Validate project namespace parent

Validate the project namespace can be added only
to user/group namespaces and project namespace must
always have a parent.

app/models/namespace.rb

@@ -531,7 +531,17 @@ def nesting_level_allowed
   end
   def validate_parent_type
-    return unless has_parent?
+    unless has_parent?
+      if project?
+        errors.add(:parent_id, 'a project namespace must have a parent')
+      end
+
+      return
+    end
+
+    if parent.project?
+      errors.add(:parent_id, 'a project namespace cannot be the parent of another namespace')
+    end
     if user?
       errors.add(:parent_id, 'a user namespace cannot have a parent')

spec/models/namespace_spec.rb

@@ -36,27 +36,34 @@
     it { is_expected.to validate_numericality_of(:max_artifacts_size).only_integer.is_greater_than(0) }
     context 'validating the parent of a namespace' do
-      context 'when the namespace has no parent' do
-        it 'allows a namespace to have no parent associated with it' do
-          namespace = build(:namespace)
-
-          expect(namespace).to be_valid
-        end
+      using RSpec::Parameterized::TableSyntax
+
+      where(:parent_type, :child_type, :error) do
+        nil         | 'User'      | nil
+        nil         | 'Group'     | nil
+        nil         | 'Project'   | 'a project namespace must have a parent'
+        'Project'   | 'User'      | 'a project namespace cannot be the parent of another namespace'
+        'Project'   | 'Group'     | 'a project namespace cannot be the parent of another namespace'
+        'Project'   | 'Project'   | 'a project namespace cannot be the parent of another namespace'
+        'Group'     | 'User'      | 'a user namespace cannot have a parent'
+        'Group'     | 'Group'     | nil
+        'Group'     | 'Project'   | nil
+        'User'      | 'User'      | 'a user namespace cannot have a parent'
+        'User'      | 'Group'     | 'a group cannot have a user namespace as its parent'
+        'User'      | 'Project'   | nil
       end
-      context 'when the namespace has a parent' do
-        it 'does not allow a namespace to have a group as its parent' do
-          namespace = build(:namespace, parent: build(:group))
-
-          expect(namespace).not_to be_valid
-          expect(namespace.errors[:parent_id].first).to eq('a user namespace cannot have a parent')
-        end
-
-        it 'does not allow a namespace to have another namespace as its parent' do
-          namespace = build(:namespace, parent: build(:namespace))
-
-          expect(namespace).not_to be_valid
-          expect(namespace.errors[:parent_id].first).to eq('a user namespace cannot have a parent')
+      with_them do
+        it 'validates namespace parent' do
+          parent = build(:namespace, type: parent_type) if parent_type
+          namespace = build(:namespace, type: child_type, parent: parent)
+
+          if error
+            expect(namespace).not_to be_valid
+            expect(namespace.errors[:parent_id].first).to eq(error)
+          else
+            expect(namespace).to be_valid
+          end
         end
       end
@@ -159,7 +166,8 @@
   describe 'handling STI', :aggregate_failures do
     let(:namespace_type) { nil }
-    let(:namespace) { Namespace.find(create(:namespace, type: namespace_type).id) }
+    let(:parent) { nil }
+    let(:namespace) { Namespace.find(create(:namespace, type: namespace_type, parent: parent).id) }
     context 'creating a Group' do
       let(:namespace_type) { 'Group' }
@@ -173,6 +181,7 @@
     context 'creating a ProjectNamespace' do
       let(:namespace_type) { 'Project' }
+      let(:parent) { create(:group) }
       it 'is valid' do
         expect(Namespace.find(namespace.id)).to be_a(Namespaces::ProjectNamespace)