Python操作SQL表上传文档时如何彻底移除多余未使用字段

现有存储业务数据的SQL表，当前上传2份文档时，数据会写入doc0、doc1字段，其余未上传的doc字段会被赋值为null保留在记录中。期望实现逻辑：仅上传N份文档时，其余未使用的doc字段需要从对应SQL记录中彻底移除，不保留值为null的字段。

def submit_quality_dept_application(request, application_id):
n = int(request.data['length'])
application = Application.objects.get(id=application_id)
application_state = application.application_state
teaching_feedback = request.FILES['teaching-feedback']
application_state['teaching_feedback'] = teaching_feedback.name
now = datetime.now()
dt_string = now.strftime("%Y-%m-%d %H:%M:%S")
application_state['edited_time'] = dt_string
for i in range(5):
    application_state[f'doc{i}'] = None
for i in range(n):
    doc = request.FILES[f'doc{i}']
    application_state[f'doc{i}'] = doc.name
    copy_to_application_directory(doc, application.id)
copy_to_application_directory(teaching_feedback, application.id)
ApplicationStep.objects.update_or_create(
    application=application, step_name=Step.STEP_7
)
Application.objects.filter(id=application_id).update(application_state=application_state)
return Response(n, status=status.HTTP_200_OK)

问题出在原有代码的字段赋值逻辑：先循环把doc0到doc4共5个字段全部赋值为None，再给前n个有实际上传的doc字段覆盖为文件名，最终剩下的5-n个字段就会以null值的形式留在存储结构中，写入数据库后就会保留这些null字段。

调整逻辑即可解决：不要先给所有doc字段赋值None，而是先把application_state中所有已存在的doc类字段全部删除，再仅为实际上传的n个doc字段赋值文件名，最终存储结构里就只会保留有实际值的doc字段，不会残留多余的null字段。

修改后的代码如下：

def submit_quality_dept_application(request, application_id):
    n = int(request.data['length'])
    application = Application.objects.get(id=application_id)
    application_state = application.application_state
    teaching_feedback = request.FILES['teaching-feedback']
    application_state['teaching_feedback'] = teaching_feedback.name
    now = datetime.now()
    dt_string = now.strftime("%Y-%m-%d %H:%M:%S")
    application_state['edited_time'] = dt_string
    # 清除所有历史doc字段，避免残留旧值或null字段
    for i in range(5):
        doc_key = f'doc{i}'
        if doc_key in application_state:
            del application_state[doc_key]
    # 仅为实际上传的文档写入对应字段
    for i in range(n):
        doc = request.FILES[f'doc{i}']
        application_state[f'doc{i}'] = doc.name
        copy_to_application_directory(doc, application.id)
    copy_to_application_directory(teaching_feedback, application.id)
    ApplicationStep.objects.update_or_create(
        application=application, step_name=Step.STEP_7
    )
    Application.objects.filter(id=application_id).update(application_state=application_state)
    return Response(n, status=status.HTTP_200_OK)

注：该方案适配Django框架JSONField类型的application_state字段，删除无对应上传文件的doc键后，存入数据库的JSON结构将不会保留这些冗余字段。如果后续文档上传上限调整，只需修改循环中range(5)的参数为对应最大值即可。

内容的提问来源于stack exchange，提问作者Yarden Yosef