Add minimal tests

app/services/imports/lettings_logs_field_import_service.rb

@@ -1,5 +1,5 @@
 module Imports
-  class LettingsLogsFieldImportService < ImportService
+  class LettingsLogsFieldImportService < LogsImportService
     def update_field(field, folder)
       case field
       when "tenancycode"

spec/fixtures/imports/logs/0ead17cb-1668-442d-898c-0d52879ff592.xml

@@ -170,7 +170,7 @@
     <Q18dyes override-field="true"/>
     <Q19void>2021-09-30</Q19void>
     <Q19repair/>
-    <Q20 override-field="">0</Q20>
+    <Q20 override-field="">21</Q20>
     <Q21a>MCB003</Q21a>
   </Group>
   <Group>

spec/services/imports/lettings_logs_field_import_service_spec.rb

@@ -249,4 +249,40 @@ RSpec.describe Imports::LettingsLogsFieldImportService do
       end
     end
   end
+
+  context "when updating offered" do
+    let(:field) { "offered" }
+
+    context "when the lettings log has no offered value" do
+      let(:lettings_log) { LettingsLog.find_by(old_id: lettings_log_id) }
+
+      before do
+        Imports::LettingsLogsImportService.new(storage_service, logger).create_logs(fixture_directory)
+        lettings_log_file.rewind
+        lettings_log.update!(offered: nil)
+      end
+
+      it "updates the lettings_log offered value" do
+        expect(logger).to receive(:info).with(/lettings log \d+'s offered value has been set to 21/)
+        expect { import_service.send(:update_field, field, remote_folder) }
+          .to(change { lettings_log.reload.offered }.from(nil).to(21))
+      end
+    end
+
+    context "when the lettings log has a different offered value" do
+      let(:lettings_log) { LettingsLog.find_by(old_id: lettings_log_id) }
+
+      before do
+        Imports::LettingsLogsImportService.new(storage_service, logger).create_logs(fixture_directory)
+        lettings_log_file.rewind
+        lettings_log.update!(offered: 18)
+      end
+
+      it "does not update the lettings_log offered value" do
+        expect(logger).to receive(:info).with(/lettings log \d+ has a value for offered, skipping update/)
+        expect { import_service.send(:update_field, field, remote_folder) }
+          .not_to(change { lettings_log.reload.offered })
+      end
+    end
+  end
 end