如何与“ project”在MongoDB查询中

Question

这是我数据的一个非常简单的示例：

[
  { _id: 1,
    title: 'other item',
    category: 'show'
  },
  { _id: 2,
    title: 'THE ITEM THAT MUST BE RETURNED FIRST',
    category: 'show'
  },
  { _id: 3,
    title: 'other item 2',
    category: 'show'
  },
  { _id: 4,
    title: 'item not matching query',
    category: 'hide'
  }
]

我正在尝试将ID与顶部的特定数字进行优先排序，并在MongoDB中尝试了相同的IT：

itemsCollection.aggregate([
{
  "$project": {
    "_id": 1,
    "category": 1,
    "title": 1,
    "priority": {
      "$eq": [
        "$_id",
        2
      ]
    }
  }
},
{
  "$sort": {
    "priority": -1
  }
}
])

我该如何在C＃中使用centregate在C＃中尝试相同的方法：

var aggregation = await collection.Aggregate()
        .Match(filterDefinition)
        .Facet(countFacet, dataFacet)
        .ToListAsync();

Answer 1

您需要一个投影和sort。有时，很难在C＃语法中写入，但是您可以使用bsondocument允许使用mongoDB查询语法。

ProjectionDefinition<BsonDocument> projection = new BsonDocument
{
    { "category", 1 },
    { "title", 1 },
    { "priority",
        new BsonDocument(
            "$eq", new BsonArray
            {
                "$_id",
                2
            }
        )
    }
};
SortDefinition<BsonDocument> sort = Builders<BsonDocument>.Sort.Descending("priority");

var result = await collection.Aggregate()
    .Match(filterDefinition)
    .Project(projection)
    .Sort(sort)
    .ToListAsync();

---

如果要返回具体类型的列表，例如list＆lt; item＆gt;，

public class Item
{
    [BsonId]
    public int Id { get; set; }
    [BsonElement("category")]
    public string Category { get; set; }
    [BsonElement("title")]
    public string Title { get; set; }
    [BsonElement("priority")]
    public bool Priority { get; set; }
}

var collection = _db.GetCollection<Item>("item");

ProjectionDefinition<Item> projection = new BsonDocument
{
    { "category", 1 },
    { "title", 1 },
    { "priority",
        new BsonDocument(
            "$eq", new BsonArray
            {
                "$_id",
                2
            }
        )
    }
};
SortDefinition<Item> sort = Builders<Item>.Sort.Descending("priority");

var result = await collection.Aggregate()
    .Match(filterDefinition)
    .Project<Item>(projection)
    .Sort(sort)
    .ToListAsync();

输出