Django-ModelFrom中修改save后的字段值

在modelform提交中，保持原未修改字段的值，views中部分代码：

project = iredmail.objects.get(id=id)
ssh_crt_name = project.ssh_crt
if request.method == 'post':
    file_obj = request.files.get("ssh_crt", "")
    i_form = iredmailform(request.post,request.files,instance=project)
    if i_form.is_valid():
        obj = i_form.save(commit=false)
        if file_obj == "":
            obj.ssh_crt = ssh_crt_name
        obj.save()