Updating rows in a table with a composite, non-integer primary key? - peewee

Question

I am using peewee ORM and have a table with a composite, non-integer primary key. The key is composed of a timestamp and an integer foreign key field. An example table and the related model are shown below. I have many tables like this for different data sets I'm working with.
class fof2_site_hourly_manual(Base):
descriptor = TextField(null=True)
qualifier = TextField(null=True)
source = ForeignKeyField(db_column='source_id', rel_model=ionosonde)
valid_time = DateTimeField()
value = FloatField(null=True)
class Meta:
db_table = 'fof2_site_hourly_manual'
primary_key = CompositeKey('valid_time','source')
class ionosonde(Base):
name = TextField()
latitude = FloatField()
longitude = FloatField()
instrument_type = TextField()
class Meta:
db_table = 'ionosonde'
Now, I can insert rows with no problem using either the save() method of an instance or the class.insert().execute() approach.
However, I can't update rows by any means I've found. Using either save() or class.update().execute() I get 'duplicate primary key' errors. Fine, of course there's a duplicate, that's why I'm trying to update.
It gets worse, I get the same error just trying to examine a row using class.get(). I can read rows using select().
Is this a known limitation in peewee? The docs do state that support for composite primary keys is pretty basic, but don't explicitly state what the limitations are.
Edit: Here are all the things I have tried so far. I have the fields and values of the product I want to insert or update a row for in a dictionary, lets call it data. For inserts:
obj = class(**data)
obj.save(force_insert=True)
or
class.insert(**data).execute()
both work. However, if the row already exists (the primary key matches the relevant fields in data) then I've tried
obj = class(**data)
obj.save()
and
class.update(**data).execute()
this last one is probably not expected to work (no where clause) but I've included in for completeness in terms of what I've tried. In desperation, I attempted to implement a select, delete then insert loop, but that also fails since:
obj = class.get(**data)
and
obj = class.get_or_create(**data)
also give duplicate key errors if the row exists! That one really bakes my noodle, I'm not even trying to write to the DB!
I can read existing data out using
query = class.select()
and constructing where clauses using the known primary key fields.
Edit:
Ha! Just tried using select() to instantiate an object for a given row, then use delete_instance() to delete it (to then re-insert with the updated data). However, it deleted the entire contents of the table! It really seems as though peewee does not support composite keys, or am I doing something very wrong?