Support M2M association in post endpoints and model generation.

[destos]

A feature like this is likely highly particular to the implementer's needs and am unsure how to implement it as piccolo and the piccolo api modules currently exist.

I'm unsure how to request the generated pydantic model to include the additional fields needed to save the m2m relations.
At the moment I would like to pass a list of IDs as a new model field that is then used to create the M2M relations. Likely the simplest interface. For others if they are storing more information in the M2M relation I can image they'd need to pass more data in that list.

Any ideas on how to implement?

[dantownsend]

Yeah, we don't really support M2M within Pydantic yet. We need to figure out what a good API for it would be.

You might want to expose the M2M in a separate endpoint. One option is to use PiccoloCRUD with the joining table. Alternatively, creating a custom endpoint, or subclassing the Pydantic model to add a field for M2M IDs.