How to Set a Primary Key in MongoDB?

In MongoDB, primary keys play an important role in uniquely identifying documents within a collection. While MongoDB doesn't have a concept of primary keys in the same way as relational databases but it allows for the creation of unique identifiers for documents.

In this article, we'll explore how to set a primary key in MongoDB by providing detailed explanations and examples to understand and implement this concept effectively.

Primary Keys in MongoDB

• In MongoDB, each document within a collection is uniquely identified by a field called the _id field.
  
• The _id field serves as the primary key for the document, ensuring its uniqueness within the collection.
  
• MongoDB automatically assigns a unique _id value to each document if one is not provided explicitly.

How to Set a Primary Key in MongoDB?

There are several methods for setting primary keys in MongoDB:

1. Using the _id Field: MongoDB automatically assigns a unique value to the _id field if not provided explicitly.
   
2. Custom _id Field: We can specify your own custom _id field with a unique value for each document.
   
3. Unique Index: We can create a unique index on a field to enforce uniqueness and use it as a primary key.

Let's explore each method with examples and outputs.

1. Using the _id Field

MongoDB automatically generates a unique _id value for each document if one is not provided explicitly. Let's see an example:

// Inserting a document without specifying the _id field
db.users.insertOne({ "name": "Alice" });

Output:

{
  "_id": ObjectId("6156d8f013c92ec9f5b8b36f"),
  "name": "Alice"
}

In this example, MongoDB automatically generates a unique ObjectId value for the _id field.

2. Using Custom _id Field

We can specify our own custom _id value for a document. This value should be unique within the collection to avoid conflicts. Let's see an example:

// Inserting a document with a custom _id field
db.users.insertOne({ "_id": 1001, "name": "Bob" });

Output:

{
  "_id": 1001,
  "name": "Bob"
}

In this example, we explicitly specify the _id field with the value 1001.

3. Using Unique Index

We can create a unique index on a field to apply uniqueness and use it as a primary key. Let's create a unique index on the email field:

// Creating a unique index on the email field
db.users.createIndex({ "email": 1 }, { unique: true });

Now, when inserting documents, MongoDB will enforce uniqueness on the email field, effectively making it act as a primary key.

Conclusion

Overall, Setting a primary key in MongoDB is essential for uniquely identifying documents within a collection. By understanding the various methods for setting primary keys and their implications, you can design your MongoDB schemas effectively and ensure data integrity within your database. Whether using the default _id field, specifying a custom _id value, or creating unique indexes, MongoDB provides flexibility in setting primary keys based on your application requirements.

What happens if I try to insert a document with a duplicate _id value?

MongoDB will throw a duplicate key error and the insertion will fail. Each _id value must be unique within a collection.

Can I change the _id field of an existing document?

No, the _id field is immutable once set. To change the _id, you would need to remove the document and insert a new one with the desired _id.

Is it possible to have multiple unique indexes in a single collection?

Yes, you can create multiple unique indexes on different fields within the same collection. This allows you to enforce uniqueness on multiple attributes of your documents.