Collections in MongoDB

In MongoDB, a collection is a grouping of MongoDB documents, analogous to a table in a relational database. However, unlike tables that enforce a strict schema (structure of rows and columns), MongoDB collections are schema-flexible, allowing documents within a collection to have different fields and data types.

Key Characteristics of Collections in MongoDB

1. No Fixed Schema

• Unlike relational databases, where every row in a table must follow a predefined structure, MongoDB collections are schema-less. This means documents within the same collection do not need to have the same structure.
• One document might have a certain set of fields, while another in the same collection could have a completely different set of fields.

Example:

{
  "_id": ObjectId("60f76c5917a1f4b4e71d3a8b"),
  "name": "Alice",
  "email": "alice@example.com"
}

{
  "_id": ObjectId("60f76c5917a1f4b4e71d3a8c"),
  "username": "bob99",
  "age": 25,
  "address": {
    "city": "New York",
    "state": "NY"
  }
}

Both documents are stored in the same collection but have different fields (name, email in one, username, age, address in another).

2. Flexible Document Structure

• Documents in a collection are stored in BSON (Binary JSON) format, which allows for more data types than plain JSON.
• You can have different structures within the same collection, supporting complex and varying data models in a single system.

3. Implicit Collection Creation

• In MongoDB, you don't need to explicitly create a collection. A collection is created automatically when the first document is inserted into it.
• Example: If you try to insert a document into a collection that doesn’t exist, MongoDB will automatically create the collection.

db.newCollection.insert({ name: "John Doe", age: 30 });
// The `newCollection` will be automatically created if it doesn't exist.

4. Naming Collections

• Collection names are case-sensitive. For example, Users and users are treated as two different collections.
• Collection names must follow these rules:
  • Cannot be an empty string.
  • Cannot include null characters.
  • Cannot start with the character system. (reserved for internal collections).
  • Maximum length of 128 characters.

5. Working with Collections

Collections are where all interactions with data in MongoDB happen. Some of the common operations involving collections include:

Inserting Documents

To insert data into a collection:

db.users.insertOne({
  name: "John Doe",
  email: "john@example.com"
});

• This command inserts a document into the users collection. If the collection doesn't exist, it will be created automatically.

Querying Documents

You can query documents from a collection using find() and related methods.

db.users.find({ name: "John Doe" });

This query searches the users collection for documents where the name field matches "John Doe".

Updating Documents

You can update documents in a collection with the updateOne() or updateMany() methods.

db.users.updateOne(
  { name: "John Doe" },
  { $set: { age: 31 } }
);

This command updates a document in the users collection where the name is "John Doe" and sets the age to 31.

Deleting Documents

You can delete documents from a collection using deleteOne() or deleteMany() methods.

db.users.deleteOne({ name: "John Doe" });

This deletes a single document where name is "John Doe".