Describe the bug
Given the following OpenAPI yaml:

openapi: 3.0.0
components:
  schemas:
    InventoryItem:
      type: object
      properties:
        item-id:
          type: string
        external-ids:
          type: array
          items:
            type: string
      oneOf:
        - required: [ 'item-id' ]
        - required: [ 'external-ids' ]

And the following generation commands:

datamodel-codegen --input inventory.yaml --input-file-type openapi --output inventory.py --output-model-type pydantic_v2.BaseModel

The resulted model is:

from __future__ import annotations
from typing import List, Optional, Union
from pydantic import BaseModel, Field, RootModel


class InventoryItem1(BaseModel):
    item_id: str = Field(..., alias='item-id')
    external_ids: Optional[List[str]] = Field(None, alias='external-ids')


class InventoryItem2(BaseModel):
    item_id: Optional[str] = Field(None, alias='item-id')
    external_ids: List[str] = Field(..., alias='external-ids')


class InventoryItem(RootModel[Union[InventoryItem1, InventoryItem2]]):
    root: Union[InventoryItem1, InventoryItem2]

The following action fails:

>>> from inventory import InventoryItem
>>> InventoryItem(**{'item-id': '123'}).item_id

Traceback (most recent call last):
  File "$PYDIR/site-packages/pydantic/main.py", line 761, in __getattr__
    raise AttributeError(f'{type(self).__name__!r} object has no attribute {item!r}')
AttributeError: 'InventoryItem' object has no attribute 'item_id'

While the following succeeds:

>>> from inventory import InventoryItem
>>> InventoryItem(**{'item-id': '123'}).root.item_id
123

Or using the following generation command for pydantic v1 models

datamodel-codegen --input inventory.yaml --input-file-type openapi --output inventory.py

The resulted model is:

from __future__ import annotations
from typing import List, Optional, Union
from pydantic import BaseModel, Field


class InventoryItem1(BaseModel):
    item_id: str = Field(..., alias='item-id')
    external_ids: Optional[List[str]] = Field(None, alias='external-ids')


class InventoryItem2(BaseModel):
    item_id: Optional[str] = Field(None, alias='item-id')
    external_ids: List[str] = Field(..., alias='external-ids')


class InventoryItem(BaseModel):
    __root__: Union[InventoryItem1, InventoryItem2]

The following action fails:

>>> from inventory import InventoryItem
>>> InventoryItem(**{'item-id': '123'}).item_id

Traceback (most recent call last):
  "$PYDIR/site-packages/pydantic/_internal/_model_construction.py", line 92, in __new__
    private_attributes = inspect_namespace(
  File "$PYDIR/site-packages/pydantic/_internal/_model_construction.py", line 316, in inspect_namespace
    raise TypeError("To define root models, use `pydantic.RootModel` rather than a field called '__root__'")
TypeError: To define root models, use `pydantic.RootModel` rather than a field called '__root__'

Expected behavior
Being able to access pydantic v2 model properties directly such as model.item_id and not model.root.item_id.

Version:

• OS: WSL2 Ubuntu20
• Python version: 3.8.10
• datamodel-code-generator version: 0.25.1 and main branch (today's pull)