from datetime import datetime, timedelta

from django.db import models


class LockManager(models.Manager):
    """
    Usage:

        # Lock 'sync' for 2 minutes.
        if Lock.objects.lock('sync', 120):
            # This code will be accessed only
            # once per 2 minutes.

    """
    def get_lock(self, name):
        try:
            return self.get(name=name)
        except Lock.DoesNotExist:
            return Lock(name=name)

    def lock(self, name, seconds):
        """
        Set new lock and return True, but if already locked, do nothing and
        return False.

        ``name``
            Lock name, any string.

        ``seconds``
            For how many seconds ``name`` must be locked.
        """
        now = datetime.now()
        lock = self.get_lock(name)
        if lock.locked and lock.locked > now:
            return False
        else:
            lock.locked = now + timedelta(seconds=seconds)
            lock.save()
            return True


class Lock(models.Model):
    name = models.CharField(max_length=16, unique=True)
    locked = models.DateTimeField()

    objects = LockManager()