ActiveRecord .exists?() and default_scope weirdness

We have a default_scope on our User class which restricts users to a set of companies:

class User < ActiveRecord::Base
  default_scope do
    c_ids = Authorization.current_company_ids
    includes(:companies).where(companies: { id: c_ids })
  end

  has_many :company_users
  has_many :companies, through: company_users
end

class CompanyUser < ActiveRecord::Base
  belongs_to :user
  belongs_to :company
  validates_uniqueness_of :company_id, scope: :user_id
end

class Company < ActiveRecord::Base
  has_many :company_users
  has_many :users, through: company_users
end

Calling User.last or User.find_by_email('mhayes@widgetworks.com') or User.find(55557) all work and scope as expected.

Calling User.exists?(id) throws an odd error:

Mysql2::Error: Unknown column 'companies.id' in 'where clause': SELECT  1 AS one FROM `users`  WHERE `companies`.`id` IN (4) AND `users`.`id` = 55557 LIMIT 1

Basically, if I'm getting this, it's saying that companies isn't a column on User, which it is. And if I even copy the sql into a where statement, it evaluates correctly.

User.where("SELECT  1 AS one FROM `users`  WHERE `companies`.`id` IN (4) AND `users`.`id` = 66668 LIMIT 1")

It makes me think there's an order of evaluation with default_scope and exists? is somehow called before default_scope.

If I call:

User.includes(:companies).where(companies: { id: [4] }).exists?(55557)

Works. And this is what the default_scope is doing, so I know the default_scope scope isn't failing.