can't update the attribute with ActiveRecord - mysql

Question

I want to swap the content in answers table with ActiveRecord.
code 1:
Archieve::Answer.find_each do |answer|
str = answer.content
dosomething() #change the value
answer.update_attribute(:content,str)
end
But It doesn't change the value of content.
code 2:
Archieve::Answer.find_each do |answer|
str = answer.content
dosomething() #change the value
answer.reload
answer.update_attributes(
:content => str
)
end
Before update the :content attributes, I reload the record every time.
It can indeed change the the value.
Why?
What's the difference between code 1 & code 2?
Source Code
###1 Post Debug Message:
Updated Post:
Changed?: false
valid?: true
errors: #<ActiveModel::Errors:0xa687568>
errors: #<ActiveModel::Errors:0xa687568 #base=#<Archieve::Answer id: 9997190932758339, user_id: 4163690810052834, question_id: 3393286738785869, content: "狗狗生病，好可怜呀，", is_correct: false, votes_count: 0, comments_count: 0, created_at: "2011-11-06 18:38:53", updated_at: "2011-11-06 18:38:53">, #messages={}>

Answer 1

possible ActiveRecord 3.1.1 bug
The OP mentioned to me that he uses require "active_record" in a stand alone script (not using rails runner).
There is no separate Rails application for his task, he just uses a script. This is not necessarily bad, and has worked in earlier ActiveRecord versions, e.g. 2.x AFAIK -- maybe this is a regression in Rails 3.1 due to a new dependency?
# the OP's require statements:
require 'rubygems'
require 'logger'
require 'yaml'
require 'uuidtools'
require 'active_record'
complete code here: https://raw.github.com/Zhengquan/Swap_Chars/master/lib/orm.rb
maybe a dependency is missing, or problem with AR 3.1.1 when initialized stand alone?
It could be a bug actually
It could be that update_attribute() triggers a bug in the dirty-tracking of attributes, which then incorrectly assumes that the object has not changed, and as a result it will not be persisted, although the implementation of update_attribute() calls save() (see code fragment below).
I've seen something like this with an older version of Mongoid -- could be that there is a similar hidden bug in your ActiveRecord version for update_attribute()
In the Rails Console monkey-patch update_attribute like this:
class ActiveRecord::Base
def update_attribute(name, value) # make sure you use the exact code of your Rails Version here
send(name.to_s + '=', value)
puts "Changed?: #{changed?}" # this produced false in the OP's scenario
puts "valid?: #{valid?}"
puts "errors: #{errors.inspect}"
save
end
end
then try to run your Code 1 again...
you shouldn't see "Changed?: false".. if it returns false, although you changed the attribute, then there is a bug in your ActiveRecord version and you should report it.
Code 1:
NOTE: check the definition of update_attribute() (singular) here:
(please read the fine-print regarding validations -- it doesn't sound like a good idea to use that method)
http://ar.rubyonrails.org/classes/ActiveRecord/Base.html#M000400
See also:
Rails: update_attribute vs update_attributes
The source code for update_attribute() looks like this:
2260: def update_attribute(name, value)
2261: send(name.to_s + '=', value)
2262: save
2263: end
it could fail if there is a bug with the dirty-tracking of attributes...
Code 2:
The second code looks correct.
There are a couple of things to also consider:
1) which attributes did you define as accessible, via attr_accessible ?
e.g. only accessible attributes will be updated via update_attributes()
http://apidock.com/rails/ActiveRecord/Base/update_attributes
2) which validations do you use?
are you sure the validations pass for the record when you call update_attribute?
See also:
http://guides.rubyonrails.org/active_record_querying.html
http://m.onkey.org/active-record-query-interface
http://api.rubyonrails.org/classes/ActiveRecord/Base.html