当数组嵌套在另一个数组中时,如何计算特定值的数量?例如,我想在下面的文档中计算“answers”的数量。查询应返回有2个苹果和1个香蕉。
{
"_id" : ObjectId("52c1d909fc7fc68ddd999a73"),
"name" : "Some survey",
"questions" : [
{
"_id" : ObjectId("52c1e250fc7fc68ddd999a75"),
"answers" :
[
{
"userId" : "some GUIDs",
"answer" : "apple"
},
{
"userId" : "some GUID",
"answer" : "apple"
},
{
"userId" : "some GUID",
"answer" : "banana"
}
],
"questionText" : "blah blah blah...",
"questionType" : "multiple choice"
}
]
}