Default to Empty Array in Postgres

Today I added an array of integers to a Postgres table, and like all such migrations, it will default to null for each new record.

This was a problem, because I wanted to use Rails 4's built-in Postgres array support to make decisions based on that data. Ruby array methods like include? will raise NoMethodError: undefined method 'include?' for nil:NilClass if that array is ever nil, which it is by default.

This led me to learn how to set the default value to an empty array using Postgres array literal syntax. I was then able to include a not null constraint as an added benefit:

# db/migrate/20160211043316_add_slack_notified.rb

def up
  execute <<-SQL
    alter table posts
      add column slack_notified integer[] not null default '{}';
  SQL
end