You can concatenate as many filters as you like to a QuerySet, and you will not hit the database until the QuerySet is evaluated. QuerySets are only evaluated in the following cases:
- The first time you iterate over them
- When you slice them, for instance, Post.objects.all()[:3]
- When you pickle or cache them
- When you call repr() or len() on them
- When you explicitly call list() on them
- When you test them in a statement, such as bool(), or , and, or if
