问无法在JavaScript中使用onload()访问表单元素

EN

{% load crispy_forms_tags %}    
<div>
    <form method="post" id="contractForm" novalidate 
        data-teachers-url="{% url 'wakemeup:ajax_load_teachers' %}" 
        data-classes-url="{% url 'wakemeup:ajax_load_classes' %}" 
        data-students-url="{% url 'wakemeup:ajax_load_students' %}" 
    >
        {% crispy form %}
    </form>
</div>

<script>
    // Update class drop-down when teacher changes
    $("#id_teacheruserid").change(function () {
        update_classes($(this))
      });

    // Update students drop-down when class changes
    $("#id_classid").change(function () {
        update_students($(this))
      });

    function update_teachers() {
        // Get form values
        var url = $("#contractForm").attr("data-teachers-url");
        var contractId = $(document.getElementById("id_contractid")).val();

        $.ajax({                             // initialize AJAX request
          url: url,                         
          data: {
            'contractid': contractId
          },
          success: function (data) {            
            $("#id_teacheruserid").html(data);  // Update "teacheruserid" field
          }
        })

        update_classes(); // Update classes drop-down
    };

    function update_classes() {
        // Get form values
        var url = $("#contractForm").attr("data-classes-url");
        var teacherUserId = $(document.getElementById("id_teacheruserid")).val();
        var contractId = $(document.getElementById("id_contractid")).val();

        $.ajax({                            // initialize AJAX request
          url: url,                         
          data: {
            'teacheruserid': teacherUserId, 
            'contractid': contractId
          },
          success: function (data) {        
            $("#id_classid").html(data);       // Update "classid" field
          }
        })

        update_students(); // Update students drop-down
    };

    function update_students() {
        // Get form values
        var url = $("#contractForm").attr("data-students-url");
        var classId = $(document.getElementById("id_classid")).val();
        var contractId = $(document.getElementById("id_contractid")).val();

        $.ajax({                                // initialize AJAX request
          url: url,                                 
          data: {
            'classid': classId,                     
            'contractid': contractId
          },
          success: function (data) {                
            $("#id_partyuserinfo").html(data);      // Update "partyuserinfo" field
          }
        })
    };

    window.onload = function() {
        // Initial load of drop-down menus
        update_teachers(); // Cascades to update_classes() and update_students()
    };      
</script>

    var teacherUserId = $(document.getElementById("id_teacheruserid")).val();