Rails uniqueness constraint and matching db unique index for null column?

Validates_uniqueness_of:product_id, :if => lambda {!self. Product_id. Nil?

}.

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.

If you are going to have NULL values for a unique column, its best to handle this at the Application level Use validates_uniqueness_of validation callback.

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. ).

It depends on your db server. As for mysql.

If you are going to have NULL values for a unique column, its best to handle this at the Application level.

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