Now, let's write code that will make sure whenever a Library Membership is created, there is no active membership for the Member.

library_membership.py

from __future__ import unicode_literals

import frappe
from frappe.model.document import Document

class LibraryMembership(Document):
    # check before submitting this document
    def before_submit(self):
        exists = frappe.db.exists(
            'Library Membership',
            {
                'library_member': self.library_member,
                # check for submitted documents
                'docstatus': 1,
                # check if the membership's end date is later than this membership's start date
                'to_date': ('>', self.from_date),
            },
        )
        if exists:
            frappe.throw('There is an active membership for this member')

We wrote our logic in the before_submit method which will run before we submit the document. We used the frappe.db.exists method to check if a Library Membership record exists with our provided filters. If it exists, we used frappe.throw to stop the execution of program with a message that will show up letting the user know the reason.

Now, try creating a Library Membership with an overlapping period and you should see an error when you submit the document.