How to create association between unsaved records - ruby-on-rails

Question

I created 3 models as below, and used cocoon nested form to create associations between them.
class Unit < ApplicationRecord
has_many :mapping_categories, -> { distinct }, dependent: :destroy, inverse_of: :unit
accepts_nested_attributes_for :mapping_categories,
allow_destroy: true,
reject_if: :all_blank
end
class MappingCategory < ApplicationRecord
belongs_to :unit
has_many :mapping_items, -> { distinct }, dependent: :destroy, inverse_of: :mapping_category
accepts_nested_attributes_for :mapping_items,
allow_destroy: true
end
class MappingItem < ApplicationRecord
belongs_to :mapping_category
has_many :mapping_item_links
has_many :linked_mapping_items, through: :mapping_item_links, dependent: :destroy
end
Each mapping_item can have many other mapping_items through a joint table. In every mapping_item section in Unit form, this association is displayed as a select input.
When creating or updating Unit, there are many mapping_categories tabs in the Unit form, and there are many mapping_items sections in each mapping_category section.
For example, I have Mapping Category A and Mapping Category B. I want to add Mapping Item 1 to Mapping Category A and Mapping Item 2 to Mapping Category B. The question is: How to create the association between Mapping Item 1 and Mapping Item 2, as these two items are not saved yet?
Thanks in advance.

Answer 1

YOU CAN DO IT
You have to write right code
user = User.new(name: 'Jons', email: 'jons#qq.ww')
bank_account = BankAccount.new(number: 'JJ123456', user: user)
bank_account.save
in this way will be saved both raws and user and bank_account
in your case:
unit = Unit.new(mapping_categories: [mapping_category])
mapping_category = MappingCategory.new(mapping_items: [mapping_item])
mapping_item = MappingItem.new
unit.save
and if you wanna use nested_attributes, you just have to build hash with attributes
params = { mapping_categories: [mapping_items: [{.....}]}] }
Unit.create(params)
but you have to figure out with right nesting

Answer 2

From my understanding of your question... You can't. These items don't yet have ids and there for can't be associated with another model.
> contact = Contact.new(full_name: "Steve", email:"example#asdf.com")
=> #<Contact id: nil, full_name: "Steve", email: "example#asdf.com", created_at: nil, updated_at: nil>
> invoice = Invoice.new(contact_id: contact.id, invoice_type: "Something")
=> #<Invoice id: nil, contact_id: nil, invoice_type: "Something" created_at: nil, updated_at: nil>
> invoice.save
=> false