I have two models that have a many-to-one relationship:

class model_a(models.Model):
  ...
  model_b_id = fields.Many2one('Some name', comodel_name='model_b')
  ...

class model_b(models.Model):
  ...
  model_a_ids = fields.One2many('Some name', comodel_name='model_a', inverse_name='model_b_id', readonly=True)
  ...

Now my goal is to have two filters, one showing only those entities of model B that have at least one entity of model A assigned (model_a_ids is not empty), and one showing only those entities of model B that have no entity of model A assigned (model_a_ids is empty).

I managed to acheive the first goal with the following filter:

<filter string="Assigned" domain="[('model_a_ids', '!=', [])]"/> 

But my attempts to create the opposite filter failed. I tried the following:

<filter string="Unassigned 1" domain="[('model_a_ids', '=', [])]"/>
<filter string="Unassigned 2" domain="['!', ('model_a_ids', '!=', [])]"/>

Both filters do not work, with both filters the result is empty (although there are entities fulfilling this criterion). And there is no error message in the log either.

How can I achieve my goal?