PostgreSQL Empty Array as Default Value

We can add a PostgreSQL array column with a default empty array using:

add column things integer[] not null default '{}';

Today I added an array of integers to a PostgreSQL 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 built-in PostgreSQL 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 null / nil, which it is by default.

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

alter table posts
  add column slack_notified integer[] not null default '{}';