MongoDB Notes

Description

• MongoDB is a BSON/JSON-based document database.
• Each document is a JSON object.
• 'Documents' are kept in 'collections', and a 'database' can have several 'collections'.
• MongoDB enforces no schema, therefore each document may contain a unique set of fields/attributes (even if they are in the same collection).

Configuration

A MongoDB server's configuration is specified in the '/etc/mongodb.conf' file.

Export Data

One way to export data from MongoDB is the 'mongoexport' command. Here's an example:

$ mongoexport --host 127.0.0.1 --db mydb --collection customers > customers.json

Import Data

One way to import data into MongoDB is the 'mongoimport' command. Here's an example:

$ mongoimport -- host 127.0.0.1 --db mydb --collection customers --file customers.json

CRUD Operations

CRUD is an acronym for Create, Read/Retrieve, Update, and Delete (this basic operations of any database).

Create

The 'insert' method/command can be used to create documents. You include the text of a JSON document as the parameter. For example:

> db.books.insert({ title: "Sixth Column", author: "Heinlein"})

Note: A collection is automatically created when the first document is 'inserted' into it. You can also use the 'save' method/command to create documents.

Read

The 'find' method/command is used to retrieve/read documents. This method/command can be simple and unconditional, or complex and specify criteria and fields to retrieve.

Here is a example of a simple use of the 'find' method/command:

> db.collection.find()

The problem with the output from this command is that it can be hard for a human to read. That is why I suggest appending the 'pretty' method. For example:

> db.collection.find().pretty()

Update

The 'update' method/command is used to modify documents. Usually, the arguments include two JSON Objects. The first JSON Object argument specifies the criteria to identify the document(s) to be modified. The second JSON Object argument replaces (or changes) the identified document(s). For example:

> db.collection.update({_id: ObjectId("52aa4ba340920f585c0000c1")}, { $set: { field: "value"})

In this example, it 'sets' the value of a field. It is possible to replace the found document(s) with a completely new document provided in the second argument.

Note: The '_id' field is automatically added to every document when they are created in MongoDB.

Delete

The 'remove' method/command is used to remove documents. While the method could be used without arguments (thereby removing all documents in the identified collection), it is usually used with an argument which specifies the criteria to identify the document(s) to be removed. For example:

> db.collection.remove({_id: ObjectId("52aa4ba340920f585c0000c1")})

Note: You can remove the collection itself by using the 'drop' method. For example:

> dv.collection.drop()

Query Operators

With the exception of Inserting/Creating documents, it is possible use query operators to identify/narrow the documents affected/retrieved by a query.

Comparison

These Query Operators are used to compare field values against values provided in the query.

• $gt - Greater Than - Ex: > db.customers.find({acct_balance:{$gt:100000}})
• $gte - Greater Than or Equal - Ex: > db.customers.find({acct_balance:{$gte:100000}})
• $in - In a Set - Ex: > db.customers.find({type:{$in:{"checking","savings"}}})
• $lt - Less Than - Ex: > db.customers.find({acct_balance:{$lt:100000}})
• $lte - Less Than or Equal - Ex: > db.customers.find({acct_balance:{$lte:100000}})
• $ne - Not Equal - Ex: > db.customers.find({acct_balance:{$ne:100000}})
• $nin - Not In a Set - Ex: > db.customers.find({type:{$nin:{"checking","savings"}}})