Collation

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• 文档结构
• 支持核对的操作
• 表现

3.4 版本中的新功能.

核对 (Collation) 允许用户来指定一个特定语言规则的字符串比较, 比如小写字母和口音标记的规则。

你可以为集合、视图或者索引指定核对，也可以给特定的支持核对操作的方法来指定核对。

文档结构

一个核对的文档结构有以下一些字段:

{
   locale: <string>,
   caseLevel: <boolean>,
   caseFirst: <string>,
   strength: <int>,
   numericOrdering: <boolean>,
   alternate: <string>,
   maxVariable: <string>,
   backwards: <boolean>
}

当指定了核对, locale 字段就是强制要带上的; 所有其他的字段都是可选的。关于这些字段的描述，参见 Collation Document。

默认的核对规则参数根据你指定的 locale 字段而不同。关于完整的默认核对参数列表以及它们相关的 locale，参见 Collation Default Parameters.

支持核对的操作

You can specify collation for the following operations:

NOTE

You cannot specify multiple collations for an operation. For example, you cannot specify different collations per field, or if performing a find with a sort, you cannot use one collation for the find and another for the sort.

表现

Local Variants

Some collation locales have variants, which employ special language-specific rules. To specify a locale variant, use the following syntax:

{ "locale" : "<locale code>@collation=<variant>" }

For example, to use the pinyin variant of the Chinese collation:

{ "locale" : "zh@collation=pinyin" }

For a complete list of all collation locales and their variants, seeCollation Locales.

Collation and Views

• You can specify a default collation for a view at creation time. If no collation is specified, the view’s default collation is the “simple” binary comparison collator. That is, the view does not inherit the collection’s default collation.
• String comparisons on the view use the view’s default collation. An operation that attempts to change or override a view’s default collation will fail with an error.
• If creating a view from another view, you cannot specify a collation that differs from the source view’s collation.
• If performing an aggregation that involves multiple views, such as with $lookup or $graphLookup , the views must have the same collation .

Collation and Index Use

To use an index for string comparisons, an operation must also specify the same collation. That is, an index with a collation cannot support an operation that performs string comparisons on the indexed fields if the operation specifies a different collation.

For example, the collection myColl has an index on a string field category with the collation locale"fr".

db.myColl.createIndex( { category: 1 }, { collation: { locale: "fr" } } )

The following query operation, which specifies the same collation as the index, can use the index:

db.myColl.find( { category: "cafe" } ).collation( { locale: "fr" } )

However, the following query operation, which by default uses the “simple” binary collator, cannot use the index:

db.myColl.find( { category: "cafe" } )

For a compound index where the index prefix keys are not strings, arrays, and embedded documents, an operation that specifies a different collation can still use the index to support comparisons on the index prefix keys.

For example, the collection myColl has a compound index on the numeric fields score and price and the string fieldcategory; the index is created with the collation locale"fr"for string comparisons:

db.myColl.createIndex(
   { score: 1, price: 1, category: 1 },
   { collation: { locale: "fr" } } )

The following operations, which use"simple"binary collation for string comparisons, can use the index:

db.myColl.find( { score: 5 } ).sort( { price: 1 } )
db.myColl.find( { score: 5, price: { $gt: NumberDecimal( "10" ) } } ).sort( { price: 1 } )

The following operation, which uses"simple"binary collation for string comparisons on the indexed category field, can use the index to fulfill only thescore:5portion of the query:

db.myColl.find( { score: 5, category: "cafe" } )