如何使用C#检查特定文档Mongodb中是否存在字段? [英] how to check if a field exists in a specific document Mongodb using C#?

问题描述

我有一个带有 asp.Net/C# 的 WebAPI，我正在使用 Mongodb.在更新特定文档之前，我需要检查文档中是否存在某个字段，如果不存在，则将该字段添加到文档中.但是我不知道如何检查文档中是否存在字段.要添加我使用此代码的字段:

I have a WebAPI with asp.Net/C# and I'm using Mongodb . Before updating a specific document, I need to check if a field exists in the document or not and if not add the filed to the document. But I don't know how I can check the existence of a field in a document. To add the field I'm using this code:

var update = Bundle.Update.Set(b => b.followers, new List<User>());
int res = Bundle.UpdateOne(Bundle.Filter.Eq(b => b._id, id), update);

提前致谢.

我尝试使用类似的东西，但它返回空值！！

I tried to use something like this but it returns null!!

var builder = Builders<BsonDocument>.Filter;               
var filter = builder.Exists("followers", false).ToBsonDocument();
var RetrievedData = Bundle.Collection().Find(filter).ToList();

推荐答案

您可以尝试以下操作:

1. 按如下方式使用 Try/Catch:

1. Use Try/Catch as follows:

var document = Bundle.Collection().Find(filter); // here is your BsonDocument
try
   {
      document["fieldNameToCheck"] // if field doesn`t exist it throws KeyNotFoundException. If there are nested objects just follow the pattern: document["fieldName"]["fieldNestedToCheck"]
   }
catch (Exception ex) when (ex is KeyNotFoundException)
   {
      // your logic for "the field wasn`t found in the document" case
   } 

使用.Contains()，如下:

Use .Contains(), as follows:

var exists = document.Contains("fieldNameToCheck");// if field exists it returns true
// If you need to check the nested fields, you can do as follows:
var nestedExists = document["fieldName"].ToBsonDocument().Contains("fieldNameToCheck"); // or:
var nestedExists = document["fieldName"]["nestedFieldNameNextLevel"].ToBsonDocument().Contains("fieldNameToCheck");  // and so on...      

并且通过使用 TryGetElement，您还可以获得此元素:

And by using TryGetElement you can additionally get this element:

BsonElement element; // it will contain found element if true for next line
var exists =  document.TryGetElement("fieldNameToCheck", out element); // returns true if element is found

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