Fix use of ActiveRecord::Base in DbCheck

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/350651

lib/gitlab/health_checks/db_check.rb

@@ -13,12 +13,14 @@ module Gitlab
         end
 
         def successful?(result)
-          result == '1'
+          result == Gitlab::Database.database_base_models.size
         end
 
         def check
           catch_timeout 10.seconds do
-            ActiveRecord::Base.connection.execute('SELECT 1 as ping')&.first&.[]('ping')&.to_s
+            Gitlab::Database.database_base_models.sum do |_, base|
+              base.connection.select_value('SELECT 1')
+            end
           end
         end
       end

spec/lib/gitlab/health_checks/db_check_spec.rb

@@ -4,5 +4,20 @@ require 'spec_helper'
 require_relative './simple_check_shared'
 
 RSpec.describe Gitlab::HealthChecks::DbCheck do
-  include_examples 'simple_check', 'db_ping', 'Db', '1'
+  include_examples 'simple_check', 'db_ping', 'Db', Gitlab::Database.database_base_models.size
+
+  context 'with multiple databases' do
+    subject { described_class.readiness }
+
+    before do
+      allow(Gitlab::Database).to receive(:database_base_models)
+        .and_return({ main: ApplicationRecord, ci: Ci::ApplicationRecord }.with_indifferent_access)
+    end
+
+    it 'checks multiple databases' do
+      expect(ApplicationRecord.connection).to receive(:select_value).with('SELECT 1').and_call_original
+      expect(Ci::ApplicationRecord.connection).to receive(:select_value).with('SELECT 1').and_call_original
+      expect(subject).to have_attributes(success: true)
+    end
+  end
 end