Use initializer to configure tenant mode

app/models/concerns/multi_tenant.rb

@@ -1,9 +1,13 @@
 module MultiTenant
   extend ActiveSupport::Concern
 
+  included do
+    cattr_accessor :multi_tenant, default: false
+  end
+
   class_methods do
     def accepting_signups?
-      ENV.fetch("MULTI_TENANT", "false") == "true" || Account.none?
+      multi_tenant || Account.none?
     end
   end
 end

config/environments/test.rb

@@ -67,4 +67,7 @@ Rails.application.configure do
 
   # Load test helpers
   config.autoload_paths += %w[ test/test_helpers ]
+
+  # Enable multi-tenant mode for tests
+  config.x.multi_tenant = true
 end

config/initializers/multi_tenant.rb

@@ -0,0 +1,3 @@
+Rails.application.config.after_initialize do
+  Account.multi_tenant = ENV["MULTI_TENANT"] == "true" || Rails.configuration.x.multi_tenant == true
+end

test/controllers/signups_controller_test.rb

@@ -67,7 +67,7 @@ class SignupsControllerTest < ActionDispatch::IntegrationTest
   test "redirects to session#new when single_tenant and user exists" do
     users(:david)
 
-    in_single_tenant_mode do
+    with_multi_tenant_mode(false) do
       untenanted do
         get new_signup_path
 

test/models/account/multi_tenant_test.rb

@@ -0,0 +1,22 @@
+require "test_helper"
+
+class MultiTenantTest < ActiveSupport::TestCase
+  test "accepting_signups? is true when multi_tenant is enabled" do
+    with_multi_tenant_mode(true) do
+      assert Account.accepting_signups?
+    end
+  end
+
+  test "accepting_signups? is false when multi_tenant is disabled and accounts exist" do
+    with_multi_tenant_mode(false) do
+      assert_not Account.accepting_signups?
+    end
+  end
+
+  test "accepting_signups? is true when multi_tenant is disabled but no accounts exist" do
+    with_multi_tenant_mode(false) do
+      Account.delete_all
+      assert Account.accepting_signups?
+    end
+  end
+end

test/test_helper.rb

@@ -1,6 +1,4 @@
 ENV["RAILS_ENV"] ||= "test"
-ENV["MULTI_TENANT"] ||= "true"
-
 require_relative "../config/environment"
 
 require "rails/test_help"

test/test_helpers/session_test_helper.rb

@@ -58,11 +58,11 @@ module SessionTestHelper
     integration_session.default_url_options[:script_name] = original_script_name
   end
 
-  def in_single_tenant_mode
-    previous_multi_tenant = ENV["MULTI_TENANT"]
-    ENV["MULTI_TENANT"] = "false"
+  def with_multi_tenant_mode(enabled)
+    previous = Account.multi_tenant
+    Account.multi_tenant = enabled
     yield
   ensure
-    ENV["MULTI_TENANT"] = previous_multi_tenant
+    Account.multi_tenant = previous
   end
 end