Django错误:"'ChoiceField'对象没有属性'is_hidden'"

Django模板在渲染时抛出'AttributeError'.我想要实现的是,在模板中,resolted forn将包含一个选择框,其中包含下面列表中的值.

这是Forms.py文件:

class CallForm (forms.ModelForm):

class Meta():
    model = Call
    widgets = {
         'employee_id' : forms.ChoiceField(choices=FormsTools.EmployeesToTuples(Employee.objects.all()))
    }

说明:

FormsTools.EmployeesToTuples(Employee.objects.all()) --> [(1, u'E1'), (2, u'E2'), (3, u'E3')]

这是django的错误:

Template error:
In template C:\virtualenv\#\#\templates\new_call.html, error at line 7
   'ChoiceField' object has no attribute 'is_hidden'
   1 : {% extends "base.html" %}


   2 : 


   3 : {% block content %}


   4 : 


   5 :         
{% csrf_token %}


   6 :             



   7 :                  {{ form.as_p }} 


   8 :             


   9 :             


   10 : 


   11 :         


   12 : 


   13 : {% endblock %}

Traceback:
File "C:\Python27\lib\site-packages\django\core\handlers\base.py" in get_response
  112.                     response = wrapped_callback(request, *callback_args, **callback_kwargs)
File "C:\virtualenv\OmegaManager2\OmegaManager\ServiceManager\views.py" in new_call
  32.         html = t.render(Context(args))
File "C:\Python27\lib\site-packages\django\template\base.py" in render
  140.             return self._render(context)
File "C:\Python27\lib\site-packages\django\template\base.py" in _render
  134.         return self.nodelist.render(context)
File "C:\Python27\lib\site-packages\django\template\base.py" in render
  840.                 bit = self.render_node(node, context)
File "C:\Python27\lib\site-packages\django\template\debug.py" in render_node
  78.             return node.render(context)
File "C:\Python27\lib\site-packages\django\template\loader_tags.py" in render
  123.         return compiled_parent._render(context)
File "C:\Python27\lib\site-packages\django\template\base.py" in _render
  134.         return self.nodelist.render(context)
File "C:\Python27\lib\site-packages\django\template\base.py" in render
  840.                 bit = self.render_node(node, context)
File "C:\Python27\lib\site-packages\django\template\debug.py" in render_node
  78.             return node.render(context)
File "C:\Python27\lib\site-packages\django\template\loader_tags.py" in render
  62.             result = block.nodelist.render(context)
File "C:\Python27\lib\site-packages\django\template\base.py" in render
  840.                 bit = self.render_node(node, context)
File "C:\Python27\lib\site-packages\django\template\debug.py" in render_node
  78.             return node.render(context)
File "C:\Python27\lib\site-packages\django\template\debug.py" in render
  88.             output = self.filter_expression.resolve(context)
File "C:\Python27\lib\site-packages\django\template\base.py" in resolve
  585.                 obj = self.var.resolve(context)
File "C:\Python27\lib\site-packages\django\template\base.py" in resolve
  735.             value = self._resolve_lookup(context)
File "C:\Python27\lib\site-packages\django\template\base.py" in _resolve_lookup
  789.                             current = current()
File "C:\Python27\lib\site-packages\django\forms\forms.py" in as_p
  241.             errors_on_separate_row = True)
File "C:\Python27\lib\site-packages\django\forms\forms.py" in _html_output
  156.             if bf.is_hidden:
File "C:\Python27\lib\site-packages\django\forms\forms.py" in is_hidden
  560.         return self.field.widget.is_hidden

Exception Type: AttributeError at /NewCall/
Exception Value: 'ChoiceField' object has no attribute 'is_hidden'

是什么导致'AttributeError'？怎么修好？谢谢.