JSON Schema - nested property is required based on conditional

Question

I tried to create a minimum viable example for this. What I want to achieve is the following:

1. if json contains post, then make both a and b required in attrs
2. if json contains patch then make neither of them required.
3. Use datamodel-codegen to generate pydantic dataclasses based on this schema.

I believe that there is a way to validate by doing the following, however I can't do it this way. For some reason the model generator being used does not properly set the dataclass properties to required/optional when defined this way.

"attrs": {
    "additionalProperties": false,
    "properties": {
        "a": {
            "$ref": "#/$defs/a"
        },
        "b": {
            "$ref": "#/$defs/b"
        }
    }
},
"create": {
    "attrs": {
        "$ref": "#/$defs/attrs",
        "required": ["a", "b"]
    }
}

leads to

class A(BaseModel):
    __root__: str


class B(BaseModel):
    __root__: str


class Attrs(BaseModel):
    class Config:
        extra = Extra.forbid

    a: Optional[A] = None
    b: Optional[B] = None


class Create(BaseModel):
    attrs: Attrs


class Update(BaseModel):
    attrs: Optional[Attrs] = None


class Model(BaseModel):
    patch: Optional[Update] = None
    post: Optional[Create] = None

This is actually incorrect because it implies that each individual attribute is optional. I assume this is because of where in the schema I defined the required property.

Instead, I would like to see if there is a way to define the required and optional properties in the definition of attrs in the json schema.

This is the schema I made as a template as an example. I tried using if and then conditions but I couldn't figure it out.

{
    "properties": {
        "patch": {
            "$ref": "#/$defs/update"
        },
        "post": {
            "$ref": "#/$defs/create"
        }
    },
    "$defs": {
        "create": {
            "properties": {
                "attrs": {
                    "$ref": "#/$defs/attrs"
                }
            }
        },
        "update": {
            "properties": {
                "attrs": {
                    "$ref": "#/$defs/attrs"
                }
            }
        },
        "attrs": {
            "additionalProperties": false,
            "properties": {
                "a": {
                    "$ref": "#/$defs/a"
                },
                "b": {
                    "$ref": "#/$defs/b"
                }
            }
        },
        "a": {
            "type": "string"
        },
        "b": {
            "type": "string"
        }
    }
}

How can I define the required properties of attrs based on the conditional, that being that the top level property contains post or patch?