We have two custom modules written for Odoo 15, each with their own models. To make things simple and avoid sharing too much information, we'll call them process and tank. A process can have many tanks, but a tank can only have one process. I used a Many2one and One2many field relationship between tanks and processes, and I've shown the field definition lines for each file below.

Whenever I upgrade either the tank parent module or process parent module, I get the error shown below, up until I push literally any commit to Odoo.sh involving the models' definition files. The error example shown below comes from the process parent module. I upgrade the module, and then get this error until I push ANY commit to Odoo.sh (which forces a reload in our case) involving the process.py file. I'm not quite sure of the root cause of the issue, and I've spent about 2 weeks off and on Googling and trying to solve this weird issue. The bandaid fix is enough for development environments, but this application goes live soon so I'd like to get it taken care of.

This is the field definition line from the process.py file. The model name is process:

tank_ids = fields.One2many('tank', 'process_id')

This is the field definition line from the tank.py file. The model name is tank:

process_id = fields.Many2one('process', 'tank_ids')

Below is the full error traceback:

Traceback (most recent call last):
  File "/home/odoo/src/odoo/odoo/api.py", line 886, in get
    return field_cache[record._ids[0]]
KeyError: 1

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/odoo/src/odoo/odoo/fields.py", line 1082, in __get__
    value = env.cache.get(record, self)
  File "/home/odoo/src/odoo/odoo/api.py", line 889, in get
    raise CacheMiss(record, field)
odoo.exceptions.CacheMiss: 'process(1,).total_cost'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/odoo/src/odoo/odoo/api.py", line 886, in get
    return field_cache[record._ids[0]]
KeyError: 1

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/odoo/src/odoo/odoo/fields.py", line 1082, in __get__
    value = env.cache.get(record, self)
  File "/home/odoo/src/odoo/odoo/api.py", line 889, in get
    raise CacheMiss(record, field)
odoo.exceptions.CacheMiss: 'process(1,).tank_ids'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/odoo/src/odoo/odoo/addons/base/models/ir_http.py", line 237, in _dispatch
    result = request.dispatch()
  File "/home/odoo/src/odoo/odoo/http.py", line 698, in dispatch
    result = self._call_function(**self.params)
  File "/home/odoo/src/odoo/odoo/http.py", line 368, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "/home/odoo/src/odoo/odoo/service/model.py", line 94, in wrapper
    return f(dbname, *args, **kwargs)
  File "/home/odoo/src/odoo/odoo/http.py", line 357, in checked_call
    result = self.endpoint(*a, **kw)
  File "/home/odoo/src/odoo/odoo/http.py", line 921, in __call__
    return self.method(*args, **kw)
  File "/home/odoo/src/odoo/odoo/http.py", line 546, in response_wrap
    response = f(*args, **kw)
  File "/home/odoo/src/odoo/addons/web/controllers/main.py", line 1282, in search_read
    return self.do_search_read(model, fields, offset, limit, domain, sort)
  File "/home/odoo/src/odoo/addons/web/controllers/main.py", line 1301, in do_search_read
    return Model.web_search_read(domain, fields, offset=offset, limit=limit, order=sort)
  File "/home/odoo/src/odoo/addons/web/models/models.py", line 62, in web_search_read
    records = self.search_read(domain, fields, offset=offset, limit=limit, order=order)
  File "/home/odoo/src/odoo/odoo/models.py", line 5064, in search_read
    result = records.read(fields, **read_kwargs)
  File "/home/odoo/src/odoo/odoo/models.py", line 3232, in read
    return self._read_format(fnames=fields, load=load)
  File "/home/odoo/src/odoo/odoo/models.py", line 3252, in _read_format
    vals[name] = convert(record[name], record, use_name_get)
  File "/home/odoo/src/odoo/odoo/models.py", line 5902, in __getitem__
    return self._fields[key].__get__(self, type(self))
  File "/home/odoo/src/odoo/odoo/fields.py", line 1131, in __get__
    self.compute_value(recs)
  File "/home/odoo/src/odoo/odoo/fields.py", line 1290, in compute_value
    records._compute_field_value(self)
  File "/home/odoo/src/odoo/odoo/models.py", line 4266, in _compute_field_value
    fields.determine(field.compute, self)
  File "/home/odoo/src/odoo/odoo/fields.py", line 87, in determine
    return needle(*args)
  File "/home/odoo/src/user/mixer_quoting/models/process.py", line 34, in _compute_total_cost
    if len(record.tank_ids) > 0:
  File "/home/odoo/src/odoo/odoo/fields.py", line 3402, in __get__
    return super().__get__(records, owner)
  File "/home/odoo/src/odoo/odoo/fields.py", line 2617, in __get__
    return super().__get__(records, owner)
  File "/home/odoo/src/odoo/odoo/fields.py", line 1108, in __get__
    recs._fetch_field(self)
  File "/home/odoo/src/odoo/odoo/models.py", line 3281, in _fetch_field
    self._read(fnames)
  File "/home/odoo/src/odoo/odoo/models.py", line 3385, in _read
    field.read(fetched)
  File "/home/odoo/src/odoo/odoo/fields.py", line 3419, in read
    group[get_id(line[inverse])].append(line.id)
  File "/home/odoo/src/odoo/odoo/fields.py", line 3411, in <lambda>
    get_id = (lambda rec: rec.id) if inverse_field.type == 'many2one' else int
Exception

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/odoo/src/odoo/odoo/http.py", line 654, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "/home/odoo/src/odoo/odoo/http.py", line 301, in _handle_exception
    raise exception.with_traceback(None) from new_cause
AttributeError: '_unknown' object has no attribute 'id'