所以。。。问题是csv编写器不理解mongo返回的“子字典”的概念。

如果我理解正确，当你查询Mongo时，你会得到这样一本字典：{

"_id": "a hex ID that correspond with the record that contains several answers",

"answers": [ ... a list with a bunch of dicts in it... ]

}

因此，当csv.DictWriter试图编写它时，它只编写了一个字典(最上面的)。它不知道(也不关心)answers是一个包含字典的列表，这些字典的值也需要写在列中(使用诸如answers.order这样的点符号访问字典中的字段只被Mongo理解，而不是csv编写器)

我知道你应该做的是“浏览”答案列表，并从列表中的每个记录(每个字典)中创建一个字典。一旦您有了一个“platten”字典列表，您就可以传递这些字典并将它们写入您的csv文件中：cursor = client.stack_overflow.stack_039.find(

{}, {'_id': 1, 'answers.order': 1, 'answers.text': 1, 'answers.answerId': 1})

# Step 1: Create the list of dictionaries (one dictionary per entry in the `answers` list)

flattened_records = []

for answers_record in cursor:

answers_record_id = answers_record['_id']

for answer_record in answers_record['answers']:

flattened_record = {

'_id': answers_record_id,

'answers.order': answer_record['order'],

'answers.text': answer_record['text'],

'answers.answerId': answer_record['answerId']

}

flattened_records.append(flattened_record)

# Step 2: Iterate through the list of flattened records and write them to the csv file

with open('stack_039.csv', 'w') as outfile:

fields = ['_id', 'answers.order', 'answers.text', 'answers.answerId']

write = csv.DictWriter(outfile, fieldnames=fields)

write.writeheader()

for flattened_record in flattened_records:

write.writerow(flattened_record)

复数的用法。answers_record与answer_record不同

创建如下文件：$ cat ./stack_039.csv

_id,answers.order,answers.text,answers.answerId

580f9aa82de54705a2520833,0,{u'en': u'Yes'},527d65de7563dd0fb98fa28c

580f9aa82de54705a2520833,1,{u'en': u'No'},527d65de7563dd0fb98fa28b

编辑：

您的查询(生成cursor = db.questions.find ({},{'_id':1, 'answers.order':1, 'answers.text':1, 'answers.answerId':1})的查询)将返回questions集合中的所有条目。如果此集合非常大，则可能需要将cursor用作iterator。

正如您可能已经意识到的，上面代码中的第一个for循环将所有记录放在一个列表中(即flattened_records列表)。您可以通过遍历cursor来执行延迟加载(而不是加载内存中的所有项，取回一个项，对它执行某些操作，获取下一个项，对它执行某些操作…)。

速度稍慢，但内存效率更高。cursor = client.stack_overflow.stack_039.find(

{}, {'_id': 1, 'answers.order': 1, 'answers.text': 1, 'answers.answerId': 1})

with open('stack_039.csv', 'w') as outfile:

fields = ['_id', 'answers.order', 'answers.text', 'answers.answerId']

write = csv.DictWriter(outfile, fieldnames=fields)

write.writeheader()

for answers_record in cursor: # Here we are using 'cursor' as an iterator

answers_record_id = answers_record['_id']

for answer_record in answers_record['answers']:

flattened_record = {

'_id': answers_record_id,

'answers.order': answer_record['order'],

'answers.text': answer_record['text'],

'answers.answerId': answer_record['answerId']

}

write.writerow(flattened_record)

它将生成与上面所示相同的.csv文件。