I am running Odoo 12.0-20201125 (Community Edition) and developing, if possible, using the developer mode in the web interface. I don't mind getting into the back-end if this problem needs it, though.

I have the following behaviour: any number of pieces of equipment (maintenance.equipment) can be assigned to any number of event records (event.event) accomplished using a many2many field.

What I would like is to be able to filter pieces of equipment to only show those not already in use on the same date as the current record (based on event start date). That is, if I have 'equipment1' and 'equipment2' already assigned to 'event1' that starts of 05/12/2020 I could not then assign 'equipment1' or 'equipment2' to 'event2' that also starts on 05/12/2020 (but I could assign 'equipment3' to 'event2', say).

To try and accomplish this, I set up another many2many field in Equipment that would show the Events to which that piece of equipment was assigned. I then accessed this field using a domain filter on the Event form view in the hope of saying 'if any of the start dates in this list of events match the start date of this Event record, filter out this piece of equipment'. I got close to the exact opposite of the desired behaviour by doing (in the Event form view):

<field name="x_equipment" widget="many2many_tags" domain="[('x_equipment_event_ids.date_begin', '=', date_begin)]"/>

this filters the field to show me only equipment that is already being used for an event with the same start date. To generate the logical opposite, I tried to filter for 'not equal to' -- like:  domain="[('x_equipment_event_ids.date_begin', '!=', date_begin)]" -- but that let through any equipment already assigned on any date. I have since tried every combination of logic I can find having scoured previous forum posts and the documentation on domain filters. (Putting '!' at the start of the expression, using 'like', 'ilike', 'not like', 'not ilike', 'in', 'not in'.)

It may be that I am missing a more obvious way to accomplish this or that I need to get into the back-end to manage it. Either way, I'd be most grateful for a solution or any help anyone can provide.