def before_save
 		if is_complete != true then
 			return false
 		end
 		true
 	end

end

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Philip Hallstrom wrote:
>>> answer
>> that?
> Hrm... something like this assuming your model has 
''completed''
> attribute.
> 
> class MyModel < ...
> 
>    attr :is_complete
> 
>    def completed=(val)
>      if completed == 0 && val == 1 then
>        is_complete = true
>      else
>        is_complete = false
>      end
>      completed = val
>    end
> 
>    def before_save
>      if is_complete != true then
>        return false
>      end
>      true
>    end
> 
> end
Thanks, that makes sense. I implemented something similar to your 
example, but for some reason the completed field now never gets saved. 
My code looks something like this:

class MyModel < ActiveRecord::Base
  attr :completed_is_changed

  # This checks to see if the completed fiel was set to true for the 
first time.
  def completed=(val)
    if completed == 0 && val == 1
      completed_is_changed = true
    else
      completed_is_changed = false
    end

    completed = val
  end

  def before_save
    if completed_is_changed == true
       #perform the action.
    end
end

The actions are performed, but the completed field value is now never 
saved to the database. I would expect that the line "completed = val" 
would take care of that, but it doesn''t seem to. Is something missing?

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In the above example there''s an "end" missing at the very
end, but other
than that, it''s basically what I''m using, minus a few
unnecessary
details.

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MenDAKE :
> My code looks something like this:
>
> class MyModel < ActiveRecord::Base
>   attr :completed_is_changed
>
>   # This checks to see if the completed fiel was set to true for the
> first time.
>   def completed=(val)
>     if completed == 0 && val == 1
>       completed_is_changed = true
>     else
>       completed_is_changed = false
>     end
>
>     completed = val
>   end
>
>   def before_save
>     if completed_is_changed == true
>        #perform the action.
>     end
> end
>
> The actions are performed, but the completed field value is
> now never saved to the database. I would expect that the
> line "completed = val" would take care of that, but it
doesn''t
> seem to. Is something missing?
You can write it like that :

def completed=(val)
  completed_is_changed = completed == 0 && val == 1
  write_attribute(:completed, val)
end

   -- Jean-François.

-- 
Ruby ( http://www.rubyfrance.org ) on Rails ( http://www.railsfrance.org )

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> Philip Hallstrom wrote:
>>>> answer
>>> that?
>> Hrm... something like this assuming your model has 
''completed''
>> attribute.
>>
>> class MyModel < ...
>>
>>    attr :is_complete
>>
>>    def completed=(val)
>>      if completed == 0 && val == 1 then
>>        is_complete = true
>>      else
>>        is_complete = false
>>      end
>>      completed = val
Ah, I suspect this is it... do something like...

self[''completed''] = val

or whatever rails does by default for this thing...
>>    end
>>
>>    def before_save
>>      if is_complete != true then
>>        return false
>>      end
>>      true
>>    end
>>
>> end
>
> Thanks, that makes sense. I implemented something similar to your
> example, but for some reason the completed field now never gets saved.
> My code looks something like this:
>
> class MyModel < ActiveRecord::Base
>  attr :completed_is_changed
>
>  # This checks to see if the completed fiel was set to true for the
> first time.
>  def completed=(val)
>    if completed == 0 && val == 1
>      completed_is_changed = true
>    else
>      completed_is_changed = false
>    end
>
>    completed = val
>  end
>
>  def before_save
>    if completed_is_changed == true
>       #perform the action.
>    end
> end
>
> The actions are performed, but the completed field value is now never
> saved to the database. I would expect that the line "completed =
val"
> would take care of that, but it doesn''t seem to. Is something
missing?
>
> -- 
> Posted via http://www.ruby-forum.com/.
>
> >
>
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That did it. Thanks very much for the help.

-- 
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