Summary

This PR adds support for Pydantic's Field(discriminator='...') syntax in event handler validation, enabling tagged unions to work correctly with the validation middleware.

Fixes #5953

Changes

• Enhanced get_field_info_annotated_type() in params.py to handle Field(discriminator) + Body() combinations
• Modified ModelField.__post_init__() in compat.py to preserve discriminator metadata when creating TypeAdapters
• Added comprehensive tests for discriminator validation and other Field features

Testing

• Added test_field_discriminator_validation() to verify discriminator functionality with tagged unions
• Added test_field_other_features_still_work() to ensure other Field features remain functional
• All existing tests pass without modification

Example Usage

from typing import Literal
from typing_extensions import Annotated
from pydantic import BaseModel, Field
from aws_lambda_powertools.event_handler import APIGatewayRestResolver
from aws_lambda_powertools.event_handler.openapi.params import Body

app = APIGatewayRestResolver(enable_validation=True)

class FooAction(BaseModel):
    action: Literal["foo"]
    foo_data: str

class BarAction(BaseModel):
    action: Literal["bar"]
    bar_data: int

# Now works with Field discriminator
Action = Annotated[FooAction | BarAction, Field(discriminator="action")]

@app.post("/actions")
def create_action(action: Annotated[Action, Body()]):
    return {"received": action.model_dump()}

Checklist

• Tests are passing
• Linting and type checking pass
• Follows project contribution guidelines
• Maintains backward compatibility

Acknowledgment

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