Rails 5.0-如何检查记录是否与另一个表关联？

Question

Before destroying a record, I'd like to check if there are any uses of it on other tables, even if the record itself has no knowledge of said uses .

For example, lets say I have a table of cost_centers , and I have a table of areas .

An area has a default cost_center . cost_centers have no connection with areas .

Which kind of validation can I use in order to prevent the user from destroying a cost_center , in order to keep an area consistent?

In other words, how can I search through the database to find out wether that record is a foreign key of some other record on any other tables?

Answer 1

When you designed your database, you've probably set all the references up. In your migrations, it would look like this: t.references :cost_center .

If so, your Cost Center Model could have a has_one relationship to each table holding the reference which, in your example, would be has_one :area .

Then, to check if it is actually used, you could have a before_destroy callback to a method that checks if any has_one definition is not null:

class CostCenter < ActiveRecord::Base
  has_one :area
  before_destroy :check_usage

  def check_usage
    !self.area.nil?
  end
end