class TasksController < ApplicationController
def incomplete
@tasks = Task.find(:all, :conditions => ['complete = ?', false])
end
end
很类似Hibernate的数据库查询hql语句，但显然我们的Rails不可能这么逊，看看改良的方法:
class TasksController < ApplicationController
def incomplete
@tasks = Task.find_all_by_complete(false)
end
end
我们的Task这个Model类没有定义find_all_by_complete啊，我们为什么可以调用这个方法呢？
请看active_record/base.rb中的一段代码:
def method_missing(method_id, *arguments)
if match = /^find_(all_by|by)_([_a-zA-Z]\w*)$/.match(method_id.to_s)
finder = determine_finder(match)
attribute_names = extract_attribute_names_from_match(match)
super unless all_attributes_exists?(attribute_names)
attributes = construct_attributes_from_arguments(attribute_names, arguments)
case extra_options = arguments[attribute_names.size]
when nil
options = { :conditions => attributes }
set_readonly_option!(options)
ActiveSupport::Deprecation.silence { send(finder, options) }
when Hash
finder_options = extra_options.merge(:conditions => attributes)
validate_find_options(finder_options)
set_readonly_option!(finder_options)
if extra_options[:conditions]
with_scope(:find => { :conditions => extra_options[:conditions] }) do
ActiveSupport::Deprecation.silence { send(finder, finder_options) }
end
else
ActiveSupport::Deprecation.silence { send(finder, finder_options) }
end
else
raise ArgumentError, "Unrecognized arguments for #{method_id}: #{extra_options.inspect}"
end
elsif match = /^find_or_(initialize|create)_by_([_a-zA-Z]\w*)$/.match(method_id.to_s)
instantiator = determine_instantiator(match)
attribute_names = extract_attribute_names_from_match(match)
super unless all_attributes_exists?(attribute_names)
if arguments[0].is_a?(Hash)
attributes = arguments[0].with_indifferent_access
find_attributes = attributes.slice(*attribute_names)
else
find_attributes = attributes = construct_attributes_from_arguments(attribute_names, arguments)
end
options = { :conditions => find_attributes }
set_readonly_option!(options)
find_initial(options) || send(instantiator, attributes)
else
super
end
end
def extract_attribute_names_from_match(match)
match.captures.last.split('_and_')
end
看看第一行代码:if match = /^find_(all_by|by)_([_a-zA-Z]\w*)$/.match(method_id.to_s)
这是一个正则表达式匹配:
^匹配一行的开始
$匹配一行的结束
\w匹配一个词语，可以是数字、字母、下划线
[]匹配括号里面符合的一个字符
*匹配0个或多个它前面的字符或短语
则([_a-zA-Z]\w*)则匹配以下划线或任意小写字母或任意大写字母开头的一个字符串
具体参考《Programming Ruby》中的Regular Expressions一章
而extract_attribute_names_from_match方法也很有意思，match.captures返回匹配的字符串组成的数组，last返回最后一个元素，如:
/^(a)_(b)_(\w*)$/.match("a_b_c_d_e_f_g").captures # => ["a", "b", "c_d_e_f_g"]
/^(a)_(b)_(\w*)$/.match("a_b_c_d_e_f_g").captures.last # =>"c_d_e_f_g"
/^find_(all_by|by)_([_a-zA-Z]\w*)$/.match("find_by_a_and_b_and_c").captures.last # => "a_and_b_and_c"
这样，第一行代码所匹配的方法名具体为find_by(或all_by)_aaaBBB形式
而extract_attribute_names_from_match允许我们匹配形式为find_by(或all_by)_aaaBBB_and_cccDDD_and_eeeFFF_and_...的方法
即我们可以无限添加查询条件，通过_and_连接字段名即可
而且可以看出，我们还可以调用find_by_columnName、find_or_initialize_by_columnName、find_or_create_by_columnName等动态方法。
补充几点：
1. 动态查询支持nil, array 以及range作为参数
比如要查询上海或者北京， 年龄在18~38之间的用户，就可以这样用：
User.find_all_by_city_and_age(['Shanghai','Beijing'], (18...38))
2. 动态查询不支持like以及单边范围的查找，比如查询名字中包括"read"，年龄大于30的老头用户：
name like '%read%' and age > 30
就无法用动态查询来做了
通过查询源代码，可以看到这样一段在做转换：
def attribute_condition(argument)
case argument
when nil then "IS ?"
when Array then "IN (?)"
when Range then "BETWEEN ? AND ?"
else "= ?"
end
end
def method_missing(method_id, *arguments, &block)
if match = DynamicFinderMatch.match(method_id)
attribute_names = match.attribute_names
super unless all_attributes_exists?(attribute_names)
if match.finder?
finder = match.finder
bang = match.bang?
# def self.find_by_login_and_activated(*args)
# options = args.extract_options!
# attributes = construct_attributes_from_arguments(
# [:login,:activated],
# args
# )
# finder_options = { :conditions => attributes }
# validate_find_options(options)
# set_readonly_option!(options)
#
# if options[:conditions]
# with_scope(:find => finder_options) do
# find(:first, options)
# end
# else
# find(:first, options.merge(finder_options))
# end
# end
self.class_eval %{
def self.#{method_id}(*args)
options = args.extract_options!
attributes = construct_attributes_from_arguments(
[:#{attribute_names.join(',:')}],
args
)
finder_options = { :conditions => attributes }
validate_find_options(options)
set_readonly_option!(options)
#{'result = ' if bang}if options[:conditions]
with_scope(:find => finder_options) do
find(:#{finder}, options)
end
else
find(:#{finder}, options.merge(finder_options))
end
#{'result || raise(RecordNotFound, "Couldn\'t find #{name} with #{attributes.to_a.collect {|pair| "#{pair.first} = #{pair.second}"}.join(\', \')}")' if bang}
end
}, __FILE__, __LINE__
send(method_id, *arguments)
elsif match.instantiator?
instantiator = match.instantiator
# def self.find_or_create_by_user_id(*args)
# guard_protected_attributes = false
#
# if args[0].is_a?(Hash)
# guard_protected_attributes = true
# attributes = args[0].with_indifferent_access
# find_attributes = attributes.slice(*[:user_id])
# else
# find_attributes = attributes = construct_attributes_from_arguments([:user_id], args)
# end
#
# options = { :conditions => find_attributes }
# set_readonly_option!(options)
#
# record = find(:first, options)
#
# if record.nil?
# record = self.new { |r| r.send(:attributes=, attributes, guard_protected_attributes) }
# yield(record) if block_given?
# record.save
# record
# else
# record
# end
# end
self.class_eval %{
def self.#{method_id}(*args)
guard_protected_attributes = false
if args[0].is_a?(Hash)
guard_protected_attributes = true
attributes = args[0].with_indifferent_access
find_attributes = attributes.slice(*[:#{attribute_names.join(',:')}])
else
find_attributes = attributes = construct_attributes_from_arguments([:#{attribute_names.join(',:')}], args)
end
options = { :conditions => find_attributes }
set_readonly_option!(options)
record = find(:first, options)
if record.nil?
record = self.new { |r| r.send(:attributes=, attributes, guard_protected_attributes) }
#{'yield(record) if block_given?'}
#{'record.save' if instantiator == :create}
record
else
record
end
end
}, __FILE__, __LINE__
send(method_id, *arguments, &block)
end
elsif match = DynamicScopeMatch.match(method_id)
attribute_names = match.attribute_names
super unless all_attributes_exists?(attribute_names)
if match.scope?
self.class_eval %{
def self.#{method_id}(*args) # def self.scoped_by_user_name_and_password(*args)
options = args.extract_options! # options = args.extract_options!
attributes = construct_attributes_from_arguments( # attributes = construct_attributes_from_arguments(
[:#{attribute_names.join(',:')}], args # [:user_name, :password], args
) # )
#
scoped(:conditions => attributes) # scoped(:conditions => attributes)
end # end
}, __FILE__, __LINE__
send(method_id, *arguments)
end
else
super
end
end