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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_active_entry(user, select_for_update=False):
 """Returns the user's currently-active entry, or None."""

    entries = apps.get_model('entries', 'Entry').no_join
    if select_for_update:
        entries = entries.select_for_update()
    entries = entries.filter(user=user, end_time__isnull=True)

    if not entries.exists():
        return None
    if entries.count() > 1:
        raise ActiveEntryError('Only one active entry is allowed.')
    return entries[0]