Rails - Aug 2007 - ActiveRecord gotcha with references?

I have this situation:

class Employee < ActiveRecord::Base
    belongs_to :designation
end

class Designation < ActiveRecord::Base
end

I do the following at the irb console:

Step 1: Find an employee
>> emp = Employee.find 3
=> #<Employee:0x35a7d34
@attributes={"designation_id"=>"3",
"id"=>"3",
"division_id"=>"4"}>

Step 2: Save the designation in a variable des
>> des = emp.designation
=> #<Designation:0x35a49b8 @attributes={"name"=>"Project
Manager",
"id"=>"3",
"division_id"=>nil}>
>> des
=> #<Designation:0x35a49b8 @attributes={"name"=>"Project
Manager",
"id"=>"3", "division_id"=>nil}>

Step 3: Replace the employee''s designation
>> emp.designation = Designation.find 1
=> #<Designation:0x359fa80
@attributes={"name"=>"Trainee",
"id"=>"1",
"division_id"=>nil}>

Step 4: Notice that the variable des''s object id has
changed
>> des
=> #<Designation:0x359fa80
@attributes={"name"=>"Trainee",
"id"=>"1",
"division_id"=>nil}>


At step 4 i would NOT EXPECT the reference of des to change as des at
step 2 had reference to object Designation:0x35a49b8

However if i change emp.designation the variable des automagically
changes to refer to the same object that emp.designation is referring
to!  It''s almost like whenever I refer to des, emp.designation seems
to be re-evaluated all over again!


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Here''s a response I got from another forum. hope that helps someone
else!

Actually, the object in des isn''t really an instance of Designation,
but
really a transparent proxy that points to it. When you execute
emp.designation=, that transparent proxy will point to another
reference. That''s why it looks like it''s changing object
reference on
a
local variable (which you can''t do in Ruby without Binding.of_caller)

Cheers

 Ola Bini (http://ola-bini.blogspot.com)
 JRuby Core Developer


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