[BUG] BackLink causes error when using model_json_schema()

[gdlmx]

Describe the bug
Generating Json Schema from a document with a BackLink field will fail with the error: PydanticInvalidForJsonSchema.

To Reproduce

from typing import List
from pydantic import Field
from beanie import Document, Link, BackLink

class Task(Document):
    detail: str
    owner:  Link["User"]

class User(Document):
    name: str
    tasks: List[BackLink["Task"]] = Field(json_schema_extra={"original_field": "owner"})

User.model_json_schema()

The last line will raise error:

PydanticInvalidForJsonSchema: Cannot generate a JsonSchema for core_schema.PlainValidatorFunctionSchema ({'type': 'with-info', 'function': <function BackLink.build_validation.<locals>.validate at 0x7f89140bdd00>})

Expected behavior
The last line (model_json_schema) should return

{
  "properties": {
    "_id": { "...":"..." },
    "name": {  "title": "Name",  "type": "string"  },
    "tasks": { "...":"..." }
  },
  "required": [ "name", "tasks" ],
  "title": "User",
  "type": "object"
}

Additional context
Python version: 3.11
Beanie version: 1.26.0
Pydantic version: 2.8.2

[gdlmx]

Possible cause of this error:

beanie/beanie/odm/fields.py

Lines 473 to 479 in 51e73eb

	@classmethod
	def __get_pydantic_core_schema__(
	cls, source_type: Any, handler: GetCoreSchemaHandler
	) -> CoreSchema: # type: ignore
	return core_schema.with_info_plain_validator_function(
	cls.build_validation(handler, source_type)
	)

See this discussion of a similar error with Pydantic v2.4

A correct implementation already exists on the Link class.

beanie/beanie/odm/fields.py

Lines 408 to 430 in bdb6a86

	@classmethod
	def __get_pydantic_core_schema__(
	cls, source_type: Any, handler: GetCoreSchemaHandler
	) -> CoreSchema: # type: ignore
	return core_schema.json_or_python_schema(
	python_schema=plain_validator(
	cls.build_validation(handler, source_type)
	),
	json_schema=core_schema.typed_dict_schema(
	{
	"id": core_schema.typed_dict_field(
	core_schema.str_schema()
	),
	"collection": core_schema.typed_dict_field(
	core_schema.str_schema()
	),
	}
	),
	serialization=core_schema.plain_serializer_function_ser_schema( # type: ignore
	lambda instance: cls.serialize(instance),
	when_used="json", # type: ignore
	),
	)

[adeelsohailahmed]

Thanks for reporting the issue, and taking the time to investigate the cause. Would you like to send a PR to fix this issue, @gdlmx?

[xalien10]

@adeelsohailahmed @gdlmx Any updated about this fix yet?

[gdlmx]

Thanks for reporting the issue, and taking the time to investigate the cause. Would you like to send a PR to fix this issue, @gdlmx?

Yes, I can prepare a PR to fix it. Could you provide me some idea about the proper json schema for BackLink?

Given this line

beanie/beanie/odm/fields.py

Line 468 in 51e73eb

return parse_obj(document_class, v)

There is no guarantee of any field in a BackLink object, is there?

[adeelsohailahmed]

There is no guarantee of any field in a BackLink object, is there?

I'm not sure if I fully understand your question here. Could you elaborate more?

[xalien10]

@gdlmx is there any update about the PR? :-)