Filtering html form options based on previous input

I'll preface this question by saying I'm not looking for code here, just an indication of whether my approach is correct before I undertake a large effort. That's why it will appear I have yet to try to code it myself.

I'm developing a django site that includes a database of precedents. I want to let certain users upload new decisions using the generic CreateView and a ModelForm built using crispy-forms. I want to make this as easy for users as possible by removing irrelevant options from html <option> fields. The part of the form I'm concerned with looks a bit like:

<select class="input-xlarge select" id="id_decisions" name="clauses">
    <option value="" selected="selected">---------</option>
    <option value="1">2012 Decisions</option>
    <option value="2">2011 Decisions</option>
    ...
</select>
<select class="input-xlarge select" id="id_clauses" name="clauses">
    <option value="" selected="selected">---------</option>
    <option value="1">2012 clause 1</option>
    <option value="2">2012 clause 2</option>
    <option value="3">2011 clause 1</option>
    ...
</select>

When someone selects 2012 decisions from the dropdown I want to filter away all options in the 'clause' option field that don't say '2012'. I suspect this means writing a javascript filter similar to the answer in this question but I don't know any javascript and therefore don't really understand what that code is doing. Before I commit to learning enough javascript to be sure I produce a good solution I just want to be sure my suspicion about how this is done is correct. Is it? Am I missing an obvious way to do this in Python using either Django or crispy-forms?

If you want the dropdown to be filtered without submitting the form and refreshing the page, then the only way to do it is with Javascript. This isn't a limitation of Django: that's just how web applications work.