Fix order checkout flow completion with custom steps

core/lib/spree/core/state_machines/order.rb

@@ -73,7 +73,7 @@ def define_state_machine!
               end
 
               event :complete do
-                transition to: :complete, from: :confirm
+                transition to: :complete, from: klass.checkout_steps.keys.last
               end
 
               if states[:payment]

core/spec/models/spree/order/state_machine_spec.rb

@@ -119,4 +119,42 @@
       end
     end
   end
+
+  describe "#complete" do
+    context "when the confirm step has been taken out of the checkout flow" do
+      let!(:payment) { create(:payment, state: 'checkout', order: order) }
+
+      before :all do
+        class Spree::Order
+          checkout_flow do
+            go_to_state :address
+            go_to_state :delivery
+            go_to_state :payment, if: ->(order) { order.payment_required? }
+            # confirm step has been removed. Payment is the last step now
+          end
+        end
+      end
+
+      after :all do
+        # restore the standard checkout flow to avoid leaking into other tests
+        class Spree::Order
+          checkout_flow do
+            go_to_state :address
+            go_to_state :delivery
+            go_to_state :payment, if: ->(order) { order.payment_required? }
+            go_to_state :confirm
+          end
+        end
+      end
+
+      before do
+        order.update!(state: 'payment')
+      end
+
+      it 'will properly transition from the last checkout flow state to complete' do
+        order.complete!
+        expect(order.state).to eq 'complete'
+      end
+    end
+  end
 end

guides/source/developers/orders/order-state-machine.html.md

@@ -25,6 +25,30 @@ The `complete` state is triggered in one of two ways:
 2. Payment is required on the order, and at least the order total has been
    received as payment.
 
+## Customizing the checkout flow
+The central steps in the checkout flow – `address`, `delivery`, `payment` and
+`confirm`, are customizable. For example, you might decide you want skip the
+`confirm` step and proceed directly from `payment` to `complete`. To modify your
+checkout flow, create a decorator similar to the following:
+
+```ruby
+# /app/models/mystore/order_decorator.rb
+
+module MyStore::OrderDecorator
+  def self.prepended(base)
+    base.checkout_flow do
+      go_to_state :address
+      go_to_state :delivery
+      go_to_state :payment, if: ->(order) { order.payment_required? }
+      # NOTE: confirm state is commented and will NOT be part of the flow
+      # go_to_state :confirm
+    end
+  end
+
+  Spree::Order.prepend self
+end
+```
+
 ## State criteria
 
 Each order state has criteria that must be met before the state can change. For