I have a JSON field (form_config) in Postgres table which contains form field structure. This is the value in that table.

[
  {
    'label': 'Age',
    'type': 'number',
    'required': True,
    'min': 0,
    'max': None
  },
  {
    'label': 'Name',
    'type': 'text',
    'required': True,
    'minLength': 0,
    'maxLength': None
  },
  {
    'label': 'City',
    'type': 'autocomplete',
    'required': True,
    'api': {
      'url': 'api/countries',
      'method': 'GET',
      'labelKey': 'name'
    }
  }
]

There is an API which fetches above data, and it returns based on Response Schema as below:

class FormConfigResponse(BaseModel):
    config : List[FormFieldModel]

where Form Field schema are as follows, using discriminator:

class FormFieldType(str, Enum):
    text        =   'text'
    number     =   'number'
    autocomplete=   'autocomplete'

class FormFieldBase(BaseModel):
    label: str
    type: FormFieldType    
    required: bool = True

class TextField(FormFieldBase):
    type: Literal[FormFieldType.text] = FormFieldType.text
    minLength: int|None
    maxLength: int|None

class NumberField(FormFieldBase):
    type: Literal[FormFieldType.number] = FormFieldType.number
    min: int|None
    max: int|None

class APIAutoCompleteConfig(BaseModel):
    url: str
    method: str
    labelKey: str

class AutoCompleteField(FormFieldBase):
    type: Literal[FormFieldType.autocomplete] = FormFieldType.autocomplete
    api: APIAutoCompleteConfig    

# Create a new model to represent the discriminated union
FormFieldModel = Annotated[Union[NumberField, TextField, AutoCompleteField], Field(discriminator='type')]

In my API handler code, when it returns the data as follows:

return FormConfigResponse(
    config = form_config
)

I get an error like so:

fastapi_app  |     content = await serialize_response(
fastapi_app  |   File "/usr/local/lib/python3.10/site-packages/fastapi/routing.py", line 155, in serialize_response
fastapi_app  |     raise ResponseValidationError(
fastapi_app  | fastapi.exceptions.ResponseValidationError: 1 validation errors:
fastapi_app  |   {'type': 'model_attributes_type', 'loc': ('response', 'config'), 'msg': 'Input should be a valid dictionary or object to extract fields from', 'input': [NumberField(label='Age', type=<FormFieldType.number: 'number'>, required=True, min=0, max=None), TextField(label='Name', type=<FormFieldType.text: 'text'>, required=True, minLength=0, maxLength=None), AutoCompleteField(label='City', type=<FormFieldType.autocomplete: 'autocomplete'>, required=True, api=APIAutoCompleteConfig(url='api/countries', method='GET', labelKey='name'))], 'url': 'https://errors.pydantic.dev/2.8/v/model_attributes_type'}

It’s saying “Input should be a valid dictionary or object to extract fields from”. Where am I going wrong?