The primary difference between embedded and referenced documents lies in how data relationships are managed in a database, particularly in document-oriented databases like MongoDB. An embedded document is a document stored inside another document, effectively establishing a "parent-child" relationship within the single document structure. In this case, when you query the parent document, you have immediate access to its child documents without needing additional lookups. For example, if you have a user document that contains a profile and several addresses, you could store the addresses directly within the user document as embedded documents.
On the other hand, a referenced document is stored separately and linked to the parent document using an identifier, such as an ID. This approach allows for better normalization of data, which means that data is separated into distinct entities, making it easier to manage and update. For instance, if you have a post document that references a user document, the post might contain a field with the user ID rather than embedding the user's entire profile within the post. With this setup, if you need to change something in the user's profile, you only need to update it in one place, without having to modify multiple posts.
Choosing between embedded and referenced documents typically depends on your application’s specific needs. Embedded documents are useful when you want to optimize read performance and the related data is unlikely to change frequently. In contrast, referenced documents are better suited for situations where data normalization is important or when you expect to update the linked documents often or need to share them across multiple parent documents. Understanding these differences can help developers design more efficient and maintainable data models tailored to their application's requirements.