Mongodb-introduction to MongoDB, Databases and collections

MongoDB stores BSON documents, i.e. data records, in collections; The collections in databases.

Databases

In MongoDB, databases hold collections of documents.

To select a database to use, in theMONGOShell, issue the use <db> statement, as in the following Example

Use MyDB

Create a Database

If A database does not exist, MongoDB creates the database when you first store data for that database. As such, you can switch to a non-existent database and perform the following operation in theMONGOShell:

> use myNewDB
switched to db myNewDB
> db.myNewCollection1.insert( { x: 1 } )
...
WriteResult({ "nInserted" : 1 })

TheInsert ()operation creates both the databasemynewdband the collectionMyNewCollection1if They does not already exist.

For a list of restrictions on the database names, see naming restrictions.

Collections

MongoDB stores documents in collections. Collections is analogous to tables in relational databases.

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 } )
WriteResult({ "nInserted" : 1 })
> db.myNewCollection3.createIndex( { y: 1 } )
2016-11-30T11:29:16.665+0800 I INDEX    [conn6] build index on: myNewDB.myNewCollection3 properties: { v: 1, key: { y: 1.0 }, name: "y_1", ns: "myNewDB.myNewCollection3" }
2016-11-30T11:29:16.665+0800 I INDEX    [conn6] 	 building index using bulk method
2016-11-30T11:29:16.667+0800 I INDEX    [conn6] build index done.  scanned 0 total records. 0 secs
{
	"createdCollectionAutomatically" : true,
	"numIndexesBefore" : 1,
	"numIndexesAfter" : 2,
	"ok" : 1
}
> 

Both theInsert ()and theCreateIndex ()operations create their respective collection if they do not ALR Eady exist.

For a list of restrictions in collection names, see naming restrictions.

Explicit Creation

MongoDB provides thedb.createcollection ()method to explicitly create a collection with various options, such as Setting the maximum size or the documentation validation rules. If you aren't specifying these options, you don't need to explicitly create the collection since MongoDB creates new Col Lections when your first store data for the collections.

To modify these collection options, seecollmod.

Document Validation

New in version 3.2.

By default, a collection does is not require it documents to the same schema; i.e. the documents in a single collection does not need to has the same set of fields and the data type for a field can dif Fer across documents within a collection.

Starting in MongoDB 3.2, however, your can enforce document validation rules for a collection during update and insert Oper Ations. See Document Validation for details.

modifying Document Structure

The structure of the documents in a collection, such as Add new fields, remove existing fields, or change the fi Eld values to a new type, update the documents to the new structure.

Mongodb-introduction to MongoDB, Databases and collections