Extract method to get the tenant root storage

This way we simplify the code a little bit and we create a method unique
to the `TenantDiskService` class, which can be used to check whether
we're using this class without using `is_a?` or similar.

app/lib/active_storage/service/tenant_disk_service.rb

@@ -2,11 +2,19 @@ require "active_storage/service/disk_service"
 
 module ActiveStorage
   class Service::TenantDiskService < Service::DiskService
+    def tenant_root
+      File.join(root, Tenant.subfolder_path)
+    end
+
+    def tenant_root_for(...)
+      File.join(root, Tenant.subfolder_path_for(...))
+    end
+
     def path_for(key)
       if Tenant.default?
         super
       else
-        super.sub(root, File.join(root, Tenant.subfolder_path))
+        super.sub(root, tenant_root)
       end
     end
   end

app/models/tenant.rb

@@ -112,10 +112,14 @@ class Tenant < ApplicationRecord
   end
 
   def self.subfolder_path
-    if default?
+    subfolder_path_for(current_schema)
+  end
+
+  def self.subfolder_path_for(schema)
+    if schema == "public"
       ""
     else
-      File.join("tenants", current_schema)
+      File.join("tenants", schema)
     end
   end
 
@@ -178,12 +182,15 @@ class Tenant < ApplicationRecord
     end
 
     def rename_storage
-      return unless ActiveStorage::Blob.service.is_a?(ActiveStorage::Service::TenantDiskService)
+      service = ActiveStorage::Blob.service
+
+      return unless service.is_a?(ActiveStorage::Service::TenantDiskService)
+
+      old_storage = service.tenant_root_for(schema_before_last_save)
 
-      old_storage = File.join(ActiveStorage::Blob.service.root, "tenants", schema_before_last_save)
       return unless File.directory?(old_storage)
 
-      new_storage = File.join(ActiveStorage::Blob.service.root, "tenants", schema)
+      new_storage = service.tenant_root_for(schema)
       File.rename(old_storage, new_storage)
     end
 

spec/lib/active_storage/service/tenant_disk_service_spec.rb

@@ -0,0 +1,11 @@
+require "rails_helper"
+
+describe ActiveStorage::Service::TenantDiskService do
+  describe "#tenant_root_for" do
+    it "returns the folder of the given tenant" do
+      service = ActiveStorage::Service::TenantDiskService.new(root: Rails.root.join("tmp", "storage"))
+
+      expect(service.tenant_root_for("megawonder")).to eq "#{Rails.root}/tmp/storage/tenants/megawonder"
+    end
+  end
+end