Allow rendering different email views per tenant

We were assigning variants in a controller, in the context of a request.
However, when sending emails, there is no request and no controller is
involved, so we also need to set the variant in the ApplicationMailer
class.

app/mailers/application_mailer.rb

@@ -3,6 +3,7 @@ class ApplicationMailer < ActionMailer::Base
   default from: proc { "#{Setting["mailer_from_name"]} <#{Setting["mailer_from_address"]}>" }
   layout "mailer"
   before_action :set_asset_host
+  before_action :set_variant
   after_action :set_smtp_settings
 
   def default_url_options
@@ -13,6 +14,10 @@ class ApplicationMailer < ActionMailer::Base
     self.asset_host ||= root_url.delete_suffix("/")
   end
 
+  def set_variant
+    lookup_context.variants = [Tenant.current_schema.to_sym]
+  end
+
   def set_smtp_settings
     unless Tenant.default?
       mail.delivery_method.settings.merge!(Tenant.current_secrets.smtp_settings.to_h)

spec/mailers/application_mailer_spec.rb

@@ -78,4 +78,22 @@ describe ApplicationMailer do
       end
     end
   end
+
+  describe "#set_variant" do
+    let(:mailer) { ApplicationMailer.new }
+
+    it "uses the default tenant by default" do
+      mailer.set_variant
+
+      expect(mailer.lookup_context.variants).to eq [:public]
+    end
+
+    it "uses the current tenant schema when defined" do
+      allow(Tenant).to receive(:current_schema).and_return("random-name")
+
+      mailer.set_variant
+
+      expect(mailer.lookup_context.variants).to eq [:"random-name"]
+    end
+  end
 end