What is collection in MongoDB, how to create and use it

Collections : MongoDB stores documents in collections. Collections are similer to tables in relational databases like Oraclle.

Create a Collection

If a collection does not exist, MongoDB creates the collection when you first store data for that collection.

db.myNewCollection2.insert( { x: 1 } )

db.myNewCollection3.createIndex( { y: 1 } )

Both the insert() and the createIndex() operations create their respective collection if they do not already exist.

Creating a collection using db.createCollection() method 

MongoDB provides the db.createCollection() method to explicitly create a collection with various options, such as setting the maximum size or the documentation validation rules. If you are not specifying these options, you do not need to explicitly create the collection since MongoDB creates new collections when you first store data for the collections.

Because MongoDB creates a collection implicitly when the collection is first referenced in a command, this method is used primarily for creating new collections that use specific options. For example, you use db.createCollection() to create a capped collection, or to create a new collection that uses document validation. db.createCollection() is also used to pre-allocate space for an ordinary collection.

Syntax:

db.createCollection(<name>, { capped: <boolean>,
                                                  autoIndexId: <boolean>,
                                                  size: <number>,
                                                  max: <number>,
                                                  storageEngine: <document>,
                                                  validator: <document>,
                                                  validationLevel: <string>,
                                                  validationAction: <string>,
                                                  indexOptionDefaults: <document> } )

Parameter    Type Description

name    string The name of the collection to create.

options    document Optional. Configuration options for creating a capped collection or                                                         for preallocating space in a new collection.

capped    boolean         Optional. To create a capped collection, specify true. If you                                                                    specify true, you must also set a maximum size in the size field.

autoIndexId    boolean          Optional. Specify false to disable the automatic creation of an                                                                index on the _id field.

size            number Specify a maximum size in bytes for a capped collection. Once a                                                           capped collection reaches its maximum size, MongoDB removes                                                           the older documents to make space for the new documents. The                                                             size field is required for capped collections and ignored for other                                                           collections. it is optional

max                   number Optional. The maximum number of documents allowed in the                                                                 capped collection. The size limit takes precedence over this limit.                                                           If a capped collection reaches the size limit before it reaches                         the maximum number of documents, MongoDB removes old                                                                 documents. If you prefer to use the max limit, ensure that the size                                                           limit, which is required for a capped collection, is sufficient to                 contain the maximum number of documents.

usePowerOf2Sizes boolean         Optional. Available for the MMAPv1 storage engine only.

noPadding boolean         Optional. Available for the MMAPv1 storage engine only.

storageEngine document    Optional. Available for the WiredTiger storage engine only.
                New in version 3.0.
                Allows users to specify configuration to the storage                                         engine on a per-collection basis when creating a collection. The                                                             value of the storageEngine option should take the following form:
               { <storage-engine-name>: <options> } Storage engin configuration                                                         specified when creating collections are validated and logged to the                 oplog during replication to support replica sets with members that                                                         use different storage engines.

validator document                        Optional. Allows users to specify validation rules or expressions                                                            for the collection. For more information, see Document Validation

validationLevel    string                Optional. Determines how strictly MongoDB applies the validation                                                        rules to existing documents during an update.

validationAction string       Optional. Determines whether to error on invalid documents or just                                                       warn about the violations but allow invalid documents to be                                                                   inserted.

Examples

>use test

switched to db test

>db.createCollection("mycollection")

{ "ok" : 1 }

>db.tutorialspoint.insert({"name" : "tutorialspoint"})

>show collections

mycol

mycollection

system.indexes

tutorialspoint

>

Create a Capped Collection

Capped collections have maximum size or document counts that prevent them from growing beyond maximum thresholds. All capped collections must specify a maximum size and may also specify a maximum document count. MongoDB removes older documents if a collection reaches the maximum size limit before it reaches the maximum document count. Consider the following example:

db.createCollection("log", { capped : true, size : 5242880, max : 5000 } )

This command creates a collection named log with a maximum size of 5 megabytes and a maximum of 5000 documents.

The following command simply pre-allocates a 2-gigabyte, uncapped collection named people:

db.createCollection("people", { size: 2147483648 } )