Rails uniqueness constraint and matching db unique index for null column

I have the following in my migration file

  def self.up
    create_table :payment_agreements do |t|
      t.boolean    :automatic, :default => true, :null => false
      t.string     :payment_trigger_on_order
      t.references :supplier
      t.references :seller
      t.references :product
      t.timestamps
    end
  end

I want to ensure that if a product_id is specified it is unique but I also want to allow null so I have the following in my model:

  validates :product_id,
            :uniqueness => true,
            :allow_nil => true

Works great but I should then add an index to the migration file

add_index :payment_agreements, :product_id, :unique => true

Obviously this will throw an exception when two null values are inserted for product_id. I could just simply omit the index in the migration but then there's the chance that开发者_运维百科 I'll get two PaymentAgreements with the same product_id as shown here: Concurrency and integrity

My question is what is the best/most common way to deal with this problem

it depends on your db server. as for mysql:

A UNIQUE index creates a constraint such that all values in the index must be distinct. An error occurs if you try to add a new row with a key value that matches an existing row. This constraint does not apply to NULL values except for the BDB storage engine. For other engines, a UNIQUE index allows multiple NULL values for columns that can contain NULL.

Some major database systems do not allow a unique index to contain multiple NULLs: unique applies to NULLs as well as non-NULLs. There are ways around this on the database level (e.g., triggers, or a computed column; see link text).

You could address this on an application level and put in a validation that checks for uniqueness if the product_id is not null.

validate :enforce_unique_product_id
def enforce_unique_product_id
  if (!self.product_id.nil? &&
      PaymentAgreement.exists?(:conditions=>['product_id = ?', self.product_id]))
    errors.add_to_base('There is already an agreement with product id " + 
                       self.product_id)
  end
end

(Update: As pointed out by zed_0xff, MySql allows multiple NULLs in a UNIQUE index in the most commonly used storage engines.)