Set up a computed field with an appropriate validator

Hi!

I am using CKAN 2.9 and I would like to have a field that is not set up by the user in the form, but computed separately and then displayed.

I started to set form_snippet to null, but by doing this, the field is always reset to null each time I try to modify the dataset.

I have been advised to use the [keep_old_value_if_missing](https://github.com/vrk-kpa/api-catalog/blob/1050ff51f04ebd061a3d73da130e662887076f7b/ckanext/ckanext-apicatalog_scheming/ckanext/apicatalog_scheming/validators.py#L99) validator. When a value for a given field on the form is missing (which is my case), the previously existing value is given.

This is a problem for me because, at the beginning, there is no previously existing value. The validator as it is now cannot work for me, unless I manage to give a random default value at the beginning.

How can I modify this validator so that it does what I want?

Start with the linked validators.py implementation of keep_old_value_if_missing and the CKAN 2.9 form handling for fields with form_snippet set to null. Reproduce dataset creation and editing with a computed field that has no initial value, then determine the expected validator behavior and add coverage for that case.

Written by the indexing model from the issue text.