$meta

The following aggregation operation performs a text search and uses the $meta operator to group by the text search score:以下聚合操作执行文本搜索，并使用$meta运算符按文本搜索得分进行分组：

db.articles.aggregate(
   [
     { $match: { $text: { $search: "cake" } } },
     { $group: { _id: { $meta: "textScore" }, count: { $sum: 1 } } }
   ]
)

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

{ "_id" : 0.75, "count" : 1 }
{ "_id" : 0.6666666666666666, "count" : 1 }
{ "_id" : 1, "count" : 2 }

For more examples, see Text Search in the Aggregation Pipeline.有关更多示例，请参阅聚合管道中的文本搜索。

The following query performs a text search for the term cake and uses the $meta operator in the projection document to include the score assigned to each matching document:以下查询对术语cake执行文本搜索，并在投影文档中使用$meta运算符来包括分配给每个匹配文档的分数：

db.articles.find(
   { $text: { $search: "cake" } },
   { score: { $meta: "textScore" } }
)

The operation returns the following documents with the text score:该操作返回以下带有文本分数的文档：

{ "_id" : 4, "title" : "some cakes", "score" : 1 }
{ "_id" : 1, "title" : "cakes and ale", "score" : 0.75 }
{ "_id" : 5, "title" : "two cakes to go", "score" : 0.6666666666666666 }
{ "_id" : 2, "title" : "more cakes", "score" : 1 }

For additional examples of "textScore" projections and sorts, see Text Search Score Examples.

The following aggregation operation finds all documents with type equal to apparel and uses the $meta operator to include the index key value for the matching document if an index was used:以下聚合操作查找type等于apparel（服装）的所有文档，如果使用了索引，则使用$meta运算符来包括匹配文档的索引键值：

db.orders.aggregate(
   [
     { $match: { type: "apparel" } },
     { $addFields: { idxKey: { $meta: "indexKey" } } }
   ]
)

The following operation finds all documents with type equal to apparel and uses the $meta operator to include the index key value for the matching document if an index was used:以下操作查找type等于apparel（服装）的所有文档，如果使用了索引，则使用$meta运算符来包括匹配文档的索引键值：

db.orders.find( { type: "apparel" }, { idxKey: { $meta: "indexKey" } } )

For example, the following operation does not use an index since no index exists on the price field to support the match condition:例如，以下操作不使用索引，因为price字段上不存在支持匹配条件的索引：

db.orders.aggregate(
   [
     { $match: { price: { $gte: NumberDecimal("10") } } },
     { $addFields: { idxKey: { $meta: "indexKey" } } }
   ]
)

For example, the following operation does not use an index since no index exists on the price field to support the match condition:例如，以下操作不使用索引，因为price字段上不存在支持匹配条件的索引：

db.orders.find(
   { price: { $gte: NumberDecimal("10") } },
   { idxKey: { $meta: "indexKey" } }
)