Cloudant Fundamentals: The _id

Unlocking the mysteries of the unique _id field (part 2 of 10)


Last time we looked at how to design a JSON document schema that models the data in your application. I didn’t mention a vital field: the document’s _id.

Every Cloudant document has an _id - if you don't supply one when you write a new document then Cloudant will generate one for you. Letting Cloudant make an _id for you is the easiest solution, but there are some cases where you might want to keep control of the _id field for yourself.

The _id field is a document's unique identifier in a database and as such, it is indexed. This means that Cloudant can retrieve a document given its _idvery quickly by consulting the index - without having to page through all the documents in the collection to find the right one.

The document _id is unique across the database. Photo by Rahul Shanbhag on Unsplash

The primary index used to write and retrieve documents by the _id field also keeps the documents in id-order on disk. We can use this to our advantage when we design your own _id values as we can employ the primary index to perform range queries e.g find documents where _id is less than, greater than or between supplied values. See the GET /db/_all_docs API endpoint for querying the primary index.

Let’s say we are storing user objects in a Cloudant database. It would be perfectly valid to store documents like this:

We omit the _id field and let Cloudant pick one. Something like e87a03636ee3d9d0943cd1f35f431fe7 will be generated on our behalf.

If we know something unique about our user, such as their email address, we could modify the document to look like this:

Now we are storing the user type and the email address in the _id field. This means we can use the primary index to add a little value. We can query the primary index to get a list of all documents whose ids start with user: and the returned list will be in email address order.

I want my auto-incrementing values back

If you’re used to relational databases, you may be familiar with auto-incrementing primary keys. The key starts at “1” for the first record and the number increments each time — easy! With Cloudant, you either get Cloudant to generate a unique id for you, or you create your own. If you want your document’s ids to be “1”, “2”, “3” etc, it’s up to you to keep track of where you’re up to!

How do I generate my own unique identifier

There are libraries that can generate unique identifiers for you such as the uuid package for Node.js:

Alternatively you could ask Cloudant to supply a list of ids for you with the GET /_uuids API call.

Can I edit an _id once it’s in the database?

Although you can edit a document body, you can’t change a document’s id. There’s nothing stopping you deleting the unwanted document and creating a new one. You can even do both the delete and the insert operations at the same time using a POST /db/_bulk_docs request.

Next time

In the next post, we’ll unlock the mysteries of the _rev token.

Originally published on May 11, 2018.