Question

I want to put some model level validation on the following table:

create_table :audios do |t|
  t.integer :library_id, :null => false
  t.string :file, :null => false, :limit => 2048
  t.string :name, :limit => 512
  t.timestamps
end

Does this mean, that my model, which (so far) looks like:

class Audio < ActiveRecord::Base
  belongs_to :library
end

Has

class Audio < ActiveRecord::Base
  validates_presence_of :library
  ...

or

class Audio < ActiveRecord::Base
  validates_presence_of :library_id
  ...

?

Answer 1

To validate the presence of an association, use its name, without _id appended:

validates_presence_of :library

It will validate two things:

1. library_id is present
2. a Library with the given id exists

Using validates_presence_of :library_id will only give you the first validation of the two.

In addition to this, the version without _id will also correctly validate if both records are new (and therefore library_id is still unset).