First go at filtering out ProjectNamespaces

For methods that expect only Group, we need to ensure
we don't accidentally return some ProjectNamespace
objects.

app/models/namespace.rb

@@ -52,7 +52,7 @@ class Namespace < ApplicationRecord
   belongs_to :owner, class_name: "User"
 
   belongs_to :parent, class_name: "Namespace"
-  has_many :children, class_name: "Namespace", foreign_key: :parent_id
+  has_many :children, -> { where(type: Group.sti_name) }, class_name: "Namespace", foreign_key: :parent_id
   has_many :custom_emoji, inverse_of: :namespace
   has_one :chat_team, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
   has_one :root_storage_statistics, class_name: 'Namespace::RootStorageStatistics'

spec/models/namespace_spec.rb

@@ -28,6 +28,16 @@ RSpec.describe Namespace do
     it { is_expected.to have_one :onboarding_progress }
     it { is_expected.to have_one :admin_note }
     it { is_expected.to have_many :pending_builds }
+
+    describe '#children' do
+      let_it_be(:group) { create(:group) }
+      let_it_be(:subgroup) { create(:group, parent: group) }
+      let_it_be(:project_namespace) { create(:project_namespace, parent: group) }
+
+      it 'excludes project namespaces' do
+        expect(group.children).to match_array([subgroup])
+      end
+    end
   end
 
   describe 'validations' do
@@ -273,8 +283,8 @@ RSpec.describe Namespace do
 
     describe '.by_parent' do
       it 'includes correct namespaces' do
-        expect(described_class.by_parent(namespace1.id)).to eq([namespace1sub])
-        expect(described_class.by_parent(namespace2.id)).to eq([namespace2sub])
+        expect(described_class.by_parent(namespace1.id)).to match_array([namespace1sub])
+        expect(described_class.by_parent(namespace2.id)).to match_array([namespace2sub])
         expect(described_class.by_parent(nil)).to match_array([namespace, namespace1, namespace2])
       end
     end