Post
    subject
    body
    source_id
    status_id

Status
    id
    label
    other_field

Source
    id
    label
    other_field


View
   create view read_only_posts as
   SELECT statuses.label as status, sources.label as source, posts.*
   from posts
   left join statuses on statuses.id = posts.status_id
   left join sources on sources.id = posts.source_id

// Post.php
class Post extends Model
{
    //
}

// ReadOnlyPost.php
class ReadOnlyPost extends Post
{
    protected $table = 'read_only_posts';
}

// Post.php Model
public function tags()
{
    return $this->morphToMany(Tag::class, 'taggable');
}

// AppServiceProvider
public function boot()
{
    Relation::morphMap([
        'properties' => Post::class,
        'properties' => ReadOnlyPost::class, <--- Can't do this