feat(fixed-charges): Add fixed charge boundaries to billing period (#4492)

## Roadmap Task

👉
https://getlago.canny.io/feature-requests/p/allow-add-ons-to-be-added-to-subscription-invoices

👉
https://getlago.canny.io/feature-requests/p/define-quantities-for-plan-charges

 ## Context

 ### What is the current situation?

 **Option 1:** User has to create a one off invoice alongside the
 subscription, it will create 2 different invoices.

 **Option 2:** User can add a recurring billable metric and use event to
 have this fee invoice on subscription renewal, but it won’t appear on
 the first billing subscription.

 ### What problem are we trying to solve?

 At subscription creation or afterward, there is no clear way to invoice
 a fixed fee that is not tied to events, aside from the subscription fee
 itself. This fee could be either a one-time charge or a recurring one.

 ## Description

 Add fixed charge boundaries to billing period serializer.
 
We need to use safe operator as existing invoice subscription does not
have fixed charges boundaries.

Author: ancorcruz

app/serializers/v1/invoices/billing_period_serializer.rb

@@ -12,6 +12,8 @@ def serialize
           subscription_to_datetime: model.to_datetime.iso8601,
           charges_from_datetime: model.charges_from_datetime.iso8601,
           charges_to_datetime: model.charges_to_datetime.iso8601,
+          fixed_charges_from_datetime: model.fixed_charges_from_datetime&.iso8601,
+          fixed_charges_to_datetime: model.fixed_charges_to_datetime&.iso8601,
           invoicing_reason: model.invoicing_reason
         }
       end

spec/factories/invoice_subscriptions.rb

@@ -15,6 +15,8 @@
       to_datetime { timestamp.end_of_month }
       charges_from_datetime { from_datetime - 1.month }
       charges_to_datetime { to_datetime.end_of_month }
+      fixed_charges_from_datetime { from_datetime.beginning_of_month }
+      fixed_charges_to_datetime { to_datetime.end_of_month }
     end
   end
 end

spec/serializers/v1/invoices/billing_period_serializer_spec.rb

@@ -10,15 +10,27 @@
   it "serializes the object" do
     result = JSON.parse(serializer.to_json)
 
-    aggregate_failures do
+    expect(result["billing_period"]["lago_subscription_id"]).to eq(invoice_subscription.subscription_id)
+    expect(result["billing_period"]["external_subscription_id"]).to eq(invoice_subscription.subscription.external_id)
+    expect(result["billing_period"]["lago_plan_id"]).to eq(invoice_subscription.subscription.plan_id)
+    expect(result["billing_period"]["subscription_from_datetime"]).to eq(invoice_subscription.from_datetime.iso8601)
+    expect(result["billing_period"]["subscription_to_datetime"]).to eq(invoice_subscription.to_datetime.iso8601)
+    expect(result["billing_period"]["charges_from_datetime"]).to eq(invoice_subscription.charges_from_datetime.iso8601)
+    expect(result["billing_period"]["charges_to_datetime"]).to eq(invoice_subscription.charges_to_datetime.iso8601)
+    expect(result["billing_period"]["fixed_charges_from_datetime"]).to eq(invoice_subscription.fixed_charges_from_datetime.iso8601)
+    expect(result["billing_period"]["fixed_charges_to_datetime"]).to eq(invoice_subscription.fixed_charges_to_datetime.iso8601)
+    expect(result["billing_period"]["invoicing_reason"]).to eq(invoice_subscription.invoicing_reason)
+  end
+
+  context "when legacy invoice subscription without fixed charges boundaries" do
+    let(:invoice_subscription) { build(:invoice_subscription, :boundaries, fixed_charges_from_datetime: nil, fixed_charges_to_datetime: nil) }
+
+    it "serializes the object without fixed charges boundaries" do
+      result = JSON.parse(serializer.to_json)
+
       expect(result["billing_period"]["lago_subscription_id"]).to eq(invoice_subscription.subscription_id)
-      expect(result["billing_period"]["external_subscription_id"]).to eq(invoice_subscription.subscription.external_id)
-      expect(result["billing_period"]["lago_plan_id"]).to eq(invoice_subscription.subscription.plan_id)
-      expect(result["billing_period"]["subscription_from_datetime"]).to eq(invoice_subscription.from_datetime.iso8601)
-      expect(result["billing_period"]["subscription_to_datetime"]).to eq(invoice_subscription.to_datetime.iso8601)
-      expect(result["billing_period"]["charges_from_datetime"]).to eq(invoice_subscription.charges_from_datetime.iso8601)
-      expect(result["billing_period"]["charges_to_datetime"]).to eq(invoice_subscription.charges_to_datetime.iso8601)
-      expect(result["billing_period"]["invoicing_reason"]).to eq(invoice_subscription.invoicing_reason)
+      expect(result["billing_period"]["fixed_charges_from_datetime"]).to be_nil
+      expect(result["billing_period"]["fixed_charges_to_datetime"]).to be_nil
     end
   end
 end