Add A Check Constraint To A Table

PostgreSQL allows you to enforce all kinds of rules about the value of a column or the relationship between two columns. These rules are defined with check constraints. ActiveRecord's migration DSL does not provide a way for adding check constraints directly. They can be added by executing a SQL statement in a migration.

class EnsurePageCountIsPositive < ActiveRecord::Migration[5.2]
  def up
    execute <<-SQL
      alter table books
        add constraint ensure_page_count_is_positive
        check (page_count > 0);
    SQL
  end

  def down
    execute <<-SQL
      alter table books
        drop constraint ensure_page_count_is_positive;
    SQL
  end

This check constraint ensures that, anytime you add or update a row in the book column, the value of page_count column is always greater than 0. This is a nice thing to enforce because it wouldn't make much sense for a book to have, say, -10 pages.

Note: these constraints will not appear in your db/schema.rb file. If you want to see what check constraints have been defined across your tables, you can crack open psql to investigate.