rails hit two

#counter_cache
in father table   sons_count:integer
in son model      belongs_to :father,:counter_cache => true

#select a category or create one
attr_accessor :new_category_one  #virtual attribute
before_save a record: method_name
def method_name
    create_fatherModelName(:name => new_category_one) unless new_category_one.blank?
end
#and valid is neccessary
def validate
    if model_id.blank? && new_level_one.blank?
      errors.add(:attr,:message =>"...")
    end  
  end  
 #in views
   <p>
 <%= f.label :level %><br />
   <%= f.collection_select(:level_id, Level.find(:all,:order => :id), :id, 
           :name,:prompt => "--select a level--") %>
     or Create: <%= f.text_field :new_level_one %>
</p> 

#paginate
   cattr_reader :per_page  #class variable
   @@per_page = 15
   def self.search(search,page)
      paginate(:page => page,
         :order => ["league_id,name"],
         :conditions => ["name like ?","%#{search}%"])
    end
#can be be do this
@category.@papers.paginate(:page => params[:page]) #hidden attr per_page use class variable

#Player.largethan returns an array includes player's age > 20
named_scope :largethan,:conditions => ["age > ?", 20 ]

#Paper belongs_to Category,then 
Category has hidden attribute: category.paper_ids  
#this is a array,in view can use like this
       <%= check_box_tag "category[product_ids][]", product.id,
           @category.products.include?(product)%>
     <%= product.name %>
#in controller action update
      @category = Category.find(params[:id])
      params[:category][:product_ids]||=[]
#in new is not need like this
@category = Category.new(params[:category]) 
#would be ok

#yield and content_for
#yield can be used in layout
<%= yield :one %>
# action's html 
<% content_for do %>
  #code
<% end %>

#ajax
<%=link_to_remote league.name,:url => league ,:method => 'get'%>
page.replace_html "showplace",:partial => "show"

#很有用的 in_groups_of,帮助表格化列出内容,同时使用了dom_id,用cycle方法来使颜色间隔
<% @tasks.in_groups_of(5,false) do |row_tasks| %>
     <tr class="<%= cycle "green","gray" %>">
       <% for task in row_tasks %>
          <td id="<%= task.dom_id %>"><%=h task.name %></td>
       <td><%= link_to 'Show', task %></td>
       <td><%= link_to 'Edit', edit_task_path(task) %></td>
       <td><%= link_to 'Destroy', task, :confirm => 'Are you sure?', :method => :delete %></td>
       <% end %>
     </tr>
  <% end %>