How to migrate data between schemas in Sanity without manual copying?

I understand your situation! You need to change the _type field from projects to uniprojects for existing documents. Unfortunately, the _type field is immutable in Sanity, meaning you cannot directly modify it through the Studio, the migration toolkit, or standard API mutations.

However, there's a reliable workaround using the export/import workflow with NDJSON files. Here's how to do it:

The Export-Modify-Import Approach

1. First, make sure your new schema is ready

Ensure your uniprojects schema is already defined in your Studio before starting. The documents need to validate against the new schema when you import them.

2. Export your dataset

sanity dataset export production

This creates a production.tar.gz file containing all your data in NDJSON format.

3. Extract and modify the NDJSON file

Extract the tar.gz archive to access the data.ndjson file inside. Then, use a text editor or command-line tool to replace the _type value. With command-line tools:

# Extract the archive
tar -xzf production.tar.gz

# Replace _type in the NDJSON file
sed -i 's/"_type":"projects"/"_type":"uniprojects"/g' data.ndjson

Or you can use a script for more control:

const fs = require('fs');

const data = fs.readFileSync('data.ndjson', 'utf8');
const lines = data.split('\n');

const modified = lines.map(line => {
  if (!line.trim()) return line;
  const doc = JSON.parse(line);
  if (doc._type === 'projects') {
    doc._type = 'uniprojects';
  }
  return JSON.stringify(doc);
});

fs.writeFileSync('modified-data.ndjson', modified.join('\n'));

4. Import the modified data

sanity dataset import modified-data.ndjson production

This will import your documents with the new _type value. Since the _id values remain the same, Sanity will update the existing documents rather than creating duplicates.

Important Considerations

• Backup first: The export you just created serves as your backup, but consider keeping it safe before importing
• The _type field is truly immutable: Even the migration toolkit's at('_type', set('uniprojects')) won't work because Sanity's underlying API prevents direct _type changes
• Document IDs are preserved: Your documents keep their original _id values, so references and relationships remain intact
• Test on a development dataset first: If you have a staging or development dataset, practice this workflow there before running it on production

Why Not Use Migrations?

While Sanity's migration toolkit with defineMigration is excellent for most schema changes (renaming fields, transforming content, etc.), it cannot modify the _type field because it's a protected system field. The export/import approach is the officially supported method for this specific use case.

This approach may seem manual, but it's actually quite fast and reliable for changing document types while preserving all your data and relationships!