Allow configuring the engine storage service independently

README.md

@@ -31,6 +31,34 @@ db:
 
 ### Customizations
 
+#### Use DB storage for selected attachments
+
+When your application uses another service by default, configure the engine's
+service separately in `config/application.rb` or an environment configuration:
+
+```rb
+config.active_storage.service = :local # Keep your existing default service
+config.active_storage_db.service = :db
+```
+
+The `active_storage_db.service` value is the name of a DB service in
+`config/storage.yml` (a string or symbol), not the adapter name `DB`.
+Select that same service on attachments that should use database storage:
+
+```rb
+class Report < ApplicationRecord
+  has_one_attached :template, service: :db
+end
+```
+
+The engine uses the configured service for downloads and direct uploads without
+changing the application's default service. The setting defaults to `nil`, which
+preserves the existing behavior of using `ActiveStorage::Blob.service`.
+An unknown service name raises the Active Storage registry's configuration error.
+This setting selects one service for all engine requests.
+
+#### Separate database connection
+
 To setup a separate database connection for the `ActiveStorageDB` migrations and files data:
 
 1. Add a different database configuration per environment to `config/database.yml`, e.g:

app/controllers/active_storage_db/files_controller.rb

@@ -33,7 +33,8 @@ def acceptable_content?(token)
     end
 
     def db_service
-      ActiveStorage::Blob.service
+      service_name = Rails.application.config.active_storage_db.service
+      service_name ? ActiveStorage::Blob.services.fetch(service_name) : ActiveStorage::Blob.service
     end
 
     def decode_verified_key

lib/active_storage_db/engine.rb

@@ -11,5 +11,8 @@ module ActiveStorageDB
 
   class Engine < ::Rails::Engine
     isolate_namespace ActiveStorageDB
+
+    config.active_storage_db = ActiveSupport::OrderedOptions.new
+    config.active_storage_db.service = nil
   end
 end

spec/requests/configured_service_spec.rb

@@ -0,0 +1,80 @@
+# frozen_string_literal: true
+
+RSpec.describe "Configured engine service" do
+  let(:data) { "Database attachment" }
+  let(:disk_service) { ActiveStorage::Blob.services.fetch(:test) }
+  let(:blob) do
+    ActiveStorage::Blob.create_and_upload!(
+      io: StringIO.new(data), filename: "template.txt", content_type: "text/plain",
+      service_name: "db"
+    )
+  end
+
+  before do
+    allow(ActiveStorage::Blob).to receive(:service).and_return(disk_service)
+    if ActiveStorage::Current.respond_to?(:url_options)
+      allow(ActiveStorage::Current).to receive(:url_options).and_return(protocol: "http://", host: "test.example.com")
+    else
+      allow(ActiveStorage::Current).to receive(:host).and_return("http://test.example.com")
+    end
+  end
+
+  shared_examples "a configured DB service" do
+    it "downloads from DB storage without changing the default service", :aggregate_failures do
+      get blob.url
+
+      expect(response).to have_http_status(:ok)
+      expect(response.body).to eq(data)
+      expect(ActiveStorage::Blob.service).to eq(disk_service)
+      expect(disk_service).not_to exist(blob.key)
+    end
+
+    it "uploads directly to DB storage without changing the default service", :aggregate_failures do
+      pending_blob = ActiveStorage::Blob.create_before_direct_upload!(
+        filename: "template.txt", byte_size: data.bytesize, content_type: "text/plain",
+        checksum: Digest::MD5.base64digest(data), service_name: "db"
+      )
+
+      put pending_blob.service_url_for_direct_upload, params: data, headers: { "Content-Type" => "text/plain" }
+
+      expect(response).to have_http_status(:no_content)
+      expect(pending_blob.download).to eq(data)
+      expect(ActiveStorage::Blob.service).to eq(disk_service)
+      expect(disk_service).not_to exist(pending_blob.key)
+    end
+  end
+
+  context "with a symbol service name" do
+    before { allow(Rails.application.config.active_storage_db).to receive(:service).and_return(:db) }
+
+    it_behaves_like "a configured DB service"
+  end
+
+  context "with a string service name" do
+    before { allow(Rails.application.config.active_storage_db).to receive(:service).and_return("db") }
+
+    it_behaves_like "a configured DB service"
+  end
+
+  context "without a configured service" do
+    before do
+      allow(Rails.application.config.active_storage_db).to receive(:service).and_return(nil)
+      allow(ActiveStorage::Blob).to receive(:service).and_return(ActiveStorage::Blob.services.fetch(:db))
+    end
+
+    it "downloads using the application's default service" do
+      get blob.url
+
+      expect(response).to have_http_status(:ok)
+      expect(response.body).to eq(data)
+    end
+  end
+
+  context "with an unknown service name" do
+    before { allow(Rails.application.config.active_storage_db).to receive(:service).and_return(:missing_db) }
+
+    it "raises the registry error instead of falling back" do
+      expect { get blob.url }.to raise_error(KeyError, /missing_db/)
+    end
+  end
+end