中文字比較

一個中文字利用3byte表示，

因此若要比較a="我愛妳"和b="妳愛我"的字首，

要比較  a[0]==b[0] && a[1]==b[1] && a[2] == b[2]

rails API

api.rubyonrails.org

included & extend

included:

the method defines the action to do when the class or module get included 

extend:

extend the class's ability by including module

ex:

module M

end

Class C

end

c= C.new

c.extend(M)

note:  use object to extend, get instance method, 

           use class to extend, get class method

class_eval

define method for class

ex:

A=Class.new

A.class_eval do

      def  test

          puts "test"

      end

end

a=A.new

a.test

--->  test

partial

include the view in other file

ex:

render :partial=> "books"

render the view in _books.html.erb (  the partial view file must 

start with _ )

ex:

render :partial => "rabbit/list"

render the view in different directory

note: the variable partial view can access is the variable the master view can access 

self & method

when there is a method ending with =,  such as test=, 

and there is another method  run

In the definition of run, there are two conditions:

(1)  test=3

      this means test is a local variable

(2) self.test=2

      this will call test method with parameter 2

Hence, when calling the method ending with = in the method definition, we can not omit self

association

use class methods( has_one, belongs_to, has_many,  has_and_belongs_to_many)

to create instance methods

has_one & belongs_to

has_many & belongs_to

has_and_belongs_to_many

the parameter for these methods:

:dependent => true

for delete.  

ex:

class Customer <>

         has_many  :orders,

                             :dependent=>true

end

if the customer A is deleted, its orders are also deleted

:order

ex:

:order=>"created_at ASC"

ascending order according to created_at field