CRUD using the shell

The mongo shell is equivalent to the administration console used by relational databases. Connecting to the mongo shell is as easy as typing the following:

$ mongo

Type it on the command line for standalone servers or replica sets. Inside the shell, we can view available databases simply by typing the following:

$ db

And connect to a database by typing the following:

> use <database_name>

The mongo shell can be used for querying and updating data in our databases. Finding documents from a collection named books is as easy as the following:

> db.books.find()
{ "_id" : ObjectId("592033f6141daf984112d07c"), "title" : "mastering mongoDB", "isbn" : "101" }

And inserting this document in the books collection can be done via the following:

> db.books.insert({title: 'mastering mongoDB', isbn: '101'})
WriteResult({ "nInserted" : 1 })

The result we get back from MongoDB informs us that the write succeeded and inserted one new document in the database.

Deleting this document has similar syntax and results:

> db.books.remove({isbn: '101'})
WriteResult({ "nRemoved" : 1 })

Try to update this same document:

> db.books.update({isbn:'101'}, {price: 30})
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })
> db.books.find()
{ "_id" : ObjectId("592034c7141daf984112d07d"), "price" : 30 }

Here, we notice a couple of things:

• First, the JSON-like formatted field in the update command is our query to search for documents to update.
• The  WriteResult object notifies us that the query matched one document and notified one document.
• Most importantly, the contents of this document were entirely replaced by the contents of the second JSON-like formatted field. We lost information on the title and ISBN!

By default, the update command in MongoDB will replace the contents of our document with the document we specify in the second argument. If we want to update the document and add new fields to it we need to use the $set operator like this:

> db.books.update({isbn:'101'}, {$set: {price: 30}})
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })

Now our document matches what we would expect:

> db.books.find()
{ "_id" : ObjectId("592035f6141daf984112d07f"), "title" : "mastering mongoDB", "isbn" : "101", "price" : 30 }

However, deleting a document can be done in several ways, the most simple of which is by its unique ObjectId:

> db.books.remove("592035f6141daf984112d07f")
WriteResult({ "nRemoved" : 1 })
> db.books.find()
>

We can see here that when there are no results, the mongo shell will not return anything other than the shell prompt itself >.