Despite some exciting advances in the field, like Node, Redis, and Go, a well-structured relational database fronted by a Rails or Sinatra (or Django, etc.) app is still one of the most effective toolsets for building things for the web. In the coming weeks, I’ll be publishing a series of posts about how to be sure that you’re taking advantage of all your RDBMS has to offer.

A “NOT NULL constraint” enforces that a database column does not accept null values. Null, according to Wikipedia, is

a special marker used in Structured Query Language (SQL) to indicate that a data value does not exist in the database. Introduced by the creator of the relational database model, E. F. Codd, SQL Null serves to fulfill the requirement that all true relational database management systems (RDBMS) support a representation of “missing information and inapplicable information.”

One could make the argument that null constraints in the database are unnecessary, since Rails includes the presence validation. What’s more, the presence validation handles blank (e.g. empty string) values that null constraints do not. For several reasons that I will lay out through the rest of this section, I contend that null constraints and presence validations should not be mutually exclusive, and in fact, if an attribute’s presence is required at the model level, its corresponding database column should always require a non-null value.

Why use non-null columns for required fields?

Data Confidence

The primary reason for using NOT NULL constraints is to have confidence that your data has no missing values. Simply using a presence validation offers no such confidence. For example, update_attribute ignores validations, as does save if you call it with the validate: false option. Additionally, database migrations that manipulate the schema with raw SQL using execute bypass validations.

Undefined method ‘foo’ for nil:NilClass

One of my biggest developer pet peeves is seeing a undefined method 'foo' for nil:NilClass come through in our error tracking service du jour. Someone assumed that a model’s association would always be present, and one way or another, that assumption turned out to be false. The merits of the Law of Demeter are beyond the scope of this post, but suffice it to say that if you’re going to say something like @athlete.team.name in your code, you better be damn sure that a) the athlete’s team_id has a value and b) it corresponds to the ID of an actual team. We’ll get to that second bit in our discussion of foreign key constraints in a later post, but the first part, ensuring that team_id has a value, demands a NOT NULL column.

Migration Issues

Another benefit of using NOT NULL constraints is that they force you to deal with data migration issues. Suppose a change request comes in to add a required age attribute to the Employee model. The easy approach would be to add the column, allow it to be null, and add a presence validation to the model. This works fine for new employees, but all of your existing employees are now in an invalid state. If, for example, an employee then attempts a password reset, updating their password_reset_token field would fail due to the missing age value.

If you’d created the age column to require a non-null value, you would have been forced to deal with the issue of existing users immediately and thus avoided this issue. That said, there’s no obvious value for what to fill in for all of the existing users’ ages, but better to have that discussion at development time than to spend weeks or months dealing with the fallout of invalid users in the system.

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I hope I’ve laid out a case for using non-null constraints for all required database fields for great justice. In the next post, I’ll show the proper way to add non-null columns to existing tables.