ruby on rails inconsistency in model persistance during has_many through association - ruby-on-rails-3

Question

I'm trying to figure out what is happening when I create a has_many through association.
models:
class Foo
has_many :bars
has_many :bazes, through: :bars
acceptes_nested_attributes_for :bars
acceptes_nested_attributes_for :bazes
end
class Bar
belongs_to :foo
belongs_to :baz
before_create :update_some_attr
def update_some_attr
if self.baz.new_record?
raise "baz is new record"
else
raise "baz is not new record"
end
end
end
class Baz
has_many :bars
end
form:
= form_for :foo do |f|
= f.fields_for :bazes do |baz_form|
# form fields for baz
= f.submit
If I raise an error within update_some_attr, and inspect the class, self.baz.new_record? returns true; and yet the ELSE condition fires, which means the Baz model is already persisted before the Bar record is created. I'm just trying to figure out why I'm getting this inconsistency while debugging.
Imgur link to _better_errors console output

Answer 1

Well first of all I have no direct answer to why the console output and the conditional statement both deliver inconsistent behaviour. However I do see opportunity to rewrite the method so the .new_record? condition can be circumvented:
You are applying a before_create callback which means that only on Model.create() the callback is fired so in that case why would one want to condition whether the instance is new? if the method should be used on existing records to update a certain attribute I would just stick to the update_attributes() method. Another method is to always require an exisiting record and change the callback to after_create this will make more sense as your method is called 'update'.
after_create :update_some_attr
def update_some_attr(*args={attribute: :name, value: "John Doe"})
args.assert_valid_keys(:attribute, :value)
attribute, value = [args[:attribute], args[:value]]
self.update_attributes(attribute => value)
end

Answer 2

I think tha you might expect something like this to happen:
Bar belongs_to Foo and to Baz, so before create Bar, ActiveRecord has to create Foo and Baz, get the ids and set it on the associations ids (foo_id and baz_id).
But i believe that the associations are giving you trouble:
Since Foo has many bars and bazs, ActiveRecord will try to save this associations before create Foo and maybe thats why Baz is already persisted on your method.
One solution (if thats the case) is to change the association between Bar and Baz to something like:
class Bar
has_many :bazs
end
class Baz
belongs_to :bar
end
But as i just realized now, you need Baz to have many Bars, so you can try to remove the association on Foo and the accepts_nested_attributes
has_many :basz, through: :bars
acceptes_nested_attributes_for :bazes
And accept the nested attributes on the Bar model:
acceptes_nested_attributes_for :baz
And you will be with something like:
class Foo
has_many :bars
acceptes_nested_attributes_for :bars
end
class Bar
belongs_to :foo
belongs_to :baz
acceptes_nested_attributes_for :baz
before_create :update_some_attr
def update_some_attr
if self.baz.new_record?
raise "baz is new record"
else
raise "baz is not new record"
end
end
end
class Baz
has_many :bars
end