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

On Mon, Oct 10, 2011 at 1:37 PM, Ewen [email protected] wrote:

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?

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

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