Up and Running with NoSQL Databases
Illustration by John Hersey

Querying attachments


From:

Up and Running with NoSQL Databases

with Joseph LeBlanc

Video: Querying attachments

Once attachments have been uploaded to your CouchDB database, you'll want to be able to reference them easily. Let's create a view to help us get all of the attachment names. First, let's navigate to the restaurant database. Then head over to the View dropdown, and select Temporary View. If you have the default function that just emits null on the document, click Run. Now, let's narrow that down to the Attachments field, add in a dot, and then underscore, and then attachments.

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Watch the Online Video Course Up and Running with NoSQL Databases
56m 57s Intermediate Nov 30, 2012

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Explore the possibilities NoSQL databases offer developers for unparalleled flexibility and performance. In this course, author Joseph LeBlanc shows how to build an entire web application with CouchDB and introduces techniques such as querying with JavaScript, working with nested data, and creating views. He also explains some of the trade-offs you might encounter with NoSQL databases, such as the complexity of partitioning data across multiple servers, and presents five commonly used NoSQL databases: CouchDB, MongoDB, Cassandra, Riak, and Redis.

Topics include:
  • What is NoSQL?
  • Installing CouchDB on Mac, Windows, and Linux
  • Storing data
  • Nesting document data
  • Specifying search criteria
  • Reducing data
  • Attaching and retrieving images
  • Deploying applications
  • Securing CouchDB
  • Understanding the CAP theorem
Subjects:
Developer IT
Software:
NoSQL CouchDB MongoDB Cassandra Riak Redis
Author:
Joseph LeBlanc

Querying attachments

Once attachments have been uploaded to your CouchDB database, you'll want to be able to reference them easily. Let's create a view to help us get all of the attachment names. First, let's navigate to the restaurant database. Then head over to the View dropdown, and select Temporary View. If you have the default function that just emits null on the document, click Run. Now, let's narrow that down to the Attachments field, add in a dot, and then underscore, and then attachments.

You'll notice that some of our documents don't have an attachments field, while the other ones do. We want to have a function that only returns the documents that have attachments. So let's go to the exercise files and get one. Open up map.js and copy the function. Then paste that function over the existing map function. The ultimate goal here is to get one file name per document.

This function is a little bit complicated, so let's step through it. First, we are detecting whether or not the document has an attachments field. If it doesn't have an attachments field, we just skip over it. First, we're going to get one file name for each document, and then emit the name field for the document as the key and the file name as the value. To actually get that file name, we have a very complicated looking loop. We are using JavaScript's for-in construct to get the object properties of the attachments field.

This gives us each of the property names as a key. And then we run two tests on this key. First, we want to make sure that this key actually belongs to this attachments object, and then we want to make sure this key does not actually point to a function. In JavaScript, we are not guaranteed the ordering of the object properties. So we use these checks to make sure that we just get the file name. Once we have that file name, we stop the loop and emit. Now let's click the Run button.

This view has everything we need. We have the document name as the key, and the file name as the value. We also have the document ID handy. This will help us construct the URLs we need to point to the images. Now, let's click Save As so we can save this view. For the design document, type menu, and for the view name type items. So now we have this as a permanent view in our system. We can also view the raw data. Remove everything from the question mark down to the root.

So now we have that same data available in JSON format. Creating a view is helpful for getting a clean list of attachment file names. With this view in place, you can now easily build applications based around it.

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