英文:
Selectively retrieving depending on existence of key in map
问题
在mongodb中，是否可以根据映射中键的存在性有选择地检索数据？如果可以，如何操作？
假设我有一个文档，例如：
{ " _id "：1234，" parentfield1 "：{" childfield1 "：{ ...}，" childfield2 "：{ ...}，" childfield5 "：{ ...}，//可能有很多childfields.. > 50}，}
我如何能够有选择地从文档中检索出一些特定的childfields，给出多个选择的选项？其中一些可能在文档中不存在。
即
输入 " childfield1 "，" childfield2 "，" childfield3 "-> 输出{ " _id "：1234，" parentfield1 "：{" childfield1 "：{ ... }，" childfield2 "：{ ... }，}，}
这是否可行？是否还可以高效地实现？任何帮助都将是很好的（使用python、go）。
英文:
Is it possible to selectively retrieve depending on the existence of keys in a map in mongodb? And if so, how do you go about doing it?
Suppose I have a document that looks like this for example..
{ "_id": 1234,"parentfield1" : {"childfield1" : { ...},"childfield2" : { ...},"childfield5" : { ...}, // There might be many childfields.. > 50},}
How would I be able to selectively retrieve from the document a/some particular childfields given multiple options to choose from? Some of which may not exist in the document.
i.e.
input "childfield1", "childfield2", "childfield3"-> output{ "_id": 1234,"parentfield1": {"childfield1" : { ... },"childfield2" : { ... },},}
Is it even doable? Is it possible to do efficiently also?
Any help would be great (python, go).
答案1
得分: 0
是的，这就是find的projection参数的目的：
<!-- language: lang-js -->
db.collection.find({_id: 1234}, {'parentfield1.childfield1': 1,'parentfield1.childfield2': 1,'parentfield1.childfield3': 1});
如果指定的字段在给定的文档中不存在，其他匹配的字段仍然会被包含在内。
如果你希望projection参数对象是动态的，可以通过编程的方式构建它。
英文:
Yes, that's the purpose of the projection parameter of find:
<!-- language: lang-js -->
db.collection.find({_id: 1234}, {'parentfield1.childfield1': 1,'parentfield1.childfield2': 1,'parentfield1.childfield3': 1});
If a specified field isn't present in a given doc, the other matching fields will still be included.
Build up the projection parameter object programmatically if you want it to be dynamic.
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