如果子文档 ID 已知，如何获取文档答案

【问题标题】：how to fetch the document if subdocument id is known如果子文档 ID 已知，如何获取文档
【发布时间】：2021-07-13 09:44:13
【问题描述】：

鉴于我们有子文档 id，我想获取文档。假设我们有多个类别，每个类别都有子类别和服务类型，所以如果我有子类别 id，那么我们如何同样获取子类别如果我拥有一种服务类型的 id，那么我该如何获取服务类型。

[
  {
    "_id": {
      "$oid": "60e958b8f907544930c7d045"
    },
    "trans": {
      "en": {
        "name": "Digital Marketing",
        "description": "Digital Marketing"
      },
      "fr": {
        "name": "Digital Marketing",
        "description": "Digital Marketing"
      }
    },
    "subcategories": [
      {
        "trans": {
          "en": {
            "name": "Social Media Advertising",
            "description": "Social Media Advertising"
          }
        },
        "$oid": "60e95a466f25a34248698a33",
        "service_type": [
          {
            "trans": {
              "en": {
                "name": "_service1",
                "description": "_service1"
              }
            },
            "_id": {
              "$oid": "60e95a466f25a34248698a30"
            }
          },
          {
            "trans": {
              "en": {
                "name": "Strategy and Planning",
                "description": "Strategy and Planning"
              }
            },
            "_id": {
              "$oid": "60e95a466f25a34248698a31"
            }
          },
          {
            "trans": {
              "en": {
                "name": "AD SETUP AND MANAGEMENT",
                "description": "AD SETUP AND MANAGEMENT"
              }
            },
            "_id": {
              "$oid": "60e95a466f25a34248698a32"
            }
          }
        ]
      },
      {
        "trans": {
          "en": {
            "name": "Social Media Marketing",
            "description": "Social Media Marketing"
          }
        },
        "$oid": "60e95a466f25a34248698a33",
        "service_type": [
          {
            "trans": {
              "en": {
                "name": "_serviceA",
                "description": "_serviceA"
              }
            },
            "_id": {
              "$oid": "60e95a466f25a34248678a78"
            }
          },
          {
            "trans": {
              "en": {
                "name": "Strategy 1",
                "description": "Strategy 1"
              }
            },
            "_id": {
              "$oid": "60e95a466f25a34248222a31"
            }
          },
          {
            "trans": {
              "en": {
                "name": "AD 2",
                "description": "AD 2"
              }
            },
            "_id": {
              "$oid": "51e85a466f25a34345498a32"
            }
          }
        ]
      }
    ]
  }
]
//similarly  we are having multiple categories then how can we fetch the subcategory with given `subcategory_id` where the subcategory is an array of object

查询已尝试

 const r = await client
      .collection("categories")
      .aggregate([
        {
          $project: {
            categories: 0,
            subcategories: {
              $filter: {
                input: "$subcategories",
                as: "subcategory",
                cond: {
                  if: {
                    $eq: ["$$subcategory._id", "60e51a116678530e84ee2e86"],
                  },
                  then: "$$subcategory",
                  else: "not found",
                },
              },
            },
          },
        },
      ])
      .toArray();
    console.log("answer", r);

    return r;

预期输出

如果我有任何一个子类别的 id 让60e95a466f25a34248698a33 那么如果我们不知道类别 id，我如何获取该子类别？

"subcategories":[
  {
    "trans": {
      "en": {
        "name": "Social Media Advertising",
        "description": "Social Media Advertising"
      }
    },
    "$oid": "60e95a466f25a34248698a33",
    "service_type": [
      {
        "trans": {
          "en": {
            "name": "_service1",
            "description": "_service1"
          }
        },
        "_id": {
          "$oid": "60e95a466f25a34248698a30"
        }
      },
      {
        "trans": {
          "en": {
            "name": "Strategy and Planning",
            "description": "Strategy and Planning"
          }
        },
        "_id": {
          "$oid": "60e95a466f25a34248698a31"
        }
      },
      {
        "trans": {
          "en": {
            "name": "AD SETUP AND MANAGEMENT",
            "description": "AD SETUP AND MANAGEMENT"
          }
        },
        "_id": {
          "$oid": "60e95a466f25a34248698a32"
        }
      }
    ]
  }
]

【问题讨论】：

标签： mongodb nosql aggregation-framework aggregate nosql-aggregation

【解决方案1】：

该查询存在 2 个问题：

您不能在同一投影中同时包含和排除字段，除非在包含投影中排除 _id。
$filter 内的 cond 表达式应计算为布尔值

$eq 运算符本身的计算结果为布尔值，因此不需要额外的 if。

要仅返回子类别字段，而不返回 _id 或任何类别字段，请使用：

.aggregate([
        {
          $project: {
            _id: 0,
            subcategories: {
              $filter: {
                input: "$subcategories",
                as: "subcategory",
                cond: {
                    $eq: ["$$subcategory._id", "60e51a116678530e84ee2e86"],
                },
              },
            },
          },
        },
      ])

【讨论】：