def taxon_parents
taxons = Taxon.where(parent_id: 0)
[['顶级分类', '0']] + taxons.map { |taxon| [taxon.name, taxon.id] }
end
<%= f.input :parent_id, as: :select, collection: taxon_parents, label: '上级导航', prompt: '上级导航', input_html: {class: 'is-invalid'}, required: true %>