class Manager
  has_many :employees
  has_many :transactions, as: :transactable
end

class Employee
  belongs_to :manager
  has_many :transactions, as: :transactable
end

class Transaction
  belongs_to :transactable, polymorphic: true
end

class Transaction

  def self.belonging_transactions(manager)
    where(
      "(transactable_type = 'Manager' AND transactable_id = ?) OR (transactable_type = 'Employee' AND transactable_id = ?)",
      manager.id,
      manager.employees.ids
    )
  end

end

Transaction.belonging_transactions(manager) # here manager is Manager class object