Tuesday, November 07, 2006

Generating ActiveRecord Data Validations Automatically

Having recently released DrySQL, which automatically generates validations for ActiveRecord data classes that enforce the constraints declared on your DB, I thought it might be useful to start a discussion about how this works (or should work).

My (possibly naive) belief is that the value of application-side validations that enforce DB constraints is twofold:
  • Allows the application to provide immediate feedback to UI/forms/whatever about validity of input without the performance overhead of querying the DB
  • Avoids wasting application and DB cycles trying to save/update invalid records
That said, let's look at the ActiveRecord validation methods, how they apply to enforcing DB constraints, and how DrySQL generates them automatically.

I don't understand how this validation is useful, but will be interested to hear others' opinions about it. To validate that a particular value is unique in a particular table requires a DB query. Why do this on the application side rather than just letting the DB perform the validation and handling the duplicate key error your DB returns? This validation can be generated automatically based on the unique key information in the DB's information schema, but I'm not sure that it should be.

These validations can be generated automatically by querying the information schema's columns table. The columns table contains the restrictions that were declared for the column when it was defined.

These validations can be generated automatically for databases that support check constraints, but DrySQL does not yet implement this. Currently, validates_inclusion_of is generated automatically only for boolean columns since we know the value must be in the set [true, false].

This validation can be generated automatically by querying the information schema's columns table and checking the column's NOT NULL constraint. Unfortunately, the implementation of this validation in ActiveRecord does not accurately mirror the DB's NOT NULL constraint. In situations where the column contains character data and has the NOT NULL constraint set, validates_presence_of will reject an empty string value even though the DB would consider this value to be valid. DrySQL does not generate this validation, but rather generates a new validation, validates_nullability_of, where appropriate.

This constraint is new, and was developed as part of DrySQL. It mirrors your DB's handling of NOT NULL constraints. This validation will reject a null value unless the column is autogenerated or has a default value specified, because your DB wouldn't reject a NULL in these situations either.

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