Multi ForeignKey not working

[collerek]

Multi ForeignKey not working

[collerek]

Ok, so this is how you go from parent model to child model in ForeigKey relation:

# you need to go through QuerySet and select related model 'chargingpads' and select all()
# will get you a list of stations (and you created only one) so yours is at index 0
# note that you CAN use instance here (charging_station.objects...) but better be explicit as this can be changed in future releases
station = await ChargingStationOrm.objects.select_related('chargingpads').all()
assert len(station[0].chargingpads) == 4

# the same but get by pk (or any other filter combination that will return one row) so no list and no need for index
# note that if you do not provide filter parameters for now the query is limited to 2 rows - plan to change that in the future
# so if you provide just get() at the end there will be only 2 pads - it's a feature not a bug as empty get should not be issued with select_related
station2 = await ChargingStationOrm.objects.select_related('chargingpads').get(pk=1)
assert len(station2.chargingpads) == 4

This is actually the only not yet implemented load (you can load child from parent: so charging_pad.charging_station.load() and child of ManyToMany with Model.many_to_many_name.all()).

You can find some explanation and discussion in #27.
Probably it will be added but still this produces additional query and this is probably not something you want performance wide -> select related composes a (sometimes complex) join in only one db query -> nested models are later extracted from result in python.

[collerek]

Oh and if you want the other way around use:

# only pads
pads = ChargingPad.object.all()
assert len(pads) == 4

# only pads and only pk field -> will complain if you try to exclude required fields
pads = ChargingPad.object.fields("id").all()
assert len(pads) == 4

# with station
pads = ChargingPad.object.select_related("charging_station").all()
assert len(pads) == 4

Note that ormar never returns you raw data. So even if you limit your query to one field (pk as it cannot be excluded - pk is always returned - note that regardless of the real name you can always set it and get it by pk alias) you will get a model with only one field populated (rest will be None). That's why if you want to exclude fields from result those fields needs to be nullable=True as otherwise pydantic will raise ValidationError. (more on this in fields() and exclude_fields() docs in queries)

So even in the first query above you can still go pads[0].charging_station.load() even if you did not provide select_related(), as the foreignkey database column is filled and ormar provides you with nested CharingStationOrm model with pk only populated (this is a special case where only pk can be set and no ValidationError is raised. If you explicitly set select_related stating that you want a nested model returned from a database and try to exclude required fields it will complain).

If you think that the documentation is not clear in this regard let me know. (Or you can issue a PR - it's always easier to start contributing with the docs). If that answers your question please close the issue :)

[EspenAlbert]

Awesome. Thank you so much for the detailed how-to :)

I feel encouraged to contribute. Let's see what happens ;)