Rails: Adding a unique constraint for a has_one association might be a good default

Copyclass User < ApplicationRecord
  has_one :session, dependent: :destroy
end

class Session < ApplicationRecord
  belongs_to :user
end

Copycreate_table :users do |t|
  t.timestamps
end

create_table :sessions do |t|
  t.references :user, null: false, foreign_key: true, index: { unique: true } # Don't forget the uniqueness here
  t.timestamps
end