Add check for add_reference method by Rails/NotNullColumn cop

[pocke]

add_reference method has same problem as add_column method(see #3415 ). So, Rails/NotNullColum cop also should detect the method.

For example

class SomeMigration < ActiveRecord::Migration
  def change
    add_reference :products, :category, null: false
  end
end

The above migration crashes other than MySQL.
Then, in MySQL, the migration does not crash, but category_id is decided by MySQL without permission.

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[bbatsov]

👍

[md5]

I'm not sure I understand why this change was made. How is it possible to have a default value for a reference column? By definition it's going to refer to the primary key of some other table, so it can't have a default.

[pocke]

By definition it's going to refer to the primary key of some other table, so it can't have a default.

The added reference column can have a default value.
For example:

default_category Category.create!(name: 'default')
add_reference :products, :category, null: false, default: default_category.id

Or we should make the reference to allow null.

add_reference :products, :category, null: true

[md5]

Fair enough, but that seems like an uncommon case. I would think it's more common to not known the default for a non-null reference. The example in the documentation of hard-coding the default to 1 certainly doesn't make sense.

[pocke]

The example in the documentation of hard-coding the default to 1 certainly doesn't make sense.

Do you mention this https://github.com/bbatsov/rubocop/pull/3782/files#diff-981212d9d67b8b33a445cb8321ae326dR18 ?
This example doesn't make sense. And I think we can update the documentation with the following.

add_reference :products, :category
# or
add_reference :products, :category, null: false, default: Category::DEFAULT_CATEGORY_ID
# or
add_reference :products, :category
Products.find_each do |product|
  product.set_category
  product.save!
end
change_column_null :products, :category_id, false

[tgaff]

This seems like more of the exception than the rule.
It's common to want to ensure that an id is always set but much less common to want it to point at a default rather than cause an error. A bank account definitely needs an owner_id, but I'd rather have it fail to open the account than point at the bank's president by default.

The non-references portion of this rule looks useful. Is there any way to configure this to not check add_reference but keep the rest of the rule?