MongoDB – Database, Collection, and Document

MongoDB – Database, Collection, and Document

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Databases, collections, documents are important parts of the MongoDB without them you are not able to store data on the MongoDB server. A Database contains a collection, and a collection contains documents and the documents contain data, they are related to each other.

In MongoDB, a database contains the collections of documents. One can create multiple databases on the MongoDB server. Learn MongoDB online training from industrial experts

View Database: To see how many databases are present in your MongoDB server, write the following statement in the mongo shell: Naming Restriction for Database: Before creating a database you should first learn about the naming restrictions for databases:

In MongoDB, the names of the database are case insensitive, but you must always remember that the database names cannot differ only by the case of the characters. For windows user, MongoDB database names cannot contain any of these following characters: /. "$*:|? For Unix and Linux users, MongoDB database names cannot contain any of these following characters: /. "$ MongoDB database names cannot contain null characters(in windows, Unix and Linux system). MongoDB database names cannot be empty and must contain less than 64 characters. Creating Database: In the mongo shell, you can create a database with the help of the following command:

use database_name This command actually switches you to the new database if the given name does not exist and if the given name exists, then it will switch you to the existing database. Now at this stage, if you use the show command to see the database list where you will find that your new database is not present in that database list because, in MongoDB, the database is actually created when you start entering data in that database.

Here, we create a new database named GeeksforGeeks using the use command. After creating a database when we check the database list we do not find our database on that list because we do not enter any data in the GeeksforGeeks database. Best MongoDB course will help you to learn more effectively.

Collection Collections are just like tables in relational databases, they also store data, but in the form of documents. A single database is allowed to store multiple collections.

Schemaless: As we know that MongoDB databases are schemaless. So, it is not necessary in a collection that the schema of one document is similar to another document. Or in other words, a single collection contains different types of documents

Naming Restrictions for Collection: Before creating a collection you should first learn about the naming restrictions for collections:

Collection name must starts with an underscore or a character. Collection name does not contain $, empty string, null character and does not begin with system. prefix. The maximum length of the collection name is 120 bytes(including the database name, dot separator, and the collection name.

Naming restriction of fields: Before moving further first you should learn about the naming restrictions for fields:

The field names are of strings. The _id field name is reserved to use as a primary key. And the value of this field must be unique, immutable and can be of any type other than an array. The field name cannot contain null characters. The top-level field names should not start with a dollar sign ($). Document Size: The maximum size of the BSON document is 16MB. It ensures that the single document does not use too much amount of RAM or bandwidth(during transmission). If a document contains more data than the specified size, then MongoDB provides a GridFS API to store such type of documents.

Important Notes –

A single document may contain duplicate fields. MongoDB always saves the order of the fields in the documents except for the _id field (which always comes in the first place) and the renaming of fields may change the order of the fields in the documents. _id Field: In MongoDB, every document store in the collection must contain a unique _id field it is just like a primary key in a relational database. The value of the _id field can be set by the user or by the system (if the user does not create an _id field, then the system will automatically generate an ObjectId for _id field). When you create a collection MongoDB automatically create a unique index on the _id field. The _id field is the first field of every document. The value of the _id field can be of any BSON type except arrays. The default value of the _id field is ObjectId.

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