DocStatus import fails

I’m following tutorial and I got to the point where I’m editing LibraryMembership class to check
if the member already has a membership active.

from __future__ import unicode_literals
import frappe
from frappe.model.document import Document
from frappe.model.docstatus import DocStatus

class LibraryMembership(Document):
	def before_submit(self):
		exists = frappe.db.exists(
			'Library Membership',
				'library_member': self.library_member,
				'docstatus': DocStatus.submitted(),
				'to_date': ('>', self.from_date),
		if exists:
			frappe.throw('There\'s an active membership for this member.')

When I give it a ride to see it in action I get import error:

ModuleNotFoundError: No module named 'frappe.model.docstatus'

I’ve checked the source code for bench to see if the imports haven’t changed (which is unlikely since I downloaded latest release) and I could clearly see the import is valid.

Any suggestions ?