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Collation

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  • Collation Document
  • Operations that Support Collation
  • Behavior
  • Restrictions

Collation allows users to specify language-specific rules for string comparison, such as rules for lettercase and accent marks.

You can specify collation for a collection or a view, an index, or specific operations that support collation.

To specify collation when you query documents in the MongoDB Atlas UI, see Specify Collation.

A collation document has the following fields:

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

When specifying collation, the locale field is mandatory; all other collation fields are optional. For descriptions of the fields, see Collation Document.

Default collation parameter values vary depending on which locale you specify. For a complete list of default collation parameters and the locales they are associated with, see Collation Default Parameters.

Field
Type
Description
locale
string

The ICU locale. See Supported Languages and Locales for a list of supported locales.

To specify simple binary comparison, specify locale value of "simple".

strength
integer

Optional. The level of comparison to perform. Corresponds to ICU Comparison Levels. Possible values are:

Value
Description
1
Primary level of comparison. Collation performs comparisons of the base characters only, ignoring other differences such as diacritics and case.
2
Secondary level of comparison. Collation performs comparisons up to secondary differences, such as diacritics. That is, collation performs comparisons of base characters (primary differences) and diacritics (secondary differences). Differences between base characters takes precedence over secondary differences.
3

Tertiary level of comparison. Collation performs comparisons up to tertiary differences, such as case and letter variants. That is, collation performs comparisons of base characters (primary differences), diacritics (secondary differences), and case and variants (tertiary differences). Differences between base characters takes precedence over secondary differences, which takes precedence over tertiary differences.

This is the default level.

4
Quaternary Level. Limited for specific use case to consider punctuation when levels 1-3 ignore punctuation or for processing Japanese text.
5
Identical Level. Limited for specific use case of tie breaker.

See ICU Collation: Comparison Levels for details.

caseLevel
boolean

Optional. Flag that determines whether to include case comparison at strength level 1 or 2.

If true, include case comparison:

  • When used with strength:1, collation compares base characters and case.

  • When used with strength:2, collation compares base characters, diacritics (and possible other secondary differences) and case.

If false, do not include case comparison at level 1 or 2. The default is false.

For more information, see ICU Collation: Case Level.

caseFirst
string

Optional. A field that determines sort order of case differences during tertiary level comparisons.

Possible values are:

Value
Description
"upper"
Uppercase sorts before lowercase.
"lower"
Lowercase sorts before uppercase.
"off"
Default value. Similar to "lower" with slight differences. See https://unicode-org.github.io/icu/userguide/strings/properties.html#customization for details of differences.
numericOrdering
boolean

Optional. Flag that determines whether to compare numeric strings as numbers or as strings.

If true, compare as numbers. For example, "10" is greater than "2".

If false, compare as strings. For example, "10" is less than "2".

Default is false.

See numericOrdering Restrictions.

alternate
string

Optional. Field that determines whether collation should consider whitespace and punctuation as base characters for purposes of comparison.

Possible values are:

Value
Description
"non-ignorable"
Whitespace and punctuation are considered base characters.
"shifted"
Whitespace and punctuation are not considered base characters and are only distinguished at strength levels greater than 3.

See ICU Collation: Comparison Levels for more information.

Default is "non-ignorable".

maxVariable
string

Optional. Field that determines up to which characters are considered ignorable when alternate: "shifted". Has no effect if alternate: "non-ignorable"

Possible values are:

Value
Description
"punct"
Both whitespace and punctuation are ignorable and not considered base characters.
"space"
Whitespace is ignorable and not considered to be base characters.
backwards
boolean

Optional. Flag that determines whether strings with diacritics sort from back of the string, such as with some French dictionary ordering.

If true, compare from back to front.

If false, compare from front to back.

The default value is false.

normalization
boolean

Optional. Flag that determines whether to check if text require normalization and to perform normalization. Generally, majority of text does not require this normalization processing.

If true, check if fully normalized and perform normalization to compare text.

If false, does not check.

The default value is false.

See https://unicode-org.github.io/icu/userguide/collation/concepts.html#normalization for details.

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.

[1](1, 2) Some index types do not support collation. See Collation and Unsupported Index Types for details.

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 unihan variant of the Chinese collation:

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

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

  • 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.

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.

Warning

Because indexes that are configured with collation use ICU collation keys to achieve sort order, collation-aware index keys may be larger than index keys for indexes without 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 field category; 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 the score: 5 portion of the query:

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

Important

Matches against document keys, including embedded document keys, use simple binary comparison. This means that a query for a key like "foo.bár" will not match the key "foo.bar", regardless of the value you set for the strength parameter.

The following indexes only support simple binary comparison and do not support collation:

Tip

To create a text or 2d index on a collection that has a non-simple collation, you must explicitly specify {collation: {locale: "simple"} } when creating the index.

When specifying the numericOrdering as true the following restrictions apply:

  • Only contiguous non-negative integer substrings of digits are considered in the comparisons.

    numericOrdering does not support:

    • +

    • -

    • decimal separators, like decimal points and decimal commas

    • exponents

  • Only Unicode code points in the Number or Decimal Digit (Nd) category are treated as digits.

  • If a digit length exceeds 254 characters, the excess characters are treated as a separate number.

Consider a collection with the following string number and decimal values:

db.c.insertMany(
[
{ "n" : "1" },
{ "n" : "2" },
{ "n" : "2.1" },
{ "n" : "-2.1" },
{ "n" : "2.2" },
{ "n" : "2.10" },
{ "n" : "2.20" },
{ "n" : "-10" },
{ "n" : "10" },
{ "n" : "20" },
{ "n" : "20.1" }
]
)

The following find query uses a collation document containing the numericOrdering parameter:

db.c.find(
{ }, { _id: 0 }
).sort(
{ n: 1 }
).collation( {
locale: 'en_US',
numericOrdering: true
} )

The operation returns the following results:

[
{ n: '-2.1' },
{ n: '-10' },
{ n: '1' },
{ n: '2' },
{ n: '2.1' },
{ n: '2.2' },
{ n: '2.10' },
{ n: '2.20' },
{ n: '10' },
{ n: '20' },
{ n: '20.1' }
]
  • numericOrdering: true sorts the string values in ascending order as if they were numeric values.

  • The two negative values -2.1 and -10 are not sorted in the expected sort order because they have unsupported - characters.

  • The value 2.2 is sorted before the value 2.10, due to the fact that the numericOrdering parameter does not support decimal values.

  • As a result, 2.2 and 2.10 are sorted in lexicographic order.

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