In Rails, how can I retrieve the object on a belongs_to association, without going through the database?

Question

Consider the following setup:

class Parent < ActiveRecord::Base
  has_many :children
end

class Child < ActiveRecord::Base
  belongs_to :parent
end

And this console session:

>> p = Parent.find 41
>> p.some_attr = 'some_value'
>> c = p.children.build
>> c.parent

By watching my log files, I can see that c.parent is querying the db for the parent object. I want instead to access the existing in-memory object (p), because I need access to the parent's some_attr value, which is not yet stored in the database. Is there any way of doing this? c.parent(force_reload=false) doesn't get me there.

Answer 1

ActiveRecord doesn't endeavor to guarantee that in-memory objects for the same database objects are all the same. This is something that DataMapper does guarantee.

I realize your example is probably simplified in order to ask your question, but just from a naive look at it -- why don't you just use p instead of c.parent?

Another possibly helpful suggestion, save the update to parent to the db:

p = Parent.find 41

# do this...
p.some_attr = 'some_value'
p.save

# OR this...
p.update_attribute(:some_attr, 'some_value')

c = p.children.build
c.parent

I'm not sure if c.parent(false) ("don't reload from the db") will do the trick here, since it's a fresh Child object. But you can try that too.

Answer 2

You could use :inverse_of to set it. Read more about it here.