005 使用with_scope

Learn how to use with_scope - a very powerful method which will allow your custom find methods to accept any find options. Just like magic!

这一节学习如何使用with_scope-一个非常强大的方法，利用这个方法可以使得自定义的find方法接受更多的参数。

 

# models/task.rb
def self.find_incomplete(options = {})
    with_scope :find => options do
        find_all_by_complete( false, :order => 'created_at DESC')
    end
end

 

过程：

 

首先是在task.rb中

 

def self.find_incomplete

    find_all_by_complete(false, :order=>'created_at DESC')

end

 

针对这个find方法

如果现在有需求，要求显示前20条

 

那么在controller中，在调用find_incomplete时，加上一个参数:limite=>20

在task.rb中修改self.find_incomplete方法：

 

# models/task.rb
def self.find_incomplete(options = {})
    with_scope :find => options do
        find_all_by_complete( false, :order => 'created_at DESC')
    end
end

 

在railsbrain文档中对with_scope的说明：

 

 

with_scope(method_scoping = {}, action = :merge) {|| ...}

Scope parameters to method calls within the block. Takes a hash of method_name => parameters hash. method_name may be   :find  or   :create.   :find  parameters may include the   :conditions,   :joins,   :include,   :offset,   :limit, and   :readonly  options.   :createparameters are an attributes hash.

在block中调用scope参数。用一个hash作为方法名的参数。方法名可以是:find或者:create. :find可以包含:conditions, :joins, :include, :offset, :limit 和:readonly这些hash选项。:create参数是一个属性hash。

  class Article < ActiveRecord::Base
    def self.create_with_scope
      with_scope(:find => { :conditions => "blog_id = 1" }, :create => { :blog_id => 1 }) do
        find(1) # => SELECT * from articles WHERE blog_id = 1 AND id = 1
        a = create(1)
        a.blog_id # => 1
      end
    end
  end

In nested scopings, all previous parameters are overwritten by the innermost rule, with the exception of   :conditions  and :include  options in   :find, which are merged.

 

  class Article < ActiveRecord::Base
    def self.find_with_scope
      with_scope(:find => { :conditions => "blog_id = 1", :limit => 1 }, :create => { :blog_id => 1 }) do
        with_scope(:find => { :limit => 10 })
          find(:all) # => SELECT * from articles WHERE blog_id = 1 LIMIT 10
        end
        with_scope(:find => { :conditions => "author_id = 3" })
          find(:all) # => SELECT * from articles WHERE blog_id = 1 AND author_id = 3 LIMIT 1
        end
      end
    end
  end

You can ignore any previous scopings by using the   with_exclusive_scope  method.

  class Article < ActiveRecord::Base
    def self.find_with_exclusive_scope
      with_scope(:find => { :conditions => "blog_id = 1", :limit => 1 }) do
        with_exclusive_scope(:find => { :limit => 10 })
          find(:all) # => SELECT * from articles LIMIT 10
        end
      end
    end
  end