Constant COLUMNS 

const COLUMNS: &str = "n.id, n.slug, n.created_at, n.published_at, n.published_version,
    n.knowledge_version, n.deleted_at, r.version AS revision, r.kind, r.title,
    r.content_md, r.content_html, r.toc_html, r.summary, r.tags, r.created_at AS revision_at";