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.


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}

This example is a good illustration of several important concepts:

  • Key/value pairs in documents are ordered—the earlier document is distinct from ...

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