Plans supported by prefetch_related

[hidaris]

prefetch_related is helpful in many situations, is there a plan to support it?

[collerek]

You have select_related that works for both ForeignKey and ManyToMany relations. It's fetching the related models in the same query (same db call) and populating the result with nested models.

[collerek]

Is this something you mean? Maybe the documentation is not clear, let me know.

[hidaris]

I need to try it later. Yes, the documentation could be clearer, because in Django select_related and prefetch_related are different.

[collerek]

Ah yeah, I think I will add prefetch_related (unless you want to create a PR, always welcome).

Right now select_related works in line with django meaning that one join is created and one query is run -> later all related models are extracted and you get a main model with populated nested models.

The difference is that in django select_related can be used only on ForeignKey relation while select_related in ormar can be used on a relation, reverse relation (so virtual foreign key -> second side of the relation) and on many2many relation. But select_related returns a QuerySet and is only run after you issue all(), get() etc.

prefetch_related should be probably implemented in a way that it's an end point (like all() - needs to be awaited).

But even without prefetch_related you can load related models for two out of 3 relation types:

• for related ForeignKey item you can load related model by await Model.related_name.load()
• for ManyToMany items the relation exposes most of the QuerySet api, so you can run await Model.many_to_many_name.all()
• currently there is no way to load reverse relation (second side of the ForeignKey). You can add and remove items, but this is not persisted in a database and no way to load items like this. Was planning to add save_related or something that will go through all nested models and save them if they are not saved yet (status of save is also not yet implemented)

Good idea, thanks.

[hidaris]

Thanks for the detailed answer, but i can't post a pr yet because I'm not familiar with the ormar implementation, or can you explain in the dev docs the proper way to get into the project and contribute?

[collerek]

OK, it took more time and was quite a lot more complicated than I initially thought but should be closed in #54.
In the end I decided to implement it working the same as django, so it's called exactly the same as select_related and returns a QuerySet.

It supports fields, exclude_fields and order_by QuerySet methods, others are called on the original/initial query anyway.

When select related and prefetch_related are called on the same related model always select_related prevails, as prefetch_related is called on a result returned from select_related.

More details in the docs.
Loading children of already loaded models will be handled in #50 (for reverse FK, other already working).

[hidaris]

Wow, very efficient, can't wait to try it!