Finding the parent of a nested object from it's model before it is saved?

Is there a way to access a nested objects parent from within that
objects model before it is saved (so I have no ID set yet)?

in artwork.rb:
   has_many artwork_attachment
   accepts_nested_attributes_for :artwork_attachment, :allow_destroy
=> true

artwork_controller.rb's new action includes Artwork.new and
artwork_attachment.build

The problem I have is that in artwork_attachment.rb I have some code
that resets the size of the paperclip attachments based on information
associated with artwork (actually artwork belongs_to line_item which
has_one product and the product has specific dimension). So how can I
refer to the parent of a nested resource before it is actually saved
and gets given an ID?

Thanks
Ewen

I'm wondering if :inverse_of will allow me to access the parent
instance from the child model before saving. Hmmmm

I faced this problem too.

A solution that seems to work is to explicitly set the relation to the parent

object in the .build method. But I am not sure if this is really kosher … A

proper solution would be welcome.

A code example:

class User

has_many :referrals

end

class Referral

belongs_to :user

end

$ rails c

Loading development environment (Rails 3.1.1.rc1)

001:0> u = User.new(:name => “Frans”)

=> #<User id: nil, name: “Frans”, created_at: nil, updated_at: nil, age: 0>

002:0> r1 = u.referrals.build(:comment => “dad”)

=> #<Referral id: nil, user_id: nil, comment: “dad”, created_at: nil, updated_at: nil>

003:0> r2 = u.referrals.build(:comment => “wize”, :user => u) ### setting the parent explicitly

=> #<Referral id: nil, user_id: nil, comment: “wize”, created_at: nil, updated_at: nil>

004:0> r1.user # I assume this is the “back link” that you are missing

=> nil

005:0> r2.user # and know we can ask the parent before it gets saved and without db query

=> #<User id: nil, name: “Frans”, created_at: nil, updated_at: nil, age: 0>

006:0> u.save

(0.4ms) BEGIN

SQL (58.4ms) INSERT INTO “users” (“age”, “created_at”, “name”, “updated_at”) VALUES ($1, $2, $3, $4) RETURNING “id” [[“age”, 0], [“created_at”, Sun, 16 Oct 2011 19:39:01 UTC +00:00], [“name”, “Frans”], [“updated_at”, Sun, 16 Oct 2011 19:39:01 UTC +00:00]]

SQL (1.1ms) INSERT INTO “referrals” (“comment”, “created_at”, “updated_at”, “user_id”) VALUES ($1, $2, $3, $4) RETURNING “id” [[“comment”, “dad”], [“created_at”, Sun, 16 Oct 2011 19:39:01 UTC +00:00], [“updated_at”, Sun, 16 Oct 2011 19:39:01 UTC +00:00], [“user_id”, 5]]

SQL (0.4ms) INSERT INTO “referrals” (“comment”, “created_at”, “updated_at”, “user_id”) VALUES ($1, $2, $3, $4) RETURNING “id” [[“comment”, “wize”], [“created_at”, Sun, 16 Oct 2011 19:39:01 UTC +00:00], [“updated_at”, Sun, 16 Oct 2011 19:39:01 UTC +00:00], [“user_id”, 5]]

(0.7ms) COMMIT

=> true

007:0> r1.user # now this works too (based on database id’s)

User Load (1.1ms) SELECT “users”.* FROM “users” WHERE “users”.“id” = 5 LIMIT 1

=> #<User id: 5, name: “Frans”, created_at: “2011-10-16 19:39:01”, updated_at: “2011-10-16 19:39:01”, age: 0>

008:0> r2.user # this works too, but does NOT call the database (danger ??)

=> #<User id: 5, name: “Frans”, created_at: “2011-10-16 19:39:01”, updated_at: “2011-10-16 19:39:01”, age: 0>

009:0> r1.user.object_id

=> 85528740

010:0> r2.user.object_id # different object for the parent user

=> 84886560

011:0> r1.user == r2.user # but equivalent user objects

=> true

Specifically in your case, that would probably adding to your artwork_attachments.build

a link to the parent artwork in memory object (before saving to db).

I assume the plural artwork_attachments

in artwork controller

@artwork = Artwork.new

@artwork.artwork_attachments.build(argument_hash.merge(:artwork => @artwork))

this then should allow in artwork_attachment.rb

belongs_to :artwork

dimension = self.artwork.line_item.product.dimension

I repeat that I am not sure that this is really aproper solution,

and I am curious for the proper solution if it exists.

Thanks,

Peter