Deprecate Spree::Payment#offsets

The old system for refunds was removed in 2014 [1].

We deprecate the `.offsets` association, but we still need to silence
the warning on the internal method `.credit_allowed` method until the
full removal.

[1] - spree/spree#4883

core/app/models/spree/payment.rb

@@ -17,6 +17,14 @@ class Payment < Spree::Base
     belongs_to :payment_method, -> { with_discarded }, class_name: 'Spree::PaymentMethod', inverse_of: :payments, optional: true
 
     has_many :offsets, -> { offset_payment }, class_name: "Spree::Payment", foreign_key: :source_id
+    # TODO: When removing the method we can also drop the `source_id` column
+    def offsets
+      Spree::Deprecation.warn <<~MSG
+        `Spree::Payment#offsets` are deprecated. Use the refund system (`Spree::Refund`) instead.
+      MSG
+
+      super
+    end
     has_many :log_entries, as: :source
     has_many :state_changes, as: :stateful
     has_many :capture_events, class_name: 'Spree::PaymentCaptureEvent'
@@ -101,7 +109,10 @@ def offsets_total
     # @return [BigDecimal] the amount of this payment minus the offsets
     #   (old-style refunds) and refunds
     def credit_allowed
-      amount - (offsets_total.abs + refunds.sum(:amount))
+      amount - (
+        (Spree::Deprecation.silence { offsets_total.abs }) +
+        refunds.sum(:amount)
+      )
     end
 
     # @return [Boolean] true when this payment can be credited

core/spec/models/spree/payment_spec.rb

@@ -1332,4 +1332,18 @@
       expect(described_class.valid).to be_empty
     end
   end
+
+  describe '::offsets' do
+    it 'is deprecated' do
+      expect(Spree::Deprecation).to receive(:warn).with(/offsets` are deprecated/)
+
+      described_class.new.offsets
+    end
+
+    it 'still calls the original method' do
+      Spree::Deprecation.silence do
+        expect(described_class.new.offsets.class.name).to eq('ActiveRecord::Associations::CollectionProxy')
+      end
+    end
+  end
 end