date_check

from __future__ import unicode_literals
import frappe
from frappe import _
from frappe.model.document import Document
from frappe.utils import nowdate

class LibraryTransaction(Document):
	def validate(self):
		last_transaction = frappe.get_list("Library Transaction",
			fields=["transaction_type", "transaction_date"],
			filters = {
				"article": self.article,
				"transaction_date": ("<=", self.transaction_date),
				"name": ("!=", self.name)
			})
		if self.transaction_type=="Issue":
			msg = _("Article {0} {1} has not been recorded as returned since {2}")
			if last_transaction and last_transaction[0].transaction_type=="Issue":
				frappe.throw(msg.format(self.article, self.article_name,
					last_transaction[0].transaction_date))
		else:
			if not last_transaction or last_transaction[0].transaction_type!="Issue":
				frappe.throw(_("Cannot return article not issued"))

		if self.transaction_date != nowdate():
			frappe.throw(_("Transaction can only be made at current date"))

Sir, its Still not working. I replaced my code with yours then also.
for eg. if i am selecting transaction_date 10-12-2014 and today it is 08-12-2014. its saving the form and not showing the given error.