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Wednesday, August 27 2008

Rails411

Disable Foreign Key Checks Under MySql

When you’ve built a database that includes foreign key constraints between parent and child tables, loading fixtures using rake load_fixtures and unit testing can fail with errors such as:

Mysql::Error: #23000Cannot add or update a child row: a foreign key constraint fails (`depot_development/line_items`, CONSTRAINT `fk_items_product` FOREIGN KEY (`product_id`) REFERENCES `products` (`id`) ON DELETE CASCADE): INSERT INTO line_items (`quantity`, `product_id`, `id`, `unit_price`) VALUES (1, 1, 1, 29.95)
OR
ActiveRecord::StatementInvalid: Mysql::Error: Cannot delete or update a parent row: a foreign key constraint fails: DELETE FROM line_items WHERE id = 1
... because the child record is being inserted into the database before the parent record. A similar error can occur when updating fixtures using the same rake task because the old parent record is being deleted before the child record.