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
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
本文转自 fsjoy1983 51CTO博客，原文链接：http://blog.51cto.com/fsjoy/131571，如需转载请自行联系原作者
