We have a field in a block which is an HTML field. we want to convert this to ST. However, how can I get all the contents which have this particular block in ST body field?
Filtering records based on a nested field is currently not supported, but it is an ongoing feature request as you can see here: Deep filtering on both CMA and CDA
There are however several ways we can do that without directly filtering all records for nested content, you can possibly get some ideas from the example script on field conversions here: GitHub - datocms/structured-text-migration-example: Some sample migrations to convert HTML/Markdown/Modular content fields into Structured Text
That also has an HTML to Structured Text conversion example.
Hope this helps!