Postgres create foreign key constraint

postgres_add_foreign_keys.rb

# add foreign key constraint
execute <<-SQL
  alter table access_tokens
  add constraint fk_access_token_user_id
  foreign key (user_id)
  references users(id)
  on delete cascade
SQL

# add unique index
execute <<-SQL
  CREATE UNIQUE INDEX <index_name>
  ON <table> (<col1>, <col2>)
SQL