Last week I wrote Using Active Support Concerns to Encapsulate Data Access and Validation, where I explained how to use Active Support concerns to define data access and validation rules for an Active Record enum attribute. The attribute mapped values to integers in the database, similar to this example:

class Project < ActiveRecord::Base
  enum status: [ :active, :archived ]
  validates :status, inclusion: { in: statuses.keys }
end

Depending on the problem that you are trying to solve, this approach could potentially lead to several drawbacks including:

1. Meaningless integer values stored in the database.
2. Unless the integer value is “limited” in a migration, Active Model validations can be by-passed through plain SQL, effectively allowing the attribute to be set to any integer.
3. Plain SQL queries need to use the integer value, not the Active Record enum.

In this blog post, I’ll show how to use the Postgres enum type with Rails to avoid the aforementioned pit falls.

Postgres Enumerated Types

Postgres supports enumerated types, which are data types that comprise a static, ordered set of values. To create an enum type, use the Postgres CREATE TYPE command. In a Rails project, generate a migration as follows rails g migration AddStatusToProjects:

class AddStatusToProjects < ActiveRecord::Migration[5.2]
  def up
    execute <<-SQL
      CREATE TYPE project_status AS ENUM ('active', 'archived');
    SQL
    add_column :projects, :status, :project_status
  end

  def down
    remove_column :projects, :status
    execute <<-SQL
      DROP TYPE project_status;
    SQL
  end
end

The migration creates a project_status enumerated type. Next, it adds a status column to the projects table of type project_status. By using the Postgres enumerated type, the status attribute is constrain at the database level to be one of active|archived. Lastly, set config.active_record.schema_format = :sql in the environment configuration files, so that the database schema includes the project_status enumerated type definition.

Active Record Enum

The initial definition of the status Active Record enum needs to be slightly modified. Simply use a hash to explicitly map the relation between the attribute and database value as follows:

class Project < ActiveRecord::Base
  enum status: { active: 'active', archived: 'archived' }
  validates :status, inclusion: { in: statuses.keys }
end

Unit Tests

Unit tests can be tremendously simplified by using the shoulda-matchers gem:

RSpec.describe Project, type: :model do
  it { should define_enum_for(:status).
      with_values(
        active: 'active',
        archived: 'archived'
      ).backed_by_column_of_type(:enum) }

  it { should allow_values(:active, :archived).for(:status) }
end

The first unit test verifies the status column is defined in the Project model and its column type is enum. Also, it makes sure the status enum values are correctly defined as specified in the status hash. Finally, the last test confirms the Project model allows the status attribute to be set using the :active and :archived symbols.

Conclusion

And that was it! The Project model defines and validates an enum for the status attribute which is mapped to meaningful string values in the database. Additionally, the status column is constrain at the database level to be one of the specified values in the Postgres enumerated type.

If you are interested in learning more about enum in Rails, I highly recommend reading Ruby on Rails - how to create perfect enum in 5 steps. This blog post, written by Błażej Pichur, has been a fantastic guide during the past few weeks to improve my understanding and usage of enum in Rails.