title: Reuse filter() queryset in Django
body: <p>I want to write a filter once and reuse it everywhere, how can I do it?</p>
  
  <p>For example: there is a model <code>Student</code> with flag <code>field</code>. I want to write a filter to get non-graduate students (<code>flag=0</code>). But in many views and functions we need list non-graduate students, and I am lazy and don't want to write the filter again and again in these views and functions, as this will make it hard to maintain the source code.</p>
  
  <p>Can I use meta in model Student? I did not find any filter related meta options. Or can I write a function to filter in model Student? In my mind, function in model only works in one Student object instead of list.</p>