I'm now trying to import an array of related objects and Sanity want's a unique _key for each entry. How can I get Sanity to just pick its own key during the import?

Great news! You don't have to manually generate _key values for every array item during import. There are a couple of approaches depending on how you're importing:

If you're using sanity dataset import with NDJSON files:

You can use the --replace flag along with missing _key values, and Sanity will automatically generate them during import. However, the most reliable approach is to add the keys to your NDJSON before importing.

For programmatic imports (recommended):

The easiest solution is to use a UUID library to generate _key values as you create your array items. Here's a quick example:

import { v4 as uuidv4 } from 'uuid';
import {createClient} from '@sanity/client'

const client = createClient({...})

const document = {
  _type: 'myDocument',
  items: [
    {
      _key: uuidv4(), // Generates a unique key automatically
      title: 'First item',
      // ... other fields
    },
    {
      _key: uuidv4(),
      title: 'Second item',
      // ... other fields
    }
  ]
}

client.create(document)

You can also use other methods to generate unique keys - they just need to be unique within the array. Some people use simple counters or slugified values, but UUIDs are the safest bet to avoid collisions.

Why _key is required:

As explained in the _key documentation, this property ensures each array item can be uniquely identified and tracked, which is crucial for Sanity's real-time collaborative editing features and proper rendering in Studio.

The key thing to remember: _key only needs to be unique within its containing array, not across your entire dataset (unlike _id for documents). So generating them on-the-fly during your import script with something like uuid is a clean, simple solution that works reliably.