$size

Compatibility兼容性

You can use $size for deployments hosted in the following environments:您可以将$size用于在以下环境中托管的部署：

• MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud：云中MongoDB部署的完全托管服务

• MongoDB Enterprise: The subscription-based, self-managed version of MongoDB：MongoDB的基于订阅的自我管理版本
• MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB：MongoDB的源代码可用、免费使用和自我管理版本

Consider the following examples:考虑以下示例：

db.collection.find( { field: { $size: 2 } } );

This query returns all documents in collection where field is an array with 2 elements. 此查询返回collection集合中的所有文档，其中field字段是一个包含2个元素的数组。For instance, the above expression will return { field: [ red, green ] } and { field: [ apple, lime ] } but not { field: fruit } or { field: [ orange, lemon, grapefruit ] }. 例如，上述表达式将返回{ field: [ red, green ] }和{ field: [ apple, lime ] }，但不会返回{ field: fruit }或{ field: [ orange, lemon, grapefruit ] }。To match fields with only one element within an array use $size with a value of 1, as follows:要匹配数组中只有一个元素的字段，请使用值为1的$size，如下所示：

db.collection.find( { field: { $size: 1 } } );

$size does not accept ranges of values. To select documents based on fields with different numbers of elements, create a counter field that you increment when you add elements to a field.不接受值的范围。要根据具有不同元素数量的字段选择文档，请创建一个计数器字段，在向字段添加元素时递增该字段。

Queries cannot use indexes for the $size portion of a query, although the other portions of a query can use indexes if applicable.查询不能对查询的$size部分使用索引，但查询的其他部分可以在适用的情况下使用索引。

Syntax语法

A $size expression has the following syntax:$size表达式具有以下语法：

{
   <field>: {
      $size: <number>
   }
}

Additional Examples其他示例

For additional examples on querying arrays, see:有关查询数组的其他示例，请参阅：

• Query an Array查询数组
• Query an Array of Embedded Documents查询嵌入式文档数组

For additional examples on querying, see Query Documents.有关查询的其他示例，请参阅查询文档。