This extension solves two use cases related to variants. I call them Ad Hoc Options and Product Customizations.
Ad Hoc Options
Use these when have numerous (possibly price-altering) product options and you don't want to create variants for each combination.
You can also restrict certain combinations of options from coexisting. These are called Ad Hoc Exclusions.
Use these when you want the ability to provide a highly customized product e.g. "Cut to length 5.82cm", "Engrave 'thanks for the memories'", "Upload my image". Full control over pricing is provided by the Spree calculator mechanism.
TODO I have only tested with V2.0, unsure if compatible with other versions as of yet.
# see the notes in Versionfile if you are using an older version of spree gem 'solidus_flexi_variants', github: 'boomerdigital/solidus_flexi_variants', branch: 'master' bundle install bundle exec rails g solidus_flexi_variants:install
Build a 'Cake' product using Ad Hoc Options and Product Customizations
Build a 'Necklace' product using Ad Hoc Options and Product Customizations
Build a 'Pizza' product using Ad Hoc Options. Note that the 'multi' option checkboxes come from a partial named after the option name (see app/views/products/ad_hoc_options/_toppings.html.erb)
See the wiki for more detail.