app/views/admin/fields/_subform.html.haml
= fields_for :field, @field do |f|
- if (as = f.object.as).present? and lookup_context.exists? "#{as}_field", 'admin/custom_fields', true
= render partial: "admin/custom_fields/#{as}_field", locals: {f: f}
- else
= render partial: "admin/custom_fields/base_field", locals: {f: f}