Feature idea: Virtual fields

[radeno]

Issue Overview

Based on this bug in #1268 (comment)

i get an idea about new field attribute property. Let name them just virtual which is boolean (default false) what allows you very interesting thing.

We are using some fields which add properties to post but that properties are not saved for post with defined fields, but in post which is addressed to.

Fox example in simple 1:N association. Yes, simple example. Something new post we associated with another post. It works fine for small datasets.

But when datasets is larger then is is very complicated to handle it.

Eg:

We have two post types. Type and Product. Forget to terms. Because we want custom post pages for TypeCategorys.

Then we define association attribute for Product

                [
                    'id'          => 'type_id',
                    'name'        => __('Type'),
                    'type'        => 'post',
                    'post_type'   => 'type',
                    'field_type'  => 'select_advanced',
                    'placeholder' => __('Types'),
                    'query_args'  => [
                        'post_status'    => 'publish',
                        'posts_per_page' => -1,
                        'order'          => 'ASC',
                        'orderby'        => 'title',
                    ],
                    'multiple' => true
                ]

Everything is fin, normalized and works. We have 200 products and 10 types.
Every product should have more than one type. And we want to add new type for better explanation of product.

Then we must open and edit every product which should have new product type. It is unfriendly, takes a lot time.

There is better solution. Reversing 1:N association but just in user interface not in data.
We could define field in Type post type:

                [
                    'id'          => '_assign_product_id',
                    'name'        => __('Assign Products'),
                    'type'        => 'post',
                    'post_type'   => 'product',
                    'field_type'  => 'select_advanced',
                    'placeholder' => __('Assign Product Id'),
                    'query_args'  => [
                        'post_status'    => 'publish',
                        'posts_per_page' => -1,
                        'order'          => 'ASC',
                        'orderby'        => 'title',
                    ],
                    'multiple' => true
                ]

And then we must add action save_post_type and make function which takes selected fields from saved field _assign_product_id, add them to another post and remove it from type post.

Is there any way how to process it in action when is everything validated and goes to save_post ?