Getting Started - MongoDB: The Definitive Guide - page 5

CHAPTER 2
Getting Started
MongoDB is very powerful, but it is still 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, roughly equivalent to a row in
a relational database management system (but much more expressive).
• Similarly, a collection can be thought of as the schema-free equivalent of a table.
• A single instance of MongoDB can host multiple independent databases , each of
which can have its own collections and permissions.
• MongoDB comes with a simple but powerful JavaScript shell , which is useful for
the administration of MongoDB instances and data manipulation.
• Every document has a special key, "_id" , that is unique across the document's
collection.
Documents
At the heart of MongoDB is the concept of a document : an ordered set of keys with
associated values. The representation of a document differs 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}