Beginners question | moving a method from controller to model

I’m learning with the Agile Web D. 4th edition book. I’ve
built a method in my controller to test that it works, and now I want to
move it to the model.


*** line_item controller ***


def create
@cart = current_cart
event = Event.find(params[:event_id])
@line_item = @cart.add_event(event)
new_capacity = event.capacity

respond_to do |format|
  if @line_item.save
    new_capacity =  new_capacity - @line_item.quantity
    event.capacity = new_capacity
    event.save

    format.html { redirect_to(@line_item.cart, :notice => 'Line item

was successfully created.’) }
format.xml { render :xml => @line_item, :status => :created,
:location => @line_item }
else
format.html { render :action => “new” }
format.xml { render :xml => @line_item.errors, :status =>
:unprocessable_entity }
end
end
end

so I started by doing this


line_item.rb***********


def decrease_seat_capacity_on_create

  new_capacity =  new_capacity - @line_item.quantity
  event.capacity = new_capacity
  event.save

end

and made a changed the line_item controller like this

respond_to do |format|
if @line_item.save
@line_item.decrease_seat_capacity_on_create

But I get the error

undefined method `quantity’ for nil:NilClass
Rails.root: c:/webtech/ServerSide/tickets_online

Application Trace | Framework Trace | Full Trace
app/models/line_item.rb:12:in decrease_seat_capacity_on_create' app/controllers/line_items_controller.rb:51:inblock in create’
app/controllers/line_items_controller.rb:49:in `create’

So I’m a bit confused about ‘quantity’. I’ve used it in another method
in the line_item.rb and it worked (much to my surprise) but it doesn’t
work in this new method I’m writing.

I really am an absolute beginner so I’m really just guessing most of the
time. Any suggestions or hints would be most appreciated.

thanks in advance for your patience.

On Sun, May 8, 2011 at 12:00 PM, linda Keating [email protected]
wrote:

 new_capacity = event.capacity
   format.xml  { render :xml => @line_item, :status => :created,

In this method @line_item does not exist because you have not created
it. So
you may need to pass to this method the line_item object or look it up.

Also, you should look at the after_save callback in ActiveRecord, that
way
you might be able to tell your LineItem model to automatically perform
this action after a successful save (if you google for activerecord
callbacks you will find a number of them, they are very useful).

See inserted comments below.

Quoting linda Keating [email protected]:

  new_capacity = event.capacity
    format.xml  { render :xml => @line_item, :status => :created,

new_capacity and event are local variables in the controller method.
They are
not accessible in the called model method. @line_item is an instance
variable
of the controller instance. It is also not accessible in the model
method.
However it references the same object as self in the model method.
Presumably, event is another model. Note: it is possible that a
LineItem has
an event method because there is an association between Events and
LineItems.

I don’t see any good reason to push this functionality into a model
method.
The only property of the model that is clearly being used is the
quantity
attribute/method. Unless the LineItem model includes a reference to an
Event
(i.e. LineItem belongs to an Event or hasMany Events). It still looks
like
poor design unless there are several factors not mentioned here.

Jeffrey

Rails.root: c:/webtech/ServerSide/tickets_online

It isn’t quantity that isn’t working, it is that @line_item is nil
(i.e.,
@line_item of the LineItem instance has not been assigned a value).
@line_item is an instance variable in the controller, not in the model.

I really am an absolute beginner so I’m really just guessing most of the
time. Any suggestions or hints would be most appreciated.

HTH,
Jeffrey

Jeffrey L. Taylor wrote in post #997405:

See inserted comments below.

Quoting linda Keating [email protected]:

  new_capacity = event.capacity
    format.xml  { render :xml => @line_item, :status => :created,

new_capacity and event are local variables in the controller method.
They are
not accessible in the called model method. @line_item is an instance
variable
of the controller instance. It is also not accessible in the model
method.
However it references the same object as self in the model method.
Presumably, event is another model. Note: it is possible that a
LineItem has
an event method because there is an association between Events and
LineItems.

I don’t see any good reason to push this functionality into a model
method.
The only property of the model that is clearly being used is the
quantity
attribute/method. Unless the LineItem model includes a reference to an
Event
(i.e. LineItem belongs to an Event or hasMany Events). It still looks
like
poor design unless there are several factors not mentioned here.

Hi Jeffrey, I was moving it into the model soley because I’ve read that
fat models and skinny controllers are the best design practice. I was
just trying to get the function to work first, and now that it’s working
I was hoping to tidy up the design. But maybe I should just leave it as
is. I have though similar functionality in the update controller that
looks like this …

def update
@line_item = LineItem.find(params[:id])
event = @line_item.event
new_quantity = params[:line_item_quantity].to_i
change_in_capacity = @line_item.quantity - new_quantity

respond_to do |format|
  if @line_item.update_attributes(params[:line_item])
    if (@line_item.quantity >0)
     event.capacity = change_in_capacity
     event.save

and I’ll be writing something similar again reversing the action in the
destroy method of line_item.

But maybe I should just leave it as is for the moment until I’ve learned
a bit more about Ruby and Rails.

Many thanks for your help and feedback. Really appreciate your
patience.

Linda

Jeffrey

Rails.root: c:/webtech/ServerSide/tickets_online

It isn’t quantity that isn’t working, it is that @line_item is nil
(i.e.,
@line_item of the LineItem instance has not been assigned a value).
@line_item is an instance variable in the controller, not in the model.

I really am an absolute beginner so I’m really just guessing most of the
time. Any suggestions or hints would be most appreciated.

HTH,
Jeffrey

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