OPTIONS

db.collection.ensureIndex()

Definition

db.collection.ensureIndex(keys, options)

Creates an index on the specified field if the index does not already exist.

The ensureIndex() method has the following fields:

Parameter Type Description
keys document For ascending/descending indexes, a document that contains pairs with the name of the field or fields to index and order of the index. A 1 specifies ascending and a -1 specifies descending. MongoDB supports several different index types including text, geospatial, and hashed indexes. See Indexing Tutorials.
options document Optional. A document that controls the creation of the index. The document contains a set of options, as described in the next table.

Warning

Index names, including their full namespace (i.e. database.collection) cannot be longer than 128 characters. See the getIndexes() field name for the names of existing indexes.

The options document has one or more of the following fields:

Parameter Type Description
background Boolean Optional. Builds the index in the background so that building an index does not block other database activities. Specify true to build in the background. The default value is false.
unique Boolean Optional. Creates a unique index so that the collection will not accept insertion of documents where the index key or keys match an existing value in the index. Specify true to create a unique index. The default value is false. This option applies only to ascending/descending indexes.
name string Optional. The name of the index. If unspecified, MongoDB generates an index name by concatenating the names of the indexed fields and the sort order.
dropDups Boolean

Optional. Creates a unique index on a field that may have duplicates. MongoDB indexes only the first occurrence of a key and removes all documents from the collection that contain subsequent occurrences of that key. Specify true to create unique index. The default value is false. This option applies only to scalar indexes.

Deprecated since version 2.6.

Warning

dropDups will delete data from your collection when building the index.

sparse Boolean Optional. If true, the index only references documents with the specified field. These indexes use less space but behave differently in some situations (particularly sorts). The default value is false. This applies only to ascending/descending indexes.
expireAfterSeconds integer Optional. Specifies a value, in seconds, as a TTL to control how long MongoDB retains documents in this collection. See Expire Data from Collections by Setting TTL for more information on this functionality. This applies only to TTL indexes.
v index version Optional. The index version number. The default index version depends on the version of mongod running when creating the index. Before version 2.0, the this value was 0; versions 2.0 and later use version 1, which provides a smaller and faster index format. Specify a different index version only in unusual situations.
weights document Optional. For text indexes, the significance of the field relative to the other indexed fields. The document contains field and weight pairs. The weight is a number ranging from 1 to 99,999 and denotes the significance of the field relative to the other indexed fields in terms of the score. You can specify weights for some or all the indexed fields. See Control Search Results with Weights to adjust the scores. The default value is 1. This applies to text indexes only.
default_language string Optional. For a text index, the language that determines the list of stop words and the rules for the stemmer and tokenizer. See Text Search Languages for the available languages and Specify a Language for Text Index for more information and examples. The default value is english. This applies to text indexes only.
language_override string Optional. For a text index, specify the name of the field in the document that contains, for that document, the language to override the default language. The default value is language.

Behaviors

The ensureIndex() method has the behaviors described here.

  • To add or change index options you must drop the index using the dropIndex() method and issue another ensureIndex() operation with the new options.

    If you create an index with one set of options, and then issue the ensureIndex() method with the same index fields and different options without first dropping the index, ensureIndex() will not rebuild the existing index with the new options.

  • If you call multiple ensureIndex() methods with the same index specification at the same time, only the first operation will succeed, all other operations will have no effect.

  • Non-background indexing operations will block all other operations on a database.

  • MongoDB will not create an index on a collection if the value of the index field in an existing document exceeds the Maximum Index Key Length. Previous versions of MongoDB would create the index but not index such documents.

    Changed in version 2.6.

Examples

Create an Ascending Index on a Single Field

The following example creates an ascending index on the field orderDate.

db.collection.ensureIndex( { orderDate: 1 } )

If the keys document specifies more than one field, then ensureIndex() creates a compound index.

Create an Index on a Multiple Fields

The following example creates a compound index on the orderDate field (in ascending order) and the zipcode field (in descending order.)

db.collection.ensureIndex( { orderDate: 1, zipcode: -1 } )

A compound index cannot include a hashed index component.

Note

The order of an index is important for supporting sort() operations using the index.

See also