Associations oddity

Rails
1

Can anyone explain this oddity to me?

Given two classes as follows:

class Question < ActiveRecord::Base
belongs_to :category
end

class Category < ActiveRecord::Base
has_many :questions
end

I get the following:

q = Question.find 6789
=> #<Question:0x37e9e70 @attributes={“id”=>“6789”,
“category_id”=>“8200”}>

cat = q.category
=> #<Category:0x37e1cc8 @attributes={“id”=>“8200”}>

q.category = nil
=> nil

cat
=> nil

The last line is the bit that’s confusing me.

I can “fix” it as follows:

q = Question.find 6789
=> #<Question:0x37e9e70 @attributes={“id”=>“6789”,
“category_id”=>“8200”}>

cat = q.category.dup
=> #<Category:0x37e1788 @attributes={“id”=>“8200”}>

q.category = nil
=> nil

cat
=> #<Category:0x37e1788 @attributes={“id”=>“8200”}>

But why do I need the “dup”? This seems to break the principle of least
surprise? Is there a better way to handle this?

Thanks in advance for your help!

paul.butcher->msgCount++

Snetterton, Castle Combe, Cadwell Park…
Who says I have a one track mind?

2

This looks very strange, doesn’t it?

What happens is that when you call

q.category

you get AssociationProxy object which pretends to be another object.
After you call

q.category = nil

this proxy pretends to be a nil object. You can see it by inspecting
the object id like

puts cat.id

It should return something other than value 4 which is an id of the nil
object.

Kent.