Thank you for this tutorial! Unfortunately I can't seem to filter based on this partner_id.sale_corder_count field. I even tried filtering my contact list using this filter without any success. It always return the total amount of customers regardless of the number I add (0,1,2, etc.) I can however add the field in the contact list view with Studio and I can see the number of Sale Orders per contact in the list view but cannot filter using it! 

Also not that the field sale_order_count is not set to be stored or indexed by default. 

Thanks for your help! :)

I have adding Salesperson / Subscription but this is not available.  What are the prerequisites, which I could be missing?

I have tried with your update but fails at finding subscription_state. 

["&", ("partner_id.sale_order_count", ">=", 0), "&", ("state", "=", "sale"), "|", ("partner_id.subscription_count", ">=", 1), ("stage_id.name", "=", "In Progress")]

I'm finding it frustrating looking at the Technical/Database/Models and Fields, as they don't show all the functions for a given object.  Where can I find a full object list and be able to traverse all functions, variables and relationships?

Where can I find documented "partner_id.sale_order_count" and where to find "subscription_state".

subscription_state -  could not find this in the fields list.

Using Odoo 16.

--Update-- 

By looking at a subscription, I see "

• QuotationIn Progress(Stage)

I then used Stage=In Progress.   However, this still doesn't answer how I can find this without all the guess work and poking around, please? Is there an addon that offers intelisense or a proper object model view?