Use Pydantic TypeAdapter for encoding/decoding JSON fields

[jmbledsoe]

Is your feature request related to a problem? Please describe.

I am using Postgres JSONB columns for storing complex data structures, and I have Pydantic models representing those data structures. Where the column itself is a Pydantic model, I can use JSONField to encode and decode the model:

• When encoding, the field uses model_dump to convert the model to a Python object here
• When decoding, the field uses the __init__ to construct a model from a Python dictionary here

However, some other columns are not themselves Pydantic models but are composite objects that include Pydantic models e.g. list[SomePydanticModel] or dict[str, SomePydanticModel]. I cannot use JSONField for these columns because the instances themselves are not Pydantic models i.e. they do not have the model_dump function and the field type is not ModelMetaclass.

Describe the solution you'd like
Pydantic provides the TypeAdapter class for working with composite objects like this. Its dump_python function behaves like model_dump, and its validate_python function behaves like a model's __init__ function. Using the functions on TypeAdapter would allow JSONField to handle composite models in the same way that it handles simple models.

Describe alternatives you've considered
I currently have an implementation of a Tortoise ORM field that uses TypeAdapter. My implementation bypasses the encoder and decoder functions, opting instead to use dump_json and validate_json directly instead. I intend to open a PR with this implementation, but I'm open to guidance to do something differently to bring this capability into Tortoise ORM.

Additional context
I am not 100% sure of the performance cost of instantiating TypeAdapter, so we may want to include some form of caching constructed instances.

[LanceMoe]

The changes there were primarily made to generate better OpenAPI documentation through TypeHints( #1702 ). I didn’t take more complex cases. If you have a better solution, I think submitting a PR would be worth a try. :)

[henadzit]

Can you please post here your implementation of your field?