How to publish custom models through plugin

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I am currently working on a plugin that spawns objects into the gazebo world. My approach is to use a transport node and messenger to publish the object, just like in the tutorials. I am running into trouble because there doesn't seem to be a way to publish an object without essentially copying out the whole SDF and publishing that. Is there a way to publish object by using only the URI and pose, just like how it's done with the <include> tag?

    // Create a new transport node, initialize with world, and create publisher
    transport::NodePtr node(new transport::Node());
    node->Init(_world->Name());
    transport::PublisherPtr factoryPub = node->Advertise<msgs::Factory>("~/factory");

    // create a model and fill it with the correct values
    msgs::Model model;

    // the line below is where I want to add URI to the model
    model.set_sdf_filename(attURI);
    model.set_name(attName);
    msgs::Set(model.mutable_pose(), ignition::math::Pose3d(10, 10, 10, 1,2,3));

    // create a string of the model sdf
    std::ostringstream newModelStr;
    newModelStr << "<sdf version='" << SDF_VERSION << "'>"
    << msgs::ModelToSDF(model)->ToString("")
    << "</sdf>";

    // publish the model
    msgs::Factory msg;
    msg.set_sdf(newModelStr.str());
    factoryPub->Publish(msg);