仅检索 MongoDB 集合中对象数组中的查询元素 [英] Retrieve only the queried element in an object array in MongoDB collection

问题描述

假设您在我的收藏中有以下文档:

Suppose you have the following documents in my collection:

{  
   "_id":ObjectId("562e7c594c12942f08fe4192"),
   "shapes":[  
      {  
         "shape":"square",
         "color":"blue"
      },
      {  
         "shape":"circle",
         "color":"red"
      }
   ]
},
{  
   "_id":ObjectId("562e7c594c12942f08fe4193"),
   "shapes":[  
      {  
         "shape":"square",
         "color":"black"
      },
      {  
         "shape":"circle",
         "color":"green"
      }
   ]
}

<小时>

进行查询:

---

Do query:

db.test.find({"shapes.color": "red"}, {"shapes.color": 1})

或

db.test.find({shapes: {"$elemMatch": {color: "red"}}}, {"shapes.color": 1})

<小时>

返回匹配的文档 (Document 1)，但总是包含 shapes 中的所有数组项:

---

Returns matched document (Document 1), but always with ALL array items in shapes:

{ "shapes": 
  [
    {"shape": "square", "color": "blue"},
    {"shape": "circle", "color": "red"}
  ] 
}

但是，我只想使用包含 color=red 的数组获取文档 (Document 1):

However, I'd like to get the document (Document 1) only with the array that contains color=red:

{ "shapes": 
  [
    {"shape": "circle", "color": "red"}
  ] 
}

我该怎么做?

推荐答案

MongoDB 2.2 的新 $elemMatch 投影运算符提供了另一种方法来改变返回的文档以仅包含 first 匹配的 shapes 元素:

MongoDB 2.2's new $elemMatch projection operator provides another way to alter the returned document to contain only the first matched shapes element:

db.test.find(
    {"shapes.color": "red"}, 
    {_id: 0, shapes: {$elemMatch: {color: "red"}}});

返回:

{"shapes" : [{"shape": "circle", "color": "red"}]}

在 2.2 中，您也可以使用 $ 投影运算符，其中投影对象字段名称中的 $ 表示该字段的第一个匹配数组元素的索引.以下返回与上述相同的结果:

In 2.2 you can also do this using the $ projection operator, where the $ in a projection object field name represents the index of the field's first matching array element from the query. The following returns the same results as above:

db.test.find({"shapes.color": "red"}, {_id: 0, 'shapes.$': 1});

MongoDB 3.2 更新

从 3.2 版本开始，您可以使用新的 $filter 聚合运算符用于在投影期间过滤数组，其好处是包括所有匹配项，而不仅仅是第一个.

Starting with the 3.2 release, you can use the new $filter aggregation operator to filter an array during projection, which has the benefit of including all matches, instead of just the first one.

db.test.aggregate([
    // Get just the docs that contain a shapes element where color is 'red'
    {$match: {'shapes.color': 'red'}},
    {$project: {
        shapes: {$filter: {
            input: '$shapes',
            as: 'shape',
            cond: {$eq: ['$$shape.color', 'red']}
        }},
        _id: 0
    }}
])

结果:

[ 
    {
        "shapes" : [ 
            {
                "shape" : "circle",
                "color" : "red"
            }
        ]
    }
]

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