Problem:
The user would like to filter a view using a taxonomy assigned to the parent post in a Toolset relationship.
Solution:
Actually, this is not possible. Toolset does not offer a way to filter a custom post type by the custom fields or taxonomies from a related post type. You can only filter with:
- The post's default(title, content, date, etc.) and custom fields.
- The post's taxonomies.
- ONE related post type in a Toolset relationship.
As a workaround, I'll suggest syncing the taxonomy from the parent post to the child posts. You can add a custom code, hooked to the save_post action, use the relationship API inside of it to synchronize the taxonomy assignment to the child posts.