[RUBY] [Rails] [Parent-child relationship] I want to register the foreign key in the child with nil when the parent is deleted.

Overview

When building the following parent-child relationships with reference When deleting the data of the parent model, I want to register the foreign key with nil without deleting the child product data, but when I delete the parent from the console, an error occurs and I do not know how to deal with it. is. -Parent model: "Category" ・ Child model: "Product"

`20201211121059_create_products.rb`


  t.references :category, null: false, foreign_key: true

Practice

It was carried out with reference to the following articles. optional: How to allow nil if set to true https://techtechmedia.com/optional-true-rails/#optional_true https://qiita.com/takuyanin/items/6f6be86d1265be21bf9e

Like this, I added dependent and optionl options to has_many and belongs_to.

`catedory.rb`


class Category < ApplicationRecord
  has_many :products, dependent: :nullify
end

`product.rb`


class Product < ApplicationRecord
  belongs_to :category, optional: true
end

In this state, register parent and child data from the console.

Example category = Category.create!(....) category.products.create!(....)

Execute Category.destroy_all as a trial. The child's foreign key is registered as nil, right? I get the following error and have trouble dealing with it.

ActiveRecord::NotNullViolation: Mysql2::Error: Column 'category_id' cannot be null

Also, when creating a new child model data with the foreign key nil, the error is different depending on whether optional: true is set/not set.

At the time of setting ActiveRecord::NotNullViolation: Mysql2::Error: Field 'category_id' doesn't have a default value

Not set ActiveRecord::RecordInvalid: Validation failed: Category must exist

Optional: true in the first place? If set to true, validation can be passed even if the foreign key does not contain a value. ↑ ** In other words, the problem is validation ** (If the default is not set, it is false, so validation is executed.)

To the solution

(Rails db: rollback is also possible with up, down notation)

class ChangeReferencesIdToProducts < ActiveRecord::Migration[6.0]
  def up
    change_column :products, :category_id, :bigint, null: true
  end
  
  def down
    change_column :products, :category_id, :bigint, null: false
  end
end

It was caused by null: false, so by changing it to null: true The child's foreign key is now registered with nil even if the parent is deleted.

I hope it helps someone.