Recursive accepts_nested_attributes_for in Rails, is it possible?

Question

I want to create a tree structure between two models, Bar and Foo.

Bar has many Foos

    Bar
  /  |  \
Foo Foo Foo
         ¦
        Bar
      /  |  \
    Foo Foo Foo

Bar can optionally belong to Foo.

Bar has many Foos, to infinity and beyond...

I configured things like this, but it doesn't seem to seed properly.

Even if I comment out any validations I have, I get the following error:

ActiveRecord::RecordInvalid: Validation failed: Foos bar must exist

I can't understand why.

class Bar < ApplicationRecord
  has_many :foos, inverse_of: :bar

  accepts_nested_attributes_for :foos
end


class Foo < ApplicationRecord
  belongs_to :bar, inverse_of: :foos

  accepts_nested_attributes_for :bar
end


class CreateFoos < ActiveRecord::Migration[6.1]
  def change
    create_table :foos do |t|
      t.text :description, null: false

      t.timestamps
    end
  end
end

class CreateBars < ActiveRecord::Migration[6.1]
  def change
    create_table :bars do |t|
      t.text :description

      t.references :foo,
        foreign_key: true,
        null: true,
        on_delete: :cascade,
        on_update: :cascade

      t.timestamps
    end
  end
end

class AddBarIdToFoosTable < ActiveRecord::Migration[6.1]
  def change
    add_reference :foos,
      :bar,
      foreign_key: true,
      null: false,
      on_delete: :cascade,
      on_update: :cascade
  end
end


Bar.create!([
  {
    description: 'Lorem ipsum...',
    foos_attributes: [
      {
        description: 'Lorem ipsum...',
        bar_attributes: {
          description: 'Lorem ipsum...',
          foos_attributes: [
            {
              description: 'Lorem ipsum...',
              bar_attributes: {
                description: 'Lorem ipsum...',
                foos_attributes: [
                  {
                    description: 'Lorem ipsum...'
                  },
                  {
                    description: 'Lorem ipsum...'
                  },
                  {
                    description: 'Lorem ipsum...'
                  },
                  {
                    description: 'Lorem ipsum...'
                  }
                ]
              }
            }
          ]
        }
      }
    ]
  }
])

why do you need accepts_nested_attributes in Foo? to create a foo with bar just pass the bar_id to foo — Mosaaleb, Nov 18 '21 at 23:46
This logic feels really circular and I'm struggling to understand what the real-world application would be here. Why would you use two different models to define the same object (i.e., a node in your tree)? IMO every element (foo or bar) should be of your node model. Here is a random blog that you might find helpful: https://www.leighhalliday.com/tree-structures-in-your-rails-models — Allison, Nov 18 '21 at 23:49
@Allison what you're seeing here is not a one-to-one mapping of what I'm trying to do. The models involved have their particular distinctions I need to keep separate — Jonathan, Nov 19 '21 at 00:57
@Mosaaleb yeah that's a good point, I'm not most familiar with Rails, coming over from another framework so I'm not completely clued up yet, I'll try removing this among other modifications — Jonathan, Nov 19 '21 at 00:58
Deleted my comment in favor of answering before I saw this, but if you have a strict parent child relationship you may want to define both a `belongs_to` and `has_many` on both classes, and the Rails Guides are pretty good if you haven't come across them before: https://guides.rubyonrails.org/association_basics.html — Allison, Nov 19 '21 at 02:06
No worries~ I'm going with belongs_to, has_many in one direction and belongs_to, has_one in the other in my situation — Jonathan, Nov 19 '21 at 02:15

score 2 · Accepted Answer · answered Nov 19 '21 at 01:49

ActiveRecord::RecordInvalid: Validation failed: Foos bar must exist

This is telling you that one of your Foo declarations requires the presence of bar
The reference to bar in your model declaration for Foo is in the belongs_to association
belongs_to is presence-validated by default in certain versions of rails; changing belongs_to :bar to belongs_to :bar, optional: true will likely resolve your issue

Ref: https://github.com/rails/rails/pull/18937/files

Recursive accepts_nested_attributes_for in Rails, is it possible?

1 Answers1