Autosave belongs_to Associations

By default, if you make a change to a belongs_to associated model and save the parent model, the associated model won't be saved. This is probably a good default, but it is overridable with the :autosave option.

class Book < ApplicationRecord
  belongs_to :author
end

book = Book.first
book.author.name # => "J.R.R. Tolkein"

book.author.name = "John Tolkein"

book.save
book.reload
book.author.name # => "J.R.R. Tolkein"

But we can change this behaviour by setting :autosave to true.

class Book < ApplicationRecord
  belongs_to :author, autosave: :true
end

book = Book.first
book.author.name # => "J.R.R. Tolkein"

book.author.name = "John Tolkein"

book.save
book.reload
book.author.name # => "John Tolkein"

While this is off by default for belongs_to associations, has_one and has_many associations have :autosave true because the foreign keys are on the associated records to ensure those FKs are kept up to date.