$bsonSize (expression operator)（表达式运算符）

Return Sizes of Documents文件返回尺寸

In mongosh, create a sample collection named employees with the following documents:在mongosh中，使用以下文档创建一个名为employees的示例集合：

 db.employees.insertMany([
   {
     "_id": 1,
     "name": "Alice", "email": "alice@company.com", "position": "Software Developer",
     "current_task": {
       "project_id": 1,
       "project_name": "Aggregation Improvements",
       "project_duration": 5,
       "hours": 20
     }
   },
   {
     "_id": 2,
     "name": "Bob", "email": "bob@company.com", "position": "Sales",
     "current_task": {
       "project_id": 2,
       "project_name": "Write Blog Posts",
       "project_duration": 2,
       "hours": 10,
       "notes": "Progress is slow. Waiting for feedback."
     }
   },
   {
     "_id": 3,
     "name": "Charlie", "email": "charlie@company.com", "position": "HR (On Leave)",
     "current_task": null
   },
   {
     "_id": 4,
     "name": "Dianne", "email": "diane@company.com", "position": "Web Designer",
     "current_task": {
       "project_id": 3,
       "project_name": "Update Home Page",
       "notes": "Need to scope this project."
     }
   }
]);

The following aggregation projects:以下聚合projects：

• The name fieldname字段
• The object_size field, which uses $bsonSize to return the size of the document in bytes. object_size字段，它使用$bsonSize返回文档的大小（以字节为单位）。The $$ROOT variable references the document currently being processed by the pipeline. $$ROOT变量引用管道当前正在处理的文档。To learn more about variables in the aggregation pipeline, see Variables in Aggregation Expressions.要了解有关聚合管道中变量的更多信息，请参阅聚合表达式中的变量。

db.employees.aggregate([
  {
    "$project": {
      "name": 1,
      "object_size": { $bsonSize: "$$ROOT" }
    }
  }
])

The operation returns the following result:该操作返回以下结果：

{ "_id" : 1, "name" : "Alice", "object_size" : 222 }
{ "_id" : 2, "name" : "Bob", "object_size" : 248 }
{ "_id" : 3, "name" : "Charlie", "object_size" : 112 }
{ "_id" : 4, "name" : "Dianne", "object_size" : 207 }

Return Combined Size of All Documents in a Collection返回集合中所有文档的组合大小

The following pipeline returns the combined size of all of the documents in the employees collection:以下管道返回employees集合中所有文档的组合大小：

db.employees.aggregate([
  {
    "$group": {
      "_id": null,
      "combined_object_size": { $sum: { $bsonSize: "$$ROOT" } }
    }
  }
])

When you specify an $group _id value of null, or any other constant value, the $group stage calculates accumulated values for all the input documents as a whole.当您将$group _id值指定为null或任何其他常数值时，$group阶段会计算所有输入文档的累积值。

The operation uses the $sum operator to calculate the combined $bsonSize of each document in the collection. 该操作使用$sum运算符计算集合中每个文档的组合$bsonSize。The $$ROOT variable references the document currently being processed by the pipeline. To learn more about variables in the aggregation pipeline, see Variables in Aggregation Expressions.$$ROOT变量引用管道当前正在处理的文档。要了解有关聚合管道中变量的更多信息，请参阅聚合表达式中的变量。

The operation returns the following result:该操作返回以下结果：

{ "_id" : null, "combined_object_size" : 789 }

Return Document with Largest Specified Field返回指定字段最大的文档

The following pipeline returns the document with the largest current_task field in bytes:以下管道返回具有以字节为单位的最大current_task字段的文档：

db.employees.aggregate([
   // First Stage
   { $project: { name: "$name", task_object_size: { $bsonSize: "$current_task" } }  },
   // Second Stage
   { $sort: { "task_object_size" : -1 } },
   // Third Stage
   { $limit: 1 }
])

First Stage第一阶段

The first stage of the pipeline projects:管道projects的第一阶段：

• The name field
• The task_object_size field, which uses $bsonSize to return the size of the document's current_task field in bytes.task_object_size字段，它使用$bsonSize返回文档的current_task字段的大小（以字节为单位）。

This stage outputs the following documents to the next stage:此阶段将以下文件输出到下一阶段：

{ "_id" : 1, "name" : "Alice", "task_object_size" : 109 }
{ "_id" : 2, "name" : "Bob", "task_object_size" : 152 }
{ "_id" : 3, "name" : "Charlie", "task_object_size" : null }
{ "_id" : 4, "name" : "Dianne", "task_object_size" : 99 }

Second Stage第二阶段

The second stage sorts the documents by task_object_size in descending order.第二阶段按task_object_size降序对文档进行排序。

This stage outputs the following documents to the next stage:此阶段将以下文件输出到下一阶段：

{ "_id" : 2, "name" : "Bob", "task_object_size" : 152 }
{ "_id" : 1, "name" : "Alice", "task_object_size" : 109 }
{ "_id" : 4, "name" : "Dianne", "task_object_size" : 99 }
{ "_id" : 3, "name" : "Charlie", "task_object_size" : null }

Third Stage第三阶段

The third stage limits the output documents to only return the document appearing first in the sort order:第三阶段将输出文档限制为仅返回排序顺序中首先出现的文档：

{ "_id" : 2, "name" : "Bob", "task_object_size" : 152 }