DEV: Allow using an AR relation as a model in services

This patch allows using an AR relation as a model in services without
fetching associated records. It will just check if the relation is empty
or not. In the former case, the execution will stop at that point, as
expected.

app/services/service/base.rb

@@ -139,13 +139,15 @@ module Service
 
       def call(instance, context)
         context[name] = super
-        raise ArgumentError, "Model not found" if !optional && context[name].blank?
+        if !optional && (!context[name] || context[name].try(:empty?))
+          raise ArgumentError, "Model not found"
+        end
         if context[name].try(:invalid?)
           context[result_key].fail(invalid: true)
           context.fail!
         end
       rescue ArgumentError => exception
-        context[result_key].fail(exception: exception)
+        context[result_key].fail(exception: exception, not_found: true)
         context.fail!
       end
     end

lib/service_runner.rb

@@ -80,7 +80,9 @@ class ServiceRunner
       default_name: "default",
     },
     on_model_not_found: {
-      condition: ->(name = "model") { failure_for?("result.model.#{name}") && result[name].blank? },
+      condition: ->(name = "model") do
+        failure_for?("result.model.#{name}") && result["result.model.#{name}"].not_found
+      end,
       key: %w[result model],
       default_name: "model",
     },

spec/lib/service_runner_spec.rb

@@ -136,6 +136,18 @@ RSpec.describe ServiceRunner do
     end
   end
 
+  class RelationModelService
+    include Service::Base
+
+    model :fake_model
+
+    private
+
+    def fetch_fake_model
+      User.where(admin: false)
+    end
+  end
+
   describe ".call(service, &block)" do
     subject(:runner) { described_class.call(service, object, &actions_block) }
 
@@ -145,7 +157,9 @@ RSpec.describe ServiceRunner do
     let(:actions) { "proc {}" }
     let(:object) do
       Class
-        .new(Chat::ApiController) do
+        .new(ApplicationController) do
+          include WithServiceHelper
+
           def request
             OpenStruct.new
           end
@@ -352,6 +366,34 @@ RSpec.describe ServiceRunner do
           end
         end
       end
+
+      context "when fetching an ActiveRecord relation" do
+        let(:service) { RelationModelService }
+
+        context "when the service does not fail" do
+          before { Fabricate(:user) }
+
+          it "does not run the provided block" do
+            expect(runner).not_to eq :no_model
+          end
+
+          it "does not fetch records from the relation" do
+            runner
+            expect(result[:fake_model]).not_to be_loaded
+          end
+        end
+
+        context "when the service fails" do
+          it "runs the provided block" do
+            expect(runner).to eq :no_model
+          end
+
+          it "does not fetch records from the relation" do
+            runner
+            expect(result[:fake_model]).not_to be_loaded
+          end
+        end
+      end
     end
 
     context "when using the on_model_errors action" do