langchain/document_loaders/json_loader.py/JSONLoader/_validate_content_key
class JSONLoader:    def _validate_content_key(self, data: Any) -> None:
        """Check if content key is valid"""
        sample = data.first()
        if not isinstance(sample, dict):
            raise ValueError(
                f"Expected the jq schema to result in a list of objects (dict), \
                    so sample must be a dict but got `{type(sample)}`"
            )

        if sample.get(self._content_key) is None:
            raise ValueError(
                f"Expected the jq schema to result in a list of objects (dict) \
                    with the key `{self._content_key}`"
            )

        if self._metadata_func is not None:
            sample_metadata = self._metadata_func(sample, {})
            if not isinstance(sample_metadata, dict):
                raise ValueError(
                    f"Expected the metadata_func to return a dict but got \
                        `{type(sample_metadata)}`"
                )