One local product's field-by-field diff against its matched Shopify product. changes only contains fields that actually differ, keyed by field atom, each holding %{current:, incoming:}. price_extreme? is true only when :price is in changes and the current/incoming ratio exceeds 3x.

base_locale is the locale diff/4 matched/compared this product against — carried on the struct (not re-derived later) so Shopify.Sync.apply_change/2 writes localized fields back into the SAME locale they were diffed against. Reading a fresh default at apply time would silently apply a change diffed in one locale into another.

create?/shopify_product are only set on a create-Change (see ProductDiff.new_product_changes/3, catalogue source only): product_uuid is nil (there is no local product yet), changes is always %{}, and shopify_product carries the raw Shopify payload Shopify.Sync.apply_change/2 hands to PhoenixKitEcommerce.Catalogue.Writer.create_from_shopify/2. A regular diff-Change never sets shopify_product.

product_id is the Shopify product's own "id" — carried on every Change (regular or create), independent of changes/opts[:only]: it isn't diffed against a current value, just carried through, so Shopify.Sync.apply_change/2 can backfill data["ecommerce"] ["shopify"]["product_id"] on a matched item even when the caller only asked to apply a subset of the comparable fields. This does NOT reach every local product on its own — diff/4 still only returns a Change when some comparable field actually differs (see its own moduledoc); product_id just rides along on whichever Changes already exist.