MongoDB is often categorized as a NoSQL database, which means it doesn't use the traditional table-based relational database structure. Instead, it employs a more flexible data storage model, allowing for varied data structures. While it is true that MongoDB can be used similarly to a key-value store, describing it solely as a key-value database simplifies its capabilities too much.
At its core, MongoDB is a document-oriented database. It stores data in flexible, JSON-like documents where fields can vary from document to document. This model allows data to be stored in a way that's more congruent with object-oriented programming, making it easier for developers to work with data.
Key-value stores are a type of database that associate a unique key with a specific value. While MongoDB does use keys (the '_id' field in each document can be considered a key) to access documents, the 'value' in MongoDB is a rich, structured document capable of holding arrays and sub-documents. This goes beyond what is traditionally expected from a key-value store, where values are usually simple strings or blobs.
To illustrate MongoDB's capabilities with an example:
In this example, while we do have a 'key' (implicitly the '_id' MongoDB provides), the 'value' (the document itself) consists of multiple fields. This illustrates how MongoDB's use case extends well beyond that of a simple key-value store to support complex, nested documents.
In conclusion, while MongoDB can technically perform the functions of a key-value store, its capabilities allow for much more complex and nuanced data modeling, making it better described as a document-oriented database.