collMod

Change Index Properties

index

The index option can change the following properties of an existing index:

Index Property	Description
expireAfterSeconds	The number of seconds that determines the expiration threshold of a TTL Collection. If successful, the command returns a document that contains: expireAfterSeconds_new, the new value for expireAfterSeconds expireAfterSeconds_old, the old value for expireAfterSeconds, if the index had a value for expireAfterSeconds before. Modifying the index option expireAfterSeconds resets the $indexStats for the index. If you use TTL indexes created before MongoDB 5.0, or if you want to sync data created in MongDB 5.0 with a pre-5.0 installation, see Indexes Configured Using NaN to avoid misconfiguration issues.
hidden	A boolean that determines whether the index is hidden or not from the query planner. If the hidden value changes, the command returns a document that contains both the old and new values for the changed property: hidden_old and hidden_new. However, if the hidden value has not changed (i.e. hiding an already hidden index or unhiding an already unhidden index), the command omits the hidden_old and hidden_new fields from the output. To hide an index, you must have featureCompatibilityVersion set to 4.4 or greater. Modifying the index option hidden resets the $indexStats for the index if the value changes. New in version 4.4.
prepareUnique	A boolean that determines whether the index will accept new duplicate entries. New duplicate entries fail with DuplicateKey errors when prepareUnique is true. The resulting index can be converted to a unique index. To convert the index, use collMod with the unique option. If an existing index is updated so that prepareUnique is true, the index is not checked for pre-existing, duplicate index entries. New in version 6.0.
unique	A boolean that determines whether or not the index is unique. When unique is true, collMod scans the keyPattern index for duplicates and then converts it to a unique index if there are no duplicate index entries. If duplicates are detected during the initial scan, collMod returns CannotConvertIndexToUnique and a list of conflicting documents. To convert an index with duplicate entries to a unique index, correct any reported conflicts and rerun collMod. To end a conversion, set prepareUnique to false. New in version 6.0.

To change index options, specify either the key pattern or name of the existing index and the index option or options you wish to change:

db.runCommand( {
   collMod: <collection>,
   index: {
      keyPattern: <index_spec> || name: <index_name>,
      expireAfterSeconds: <number>,  // Set the TTL expiration threshold
      hidden: <boolean>,             // Change index visibility in the query planner
      prepareUnique: <boolean>,      // Reject new duplicate index entries
      unique: <boolean>              // Convert an index to a unique index
   }
} )

If the index does not exist, the command errors with the message "cannot find index <name|keyPattern> for ns <db.collection>".

Tip

See also:

• Hidden Indexes

Validate Documents

validator

validator allows users to specify validation rules or expressions for a collection. For more information, see Schema Validation.

The validator option takes a document that specifies the validation rules or expressions. You can specify the expressions using the same operators as the query operators with the exception of $near, $nearSphere, $text, and $where.

Note

• Validation occurs during updates and inserts. Existing documents do not undergo validation checks until modification.

• You cannot specify a validator for collections in the admin, local, and config databases.

• You cannot specify a validator for system.* collections.

validationLevel

The validationLevel determines how strictly MongoDB applies the validation rules to existing documents during an update.

validationLevel	Description
"off"	No validation for inserts or updates.
"strict"	Default Apply validation rules to all inserts and all updates.
"moderate"	Apply validation rules to inserts and to updates on existing valid documents. Do not apply rules to updates on existing invalid documents.

To see an example that uses validationLevel, see Specify Validation Level for Existing Documents.

validationAction

The validationAction option determines whether to error on invalid documents or just warn about the violations but allow invalid documents.

Important

Validation of documents only applies to those documents as determined by the validationLevel.

To see an example that uses validationAction, see Choose How to Handle Invalid Documents.

Modify Views

viewOn

The underlying source collection or view for the view. The view definition is determined by applying the specified pipeline to this source.

Required if modifying a view on a MongoDB deployment that is running with access control.

pipeline

The aggregation pipeline that defines the view.

Note

A view definition pipeline cannot include the $out or the $merge stage. This restriction also applies to embedded pipelines, such as pipelines used in $lookup or $facet stages.

Required if modifying a view on a MongoDB deployment that is running with access control.

The view definition is public; i.e. db.getCollectionInfos() and explain operations on the view will include the pipeline that defines the view. As such, avoid referring directly to sensitive fields and values in view definitions.

db.runCommand( {
   collMod: "myView",
   viewOn: "activities",
   pipeline: [
     { $match: { status: "Q" } },
     { $project: { user: 1, date: 1, description: 1} } ]
} )

Modify Time Series Collections

expireAfterSeconds

Note

This is distinct from using the index option with the expireAfterSeconds property to change the expiration time for a TTL Collection.

To enable automatic document removal or modify the current expiration interval for a time series collection, change the expireAfterSeconds value:

db.runCommand( {
   collMod: <collection>,
   expireAfterSeconds: <number> || "off"
} )

Set expireAfterSeconds to "off" to disable automatic removal, or a non-negative decimal number (>=0) to specify the number of seconds after which documents expire.

timeseries-granularity

To modify the granularity of a time series collection, change the timeseries.granularity value:

db.runCommand({
   collMod: "weather24h",
   timeseries: { granularity: "seconds" || "minutes" || "hours" }
})

You cannot decrease granularity. For example, if granularity is set to minutes, you can increase it to hours, but you cannot decrease it to seconds.

The following table shows the maximum time interval included in one bucket of data when using a given granularity value:

granularity	Covered Time Span
"seconds" (default)	one hour
"minutes"	24 hours
"hours"	30 days

Important

As with all system collections, do not run any operations on the underlying system.buckets collection.

Tip

See also:

Set Granularity for Time Series Data

Resize a Capped Collection

Starting in MongoDB 6.0, you can resize a capped collection. To change a capped collection's maximum size in bytes, use the cappedSize option. To change the maximum number of documents in an existing capped collection, use the cappedMax option.

cappedSize

Specifies a new maximum size, in bytes, for a capped collection. cappedSize must be greater than 0 and less than 1e+15 (1 PB).

cappedMax

Specifies a new maximum number of documents in a capped collection. Setting cappedMax less than or equal to 0 implies no limit.

For example, the following command sets the maximum size of a capped collection to 100000 bytes and sets the maximum number of documents in the collection to 500:

db.runCommand( {
   collMod: <collection>,
   cappedSize: 100000,
   cappedMax: 500
} )

Change Streams with Document Pre- and Post-Images

changeStreamPreAndPostImages

Starting in MongoDB 6.0, you can use change stream events to output the version of a document before and after changes (the document pre- and post-images):

• The pre-image is the document before it was replaced, updated, or deleted. There is no pre-image for an inserted document.

• The post-image is the document after it was inserted, replaced, or updated. There is no post-image for a deleted document.

• Enable changeStreamPreAndPostImages for a collection using db.createCollection(), create, or collMod.

To use collMod to enable change stream pre- and post-images for a collection, use the changeStreamPreAndPostImages field:

db.runCommand( {
   collMod: <collection>,
   changeStreamPreAndPostImages: { enabled: <boolean> }
} )

To enable change stream pre- and post-images for a collection, set changeStreamPreAndPostImages to true. For example:

db.runCommand( {
   collMod: "orders",
   changeStreamPreAndPostImages: { enabled: true }
} )

To disable change stream pre- and post-images for a collection, set changeStreamPreAndPostImages to false. For example:

db.runCommand( {
   collMod: "orders",
   changeStreamPreAndPostImages: { enabled: false }
} )

Pre- and post-images are not available for a change stream event if the images were:

• Not enabled on the collection at the time of a document update or delete operation.

• Removed after the pre- and post-image retention time set in expireAfterSeconds.

  • The following example sets expireAfterSeconds to 100 seconds:

    use admin
    db.runCommand( {
       setClusterParameter:
          { changeStreamOptions: { preAndPostImages: { expireAfterSeconds: 100 } } }
    } )

  • The following example returns the current changeStreamOptions settings, including expireAfterSeconds:

    db.adminCommand( { getClusterParameter: "changeStreamOptions" } )

  • Setting expireAfterSeconds to off uses the default retention policy: pre- and post-images are retained until the corresponding change stream events are removed from the oplog.

  • If a change stream event is removed from the oplog, then the corresponding pre- and post-images are also deleted regardless of the expireAfterSeconds pre- and post-image retention time.

Additional considerations:

• Enabling pre- and post-images consumes storage space and adds processing time. Only enable pre- and post-images if you need them.

• Limit the change stream event size to less than 16 megabytes. To limit the event size, you can:

  • Limit the document size to 8 megabytes. You can request pre- and post-images simultaneously in the change stream output if other change stream event fields like updateDescription are not large.

  • Request only post-images in the change stream output for documents up to 16 megabytes if other change stream event fields like updateDescription are not large.

  • Request only pre-images in the change stream output for documents up to 16 megabytes if:

    • document updates affect only a small fraction of the document structure or content, and

    • do not cause a replace change event. A replace event always includes the post-image.

• To request a pre-image, you set fullDocumentBeforeChange to required or whenAvailable in db.collection.watch(). To request a post-image, you set fullDocument using the same method.

• Pre-images are written to the config.system.preimages collection.

  • The config.system.preimages collection may become large. To limit the collection size, you can set expireAfterSeconds time for the pre-images as shown earlier.

  • Pre-images are removed asynchronously by a background process.

Write Concern

Optional. A document expressing the write concern of the collMod command.

Omit to use the default write concern.

Resource Locking

The collMod command obtains an exclusive lock on the parent database of the specified collection for the duration of the operation. All subsequent operations on the database and all its collections must wait until collMod releases the lock.

Change Expiration Value for Indexes

The following example updates the expireAfterSeconds property of an existing TTL index { lastAccess: 1 } on a collection named user_log. The current expireAfterSeconds property for the index is set to 1800 seconds (or 30 minutes) and the example changes the value to 3600 seconds (or 60 minutes).

db.runCommand({
   collMod: "user_log",
   index: {
      keyPattern: { lastAccess: 1 },
      expireAfterSeconds: 3600
   }
})

If successful, the operation returns a document that includes both the old and new value for the changed property:

{ "expireAfterSeconds_old" : 1800, "expireAfterSeconds_new" : 3600, "ok" : 1 }

Hide an Index from the Query Planner

The following example hides an existing index on the orders collection. Specifically, the operation hides the index with the specification { shippedDate: 1 } from the query planner.

db.runCommand({
   collMod: "orders",
   index: {
      keyPattern: { shippedDate: 1 },
      hidden: true
   }
})

If successful, the operation returns a document that includes both the old and new value for the changed property:

{ "hidden_old" : false, "hidden_new" : true, "ok" : 1 }

To hide a text index, you must specify the index by name and not by keyPattern.

Convert an Existing Index to a Unique Index

Create the apples collection:

db.apples.insertMany( [
   { type: "Delicious", quantity: 12 },
   { type: "Macintosh", quantity: 13 },
   { type: "Delicious", quantity: 13 },
   { type: "Fuji", quantity: 15 },
   { type: "Washington", quantity: 10 },
] )

Add a single field index on type:

db.apples.createIndex( { type: 1 } )

Prepare the index on the type field for conversion:

db.runCommand( {
    collMod: "apples",
    index: {
       keyPattern: { type: 1 },
       prepareUnique: true
    }
 } )

The existing index may contain duplicate entries, but it will not accept new documents that duplicate an index entry when prepareUnique is true.

Try to insert a document with a duplicate index value:

db.apples.insertOne( { type: "Delicious", quantity: 200 } )

The operation returns an error. The index will not accept new duplicate entries.

Use the unique``option to convert the index to a unique index. ``collMod checks the collection for duplicate index entries before converting the index:

db.runCommand( {
    collMod: "apples",
    index: {
       keyPattern: { type: 1 },
       unique: true
    }
 } )

The response to this operation varies by driver. You will always receive an error message about the duplicate entries.

"errmsg" : "Cannot convert the index to unique. Please resolve
            conflicting documents before running collMod again."

Some drivers also return a list of ObjectIds for the duplicate entries:

{
     "ok" : 0,
     "errmsg" : "Cannot convert the index to unique. Please resolve \
                 conflicting documents before running collMod again.",
     "code" : 359,
     "codeName" : "CannotConvertIndexToUnique",
     "violations" : [
             {
                     "ids" : [
                             ObjectId("62a2015777e2d47c4da33146"),
                             ObjectId("62a2015777e2d47c4da33148")
                     ]
             }
     ]
}

To complete the conversion, modify the duplicate entries to remove any conflicts and re-run collMod() with the unique option.