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How to update a field programmatically when saving a document in Sanity?

6 replies
Last updated: Mar 6, 2023
Afternoon, does anyone know if there’s some way I can update some data programmatically when updating a document? e.g I’d like to update a 
readingTime
hidden field when a user saves a blog post based on the text content in said blog post. Cheers!
AI Update

Hey! Great question. For automatically updating a field like readingTime when a blog post is saved, you have a couple of approaches:

Best Option: Sanity Functions

The modern, recommended way to handle this is with Sanity Functions. These are serverless functions that run on Sanity's infrastructure and can be triggered by document events like publishing or updating.

Here's why Functions are great for your use case:

  • No external infrastructure needed
  • Automatically triggered when documents change
  • Can calculate reading time based on content and update the field programmatically
  • Works across all your datasets

You'd set up a function that triggers on document publish/update, calculates the reading time from your text content, and patches the document with the new readingTime value. Check out the Functions quickstart guide to get started - you can initialize it with the Sanity CLI (v4.5.0+) and deploy it directly.

One important note: Functions don't currently prevent recursive loops, so you'll want to be careful that your function doesn't trigger itself when it updates the readingTime field. You can handle this by checking if the field actually needs updating before patching.

Alternative: Custom Input Component

If you want the calculation to happen client-side in the Studio, you could create a custom input component that calculates reading time whenever the text field changes and updates the hidden field. However, this approach means the calculation only happens when someone is actively editing in the Studio, not when documents are created/updated via API.

For your use case with a hidden field that should auto-update on save, Sanity Functions are definitely the way to go - they'll ensure the reading time is always calculated consistently regardless of how the document is updated.

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6 replies
Thanks, User. Unfortunately this doesn't seem to be able to be used with GraphQL. I was hoping for some kind of 
onAfterUpdate
callback in the schema or something.
Hum. I guess you could create a webhook to write a patch to the dataset whenever the relevant document is published?
However, this seems quite roundabout to me, since if you want to compute the readingTime for a field you could do it
directly in GROQ .
You could additionally have a field in the editor in case you want to override it as well.
Yeah, that's a bit convoluted for the use-case.
You could use a custom input component or a document action. For a custom input, you could use 
renderDefault
so that you don’t actually have to reimplement the component or logic of the field, but use something like a 
useEffect
to write to your hidden field. It would likely write on every keystroke and may impact performance within the Studio. If you use a document action, you can replace the default Publish action so that it updates the field, then completes the action .
Ah, document action would possibly be a goer. Would be good if actions could be run against the normal publish action.

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