Railsdocs Rails Documentation Project website

In importing legacy databases, it can be helpful to go behind ActiveRecord’s back and work directly through MySQL. But there’s little or no documentation of the Model.connection.execute(“[SQL statement]”) method, and NO description of the syntax: it’s a class method

For example, Model.delete_all empties the table, but autoincrementing fields like :id keep on incrementing from whatever number they were on. Model.connection.execute(“TRUNCATE TABLE models”) effectively deletes the table and recreates it with all counters reset. This would prevent :id integer overflow if one kept reimporting a large table.

Other SQL statements could be used to generate :id fields from existing legacy table key fields not in integer format. That way, one doesn’t have to dig through the entire imported database to look up valid foreign key links: calculate them instead. (According to the Agile Web Development book, Rails can’t save a record unless the :id field is valid, so generating a new :id field appears to be verboten.)