Rails - Jul 2006 - Question re: Computing a field in a DB record

I''m just learning Rails and have encountered a problem trying to set  
a field in a DB record based on the value of another form input  
field. Using the scaffold code, the create works as expected and the  
update (edit) also works. However, one of the DB fields (amount) is  
supposed to be computer from another (units): if the category field  
is "Mileage", amount= units * Fed_mileage_rate, otherwise amount=
units.

To accomplish this, I removed the amount field from the input form,  
as it is never entered directly, and put in an if statement to  
accomplish the computation above. When I test this by doing a create  
or an update, and units field is correct, but the amount is not.  
(Strangely, it seems to me, if I repeat the update operation, the  
amount is then correct.)

I feel that I may not be referring to the parameter or the DB field  
correctly, or that update_attributes does not work as I expect.

I would appreciate it if someone with experience would look at this  
and tell me what is wrong.

The scaffold code (with simple updates from the form with no  
calculation) of the amount works:

   def update
     @expense = Expense.find(params[:id])
     if @expense.update_attributes(params[:expense])
       flash[:notice] = ''Expense was successfully updated.''
       redirect_to :action => ''show'', :id => @expense
     else
       render :action => ''edit''
     end
   end

The modified code, which tries but fails to set calculate amount from  
units is here:

   def update
     @expense = Expense.find(params[:id])
     if @expense.category == "Mileage"
         @expense.amount= @expense.units * Fed_mileage_rate
     else
         @expense.amount= @expense.units
     end
	if @expense.update_attributes(params[:expense])
         flash[:notice] = ''Expense was successfully updated.''
		redirect_to :action => ''show'', :id => @expense
	else
       render :action => ''edit''
     end
   end

Any enlightenment would be most welcomed.

Thanks,
Tom Bajzek

>The modified code, which tries but fails to set calculate amount
>from units is here:
>
>   def update
>     @expense = Expense.find(params[:id])
>     if @expense.category == "Mileage"
>         @expense.amount= @expense.units * Fed_mileage_rate
>     else
>         @expense.amount= @expense.units
>     end
The code above does all it''s calculations based on data that is in your
database already.  You reload the object from the database, but don''t  
use the changed values passed back in the params hash to do your  
calculations.
>	if @expense.update_attributes(params[:expense])
>         flash[:notice] = ''Expense was successfully
updated.''
>		redirect_to :action => ''show'', :id => @expense
>	else
>       render :action => ''edit''
>     end
>   end
>
>Any enlightenment would be most welcomed.
>
>Thanks,
>Tom Bajzek
>_______________________________________________
>Rails mailing list
>Rails@lists.rubyonrails.org
>http://lists.rubyonrails.org/mailman/listinfo/rails

_Kevin

-- 
Posted with http://DevLists.com.  Sign up and save your mailbox.