dev wrote:
> For some reason I am tripping up on the use of self and new.
>
> class LineItem < ActiveRecord::Base
> belongs_to :order
> belongs_to :product
>
> def self.from_cart_item(cart_item)
> li = self.new
> li.product = cart_item.product
> li.quantity = cart_item.quantity
> li.total_price = cart_item.price
> li
> end
> end
This pattern looks very much like the NSCoding protocol pattern that I'm
familiar with from the Cocoa frameworks. Compare what you have with this
Objective-C example:
- (id)initWithCoder:(NSCoder *)coder {
self = [super initWithCoder:coder];
mapName = [[coder decodeObjectForKey:@"MVMapName"] retain];
legendView = [[coder decodeObjectForKey:@"MVLegend"] retain];
auxiliaryView = [coder decodeObjectForKey:@"MVAuxView"];
magnification = [coder decodeFloatForKey:@"MVMagnification"];
return self;
}
This implementation is slightly more abstract, and implemented as an
instance method rather than a class method, but it serves essentially
the same purpose.
To use this method you would do this:
id myObject = [[MyObject alloc] initWithCoder:coder];
Notice that Obj-C instance methods begin with a minus sign (-) and class
methods begin with a plus sign (+). Since this is an instance method the
instance must be allocated before being initialized. This alloc-init
process is roughly equivalent to Ruby's "new" method.
One way to declare a class method in Ruby is to begin it with "self."
Which is what you are doing in your code (self.from_cart_item). In order
to use class methods you send messages to the class rather than an
instance of the class:
my_line_item = LineItem.from_cart_item # LineItem class is the receiver
of message from_cart_item
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