In PHP, I can set an attribute (that is not a column in database) to a model. E.g.(PHP code),
$user = new User; $user->flag = true; But in rails, when I set any attribute that doesn't exist in database, it will throw an error undefined method flag. There is attr_accessor method, but what will happen if I need about ten temp attributes?
but what will happen if I need about ten temp attributes?
#app/models/user.rb class User < ActiveRecord::Base attr_accessor :flag, :other_attribute, :other_attribute2, :etc... end attr_accessor creates "virtual" attributes in Rails -- they don't exist in the database, but are present in the model.
As with attributes from the db, attr_accessor just creates a set of setter & getter (instance) methods in your class, which you can call & interact with when the class is initialized:
#app/models/user.rb class User < ActiveRecord::Base attr_accessor :flag # getter def flag @flag end # setter def flag=(val) @flag = val end end This is expected because it's how ActiveRecord works by design. If you need to set arbitrary attributes, then you have to use a different kind of objects.
For example, Ruby provides a library called OpenStruct that allows you to create objects where you can assign arbitrary key/values. You may want to use such library and then convert the object into a corresponding ActiveRecord instance only if/when you need to save to the database.
Don't try to model ActiveRecord to behave as you just described because it was simply not designed to behave in that way. That would be a cargo culting error from your current PHP knowledge.
OpenStruct lib until you told me. I combined your solution with @Nimir solution and it work perfectly :D attr_accessor :flag after_initialize do |user| self.flag = OpenStruct.new end And I can set/get the temp flag attribute by using @user.flag.hello = "Hello World" ps: sr, I don't know how to format the codeAs the guys explained, attr_accessor is just a quick setter and getter.
We can set our Model attr_accessor on record initializing to be a Ruby#Hash for example using ActiveRecord#After_initilize method so we get more flexibility on temporarily storing values (idea credit to this answer).
Something like:
class User < ActiveRecord::Base attr_accessor :vars after_initialize do |user| self.vars = Hash.new end end Now you could do:
user = User.new #set user.vars['flag'] = true #get user.vars['flag'] #> true All that attr_accessor does is add getter and setter methods which use an instance variable, eg this
attr_accessor :flag will add these methods:
def flag @flag end def flag=(val) @flag = val end You can write these methods yourself if you want, and have them do something more interesting than just storing the value in an instance var, if you want.
If you need temp attributes you can add them to the singleton object.
instance = Model.new class << instance attr_accessor :name end 
