def _load_has_relations(
        self, attrs, resource_name, resource_model, service_context
    ):
        """
        Load related resources, which are defined via a ``has``
        relationship but conceptually come in two forms:

        1. A reference, which is a related resource instance and can be
           ``None``, such as an EC2 instance's ``vpc``.
        2. A subresource, which is a resource constructor that will always
           return a resource instance which shares identifiers/data with
           this resource, such as ``s3.Bucket('name').Object('key')``.
        """
        for reference in resource_model.references:
            # This is a dangling reference, i.e. we have all
            # the data we need to create the resource, so
            # this instance becomes an attribute on the class.
            attrs[reference.name] = self._create_reference(
                reference_model=reference,
                resource_name=resource_name,
                service_context=service_context,
            )

        for subresource in resource_model.subresources:
            # This is a sub-resource class you can create
            # by passing in an identifier, e.g. s3.Bucket(name).
            attrs[subresource.name] = self._create_class_partial(
                subresource_model=subresource,
                resource_name=resource_name,
                service_context=service_context,
            )

        self._create_available_subresources_command(
            attrs, resource_model.subresources
        )

        Load related resources, which are defined via a ``has``
        relationship but conceptually come in two forms:

        1. A reference, which is a related resource instance and can be
           ``None``, such as an EC2 instance's ``vpc``.
        2. A subresource, which is a resource constructor that will always
           return a resource instance which shares identifiers/data with
           this resource, such as ``s3.Bucket('name').Object('key')``.
        
boto3/resources/factory.py