Use shortened `type` values for the `namespace` table

app/models/group.rb

@@ -18,6 +18,10 @@ class Group < Namespace
   include EachBatch
   include BulkMemberAccessLoad
 
+  def self.sti_name
+    'Group'
+  end
+
   has_many :all_group_members, -> { where(requested_at: nil) }, dependent: :destroy, as: :source, class_name: 'GroupMember' # rubocop:disable Cop/ActiveRecordDependent
   has_many :group_members, -> { where(requested_at: nil).where.not(members: { access_level: Gitlab::Access::MINIMAL_ACCESS }) }, dependent: :destroy, as: :source # rubocop:disable Cop/ActiveRecordDependent
   alias_method :members, :group_members

app/models/namespace.rb

@@ -18,6 +18,11 @@ class Namespace < ApplicationRecord
 
   ignore_column :delayed_project_removal, remove_with: '14.1', remove_after: '2021-05-22'
 
+  # Tells ActiveRecord not to store the full class name, in order to space some space
+  # https://gitlab.com/gitlab-org/gitlab/-/merge_requests/69794
+  self.store_full_sti_class = false
+  self.store_full_class_name = false
+
   # Prevent users from creating unreasonably deep level of nesting.
   # The number 20 was taken based on maximum nesting level of
   # Android repo (15) + some extra backup.
@@ -131,6 +136,21 @@ class Namespace < ApplicationRecord
   attr_writer :root_ancestor, :emails_disabled_memoized
 
   class << self
+    def sti_class_for(type_name)
+      case type_name
+      when 'Group'
+        Group
+      when 'Project'
+        Namespaces::ProjectNamespace
+      when 'User'
+        # TODO: We create a normal Namespace until
+        #       https://gitlab.com/gitlab-org/gitlab/-/merge_requests/68894 is ready
+        Namespace
+      else
+        Namespace
+      end
+    end
+
     def by_path(path)
       find_by('lower(path) = :value', value: path.downcase)
     end
@@ -227,15 +247,23 @@ class Namespace < ApplicationRecord
   end
 
   def kind
-    type == 'Group' ? 'group' : 'user'
-  end
+    return 'group' if group?
+    return 'project' if project?
 
-  def user?
-    kind == 'user'
+    'user' # defaults to user
   end
 
   def group?
-    type == 'Group'
+    type == Group.sti_name
+  end
+
+  def project?
+    type == Namespaces::ProjectNamespace.sti_name
+  end
+
+  def user?
+    # That last bit ensures we're considered a user namespace as a default
+    type.nil? || type == Namespaces::UserNamespace.sti_name || !(group? || project?)
   end
 
   def find_fork_of(project)

app/models/namespaces/project_namespace.rb

@@ -3,5 +3,9 @@
 module Namespaces
   class ProjectNamespace < Namespace
     has_one :project, foreign_key: :project_namespace_id, inverse_of: :project_namespace
+
+    def self.sti_name
+      'Project'
+    end
   end
 end

app/models/namespaces/user_namespace.rb

+# frozen_string_literal: true
+
+# TODO: currently not created/mapped in the database, will be done in another issue
+#       https://gitlab.com/gitlab-org/gitlab/-/issues/337102
+module Namespaces
+  class UserNamespace < Namespace
+    def self.sti_name
+      'User'
+    end
+  end
+end

ee/spec/helpers/billing_plans_helper_spec.rb

@@ -146,21 +146,20 @@ RSpec.describe BillingPlansHelper do
     end
 
     context 'when the namespace belongs to a user' do
-      let(:group) { build(:group, type: 'user') }
+      let(:namespace) { build(:namespace) }
 
       context 'when the namespace is free plan' do
         it 'returns attributes with free_personal_namespace true' do
-          expect(helper.subscription_plan_data_attributes(group, plan))
+          expect(helper.subscription_plan_data_attributes(namespace, plan))
             .to include(free_personal_namespace: 'true')
         end
       end
 
       context 'when the namespace is paid plan' do
-        let(:group) { build(:group, type: 'user') }
-        let!(:gitlab_subscription) { build(:gitlab_subscription, :ultimate, namespace: group) }
+        let!(:gitlab_subscription) { build(:gitlab_subscription, :ultimate, namespace: namespace) }
 
         it 'returns attributes with free_personal_namespace false' do
-          expect(helper.subscription_plan_data_attributes(group, plan))
+          expect(helper.subscription_plan_data_attributes(namespace, plan))
             .to include(free_personal_namespace: 'false')
         end
       end

spec/models/namespace_spec.rb

@@ -157,6 +157,63 @@ RSpec.describe Namespace do
     end
   end
 
+  describe 'handling STI', :aggregate_failures do
+    let(:namespace_type) { nil }
+    let(:namespace) { Namespace.find(create(:namespace, type: namespace_type).id) }
+
+    context 'creating a Group' do
+      let(:namespace_type) { 'Group' }
+
+      it 'is valid' do
+        expect(namespace).to be_a(Group)
+        expect(namespace.kind).to eq('group')
+        expect(namespace.group?).to be_truthy
+      end
+    end
+
+    context 'creating a ProjectNamespace' do
+      let(:namespace_type) { 'Project' }
+
+      it 'is valid' do
+        expect(Namespace.find(namespace.id)).to be_a(Namespaces::ProjectNamespace)
+        expect(namespace.kind).to eq('project')
+        expect(namespace.project?).to be_truthy
+      end
+    end
+
+    context 'creating a UserNamespace' do
+      let(:namespace_type) { 'User' }
+
+      it 'is valid' do
+        # TODO: We create a normal Namespace until
+        #       https://gitlab.com/gitlab-org/gitlab/-/merge_requests/68894 is ready
+        expect(Namespace.find(namespace.id)).to be_a(Namespace)
+        expect(namespace.kind).to eq('user')
+        expect(namespace.user?).to be_truthy
+      end
+    end
+
+    context 'creating a default Namespace' do
+      let(:namespace_type) { nil }
+
+      it 'is valid' do
+        expect(Namespace.find(namespace.id)).to be_a(Namespace)
+        expect(namespace.kind).to eq('user')
+        expect(namespace.user?).to be_truthy
+      end
+    end
+
+    context 'creating an unknown Namespace type' do
+      let(:namespace_type) { 'One' }
+
+      it 'defaults to a Namespace' do
+        expect(Namespace.find(namespace.id)).to be_a(Namespace)
+        expect(namespace.kind).to eq('user')
+        expect(namespace.user?).to be_truthy
+      end
+    end
+  end
+
   describe 'scopes', :aggregate_failures do
     let_it_be(:namespace1) { create(:group, name: 'Namespace 1', path: 'namespace-1') }
     let_it_be(:namespace2) { create(:group, name: 'Namespace 2', path: 'namespace-2') }