DB service: add compose method for Rails 7 only

Author: Mattia Roccoberton

.rubocop.yml

@@ -36,7 +36,7 @@ RSpec/MultipleExpectations:
 
 RSpec/MultipleMemoizedHelpers:
   # Default is 5
-  Max: 10
+  Max: 12
 
 RSpec/NestedGroups:
   # Default is 3

lib/active_storage/service/db_service_rails70.rb

@@ -2,6 +2,19 @@
 
 module ActiveStorage
   module DBServiceRails70
+    def compose(source_keys, destination_key, **)
+      buffer = nil
+      source_keys.each do |source_key|
+        data = ::ActiveStorageDB::File.find_by!(ref: source_key).data
+        if buffer
+          buffer << data
+        else
+          buffer = data
+        end
+      end
+      ::ActiveStorageDB::File.create!(ref: destination_key, data: buffer) if buffer
+    end
+
     private
 
     def current_host

spec/service/active_storage/service/db_service_spec.rb

@@ -48,6 +48,22 @@
     end
   end
 
+  if Rails::VERSION::MAJOR >= 7
+    describe '.compose' do
+      subject(:compose) { service.compose(%w[key1 key2 key3], 'dest_key') }
+
+      let!(:db_file1) { create(:active_storage_db_file, ref: 'key1', data: 'first file') }
+      let!(:db_file2) { create(:active_storage_db_file, ref: 'key2', data: 'second file') }
+      let!(:db_file3) { create(:active_storage_db_file, ref: 'key3', data: 'third file') }
+
+      it 'composes the source files' do
+        expect { compose }.to change { ::ActiveStorageDB::File.where(ref: 'dest_key').count }.by(1)
+        expect(compose).to be_kind_of ::ActiveStorageDB::File
+        expect(compose.data).to eq [db_file1.data, db_file2.data, db_file3.data].join
+      end
+    end
+  end
+
   describe '.delete' do
     subject(:delete) { service.delete(key) }