$meta (aggregation)¶

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Definition
Behavior
Examples

Definition¶

$meta¶

2.6 新版功能.

Returns the metadata associated with a document in a pipeline operations, e.g. "textScore" when performing text search.

A $meta expression has the following syntax:

{ $meta: <metaDataKeyword> }

The $meta expression can specify the following keyword as the <metaDataKeyword>:

Keyword	Description	Sort Order
`"textScore"`	Returns the score associated with the corresponding `$text` query for each matching document. The text score signifies how well the document matched the search term or terms. If not used in conjunction with a `$text` query, returns a score of null.	Descending

Behavior¶

The { $meta: "textScore" } expression is the only expression that the $sort stage accepts.

Although available for use everywhere expressions are accepted in the pipeline, the { $meta: "textScore" } expression is only meaningful in a pipeline that includes a $match stage with a $text query.

Views do not support text search.

Examples¶

Consider an articles collection with the following documents:

{ "_id" : 1, "title" : "cakes and ale" }
{ "_id" : 2, "title" : "more cakes" }
{ "_id" : 3, "title" : "bread" }
{ "_id" : 4, "title" : "some cakes" }

The following aggregation operation performs a text search and use the $meta operator to group by the text search score:

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

The operation returns the following results:

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

For more examples, see 在聚合管道中使用文本搜索.

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