Allow List Params Anywhere With Strong Params

The intended use of StrongParams is to prevent unintended params from getting through a controller action during mass assignment.

This can be put to use other places in your Rails app, such as a service object, where mass assignment is used to update records.

class BookTitleUpdater
  ALLOW_LIST = [:title].freeze

  def self.run(data)
    params = ActionController::Parameters.new(data).permit(*ALLOW_LIST)

    Book.find(data[:id]).update!(params)
  end
end

This helps prevent other values from getting inadvertently updated on the book record.

> ALLOW_LIST = [:title]
> data = { title: "Legacy Code", author_id: 22 }
> params = ActionController::Parameters.new(data).permit(*ALLOW_LIST)
> params.to_h
#=> { title: "Legacy Code" }

The author_id value is ignored and won't be passed to the #update call.