MongoDB - Database, Collection, and Document

MongoDB is a NoSQL database that stores data in a flexible structure. A database is like a container for data, a collection is a group of related records, and a document is a single record stored in JSON-like format.

• MongoDB can have many databases on one server, each separate from the others.
• It creates three default databases: admin, local, and config (used internally).
• A new database is made automatically when you insert data into it or switch using use <dbname>.

View Database

To see how many databases are present in your MongoDB server, write the following statement in the mongo shell:

show dbs

Here, we freshly started MongoDB, so we do not have a database except for these three default databases, i.e., admin, config, and local.

The use GeeksforGeeks command creates and switches to a database, but it does not appear in the database list until at least one document is inserted.

Naming Restriction for Database

Before creating a database we should first learn about the naming restrictions for databases:  

• Database names must be case-sensitive.
• The names cannot contain special characters such as /, ., $, *, |, etc.
• MongoDB database names cannot contain null characters(in windows, Unix, and Linux systems).
• MongoDB database names cannot be empty and must contain less than 64 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: 

/\. "$

Example:

use LibraryDB  # Switch to the LibraryDB database

Collection in MongoDB

A Collection in MongoDB is similar to a table in relational databases. It holds a group of documents and is a part of a database. Collections provide structure to data, but like the rest of MongoDB, they are schema-flexible.

• Schema-flexible: MongoDB collections can store documents with different structures, and documents in the same collection don’t need to follow a uniform schema.
• Multiple Collections: A database can have multiple collections, each holding different types of documents.

Naming Restrictions for Collection

Before creating a collection we should first learn about the naming restrictions for collections: 

• Collection name must start with an underscore(`_`) or a letter (a-z or A-Z)
• Collection names must not start with a number, contain $, null characters, or begin with the reserved system. prefix.
• The maximum length of the collection name is 120 bytes(including the database name, dot separator, and the collection name).

Example:

db.books.insertOne({ title: "Learn MongoDB", author: "Jane Doe", year: 2023 })

Creating collection

After creating database now we create a collection to store documents. The collection is created using the following syntax: 

db.collection_name.insertOne({..})

Here, insertOne() function is used to store single data in the specified collection. And in the curly braces {} we store our data or in other words, it is a document.  

• Creates the Author collection.
• Inserts a document using insertOne().
• { name: "Alen" } is a document with name as the field.
• MongoDB confirms insertion with acknowledged: true.
• A unique _id is automatically generated (custom IDs are allowed).

Document in MongoDB

MongoDB stores data as BSON documents, a binary form of JSON that supports additional data types.

• Data is stored as BSON (Binary JSON) documents.
• BSON supports more data types than JSON.
• Documents use field–value (key–value) pairs.
• Field values can be of any BSON-supported type.

Syntax:

{ 
field1: value1
field2: value2
....
fieldN: valueN
}

Document Structure

A document in MongoDB is a flexible data structure made up of field-value pairs. For instance:

{
  title: "MongoDB Basics",
  author: "John Doe",
  year: 2025
} 

Naming restriction for Document Fields

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

• Fields in documents must be named with strings
• The _id field is a reserved primary key that must be unique, immutable, and cannot be an array.
• The field name cannot contain null characters.
• The top-level field names should not start with a dollar sign($).

Document Size

• Maximum BSON document size: 16MB, preventing excessive RAM or bandwidth usage.
• For larger data, MongoDB provides GridFS.
• Documents can contain duplicate fields.
• Field order in MongoDB documents is preserved as inserted. The _id field is not required to be the first field. Renaming fields may affect their order.

_id Field in MongoDB

Every document must have a unique _id field, acting like a primary key.

• Automatic ObjectId: If _id is not provided, MongoDB generates a unique ObjectId.
• Custom _id: Users can assign a custom unique value for _id.

Example with ObjectId

The name, branch, course, and paid fields are strings, the amount field is an integer, and the _id field is system-generated.

Example with Custom _id

• The _id field is explicitly created by the user.
• Always paste the data inside the function before closing the parenthesis.
• Closing the parenthesis early results in a syntax error.

Differences Between Databases, Collections, and Documents

MongoDB organizes data hierarchically into databases, collections, and documents, each serving a distinct purpose.

• Database: A container for collections, providing structure and logical isolation for data.
• Collection: A group of documents within a database, similar to a table in relational databases.
• Document: A single data record within a collection, stored as a BSON object.

Creating a Database, Collection, and Document

Here’s how you can create a database, collection, and document in MongoDB step by step:

1. Create or Switch to a Database

use LibraryDB

2. Create a Collection and Insert a Document

db.books.insertOne({
  title: "MongoDB for Beginners",
  author: "Alice Johnson",
  year: 2023
})

3. Verify the Insertion

db.books.find()

This will display the document stored in the books collection within the LibraryDB database.

Note: MongoDB automatically creates a unique index on the _id field for every collection. This index helps MongoDB quickly find documents based on their unique identifier.