MongoDB: The Definitive Guide, 2nd Edition

Chapter 2. Getting Started

MongoDB is powerful but easy to get started with. In this chapter we’ll introduce some of the basic concepts of MongoDB:

• A document is the basic unit of data for MongoDB and is roughly equivalent to a row in a relational database management system (but much more expressive).

• Similarly, a collection can be thought of as a table with a dynamic schema.

• A single instance of MongoDB can host multiple independent databases, each of which can have its own collections.

• Every document has a special key, "_id", that is unique within a collection.

• MongoDB comes with a simple but powerful JavaScript shell, which is useful for the administration of MongoDB instances and data manipulation.

Documents

At the heart of MongoDB is the document: an ordered set of keys with associated values. The representation of a document varies by programming language, but most languages have a data structure that is a natural fit, such as a map, hash, or dictionary. In JavaScript, for example, documents are represented as objects:

{"greeting" : "Hello, world!"}

This simple document contains a single key, "greeting", with a value of "Hello, world!". Most documents will be more complex than this simple one and often will contain multiple key/value pairs:

{"greeting" : "Hello, world!", "foo" : 3}

As you can see from the example above, values in documents are not just “blobs.” They can be one of several different data types (or even an entire embedded document—see Embedded Documents). In this example ...