使用python将数据从mongodb导出到csv

所以...问题是csv当mongo返回时，作者不理解“ subdictionaries”的概念。

如果我理解正确，当您查询Mongo时，您会得到如下字典：

{
   "_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尝试编写该字典时，它只写了一部字典（最上面的字典）。它不知道（也不在乎）answers列表中是否包含字典，这些字典的值也需要写在列中（使用点表示法访问字典中的字段，例如，answers.order仅Mongo可以理解，而不是csv编写器可以理解）

我了解您应该做的是“遍历”答案列表，并从该列表中的每个记录（每个字典）中创建一个字典。获得“拼合”字典的列表后，您可以传递这些字典并将其写入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)

ch用复数形式。answers_record与...不同answer_record

创建一个像这样的文件：

$ 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（产生的查询cursor = db.questions.find ({},{'_id':1, 'answers.order':1, 'answers.text':1, 'answers.answerId':1})）将返回questions集合中的所有条目。如果此集合非常大，则可能要使用cursor作为迭代器。

您可能已经意识到，for上面我的代码中的第一个循环将所有记录放入一个列表（flattened_records列表）中。您可以通过迭代来进行延迟加载cursor（而不是加载内存中的所有项目，获取一个，对其进行处理，获取下一个，对其进行处理...）。

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)

它将产生与上述相同的.csv文件。