Fix seeding of default plan

Only insert default plan if it does not exist. The default plan could
exist if Plan.default has been called.

app/models/plan.rb

@@ -14,7 +14,7 @@ class Plan < ApplicationRecord
     Gitlab::SafeRequestStore.fetch(:plan_default) do
       # find_by allows us to find object (cheaply) against replica DB
       # safe_find_or_create_by does stick to primary DB
-      find_by(name: DEFAULT) || safe_find_or_create_by(name: DEFAULT)
+      find_by(name: DEFAULT) || safe_find_or_create_by(name: DEFAULT) { |plan| plan.title = DEFAULT.titleize }
     end
   end
 

ee/db/fixtures/production/027_plans.rb

 # frozen_string_literal: true
 
 Gitlab::Seeder.quiet do
-  Plan.create!(name: Plan::DEFAULT, title: Plan::DEFAULT.titleize)
+  # The default plan could already be created if Plan.default was called
+  Plan.safe_find_or_create_by!(name: Plan::DEFAULT) { |plan| plan.title = Plan::DEFAULT.titleize }
   Plan.create!(name: Plan::FREE, title: Plan::FREE.titleize) if Gitlab.com?
 end

spec/models/plan_spec.rb

@@ -15,6 +15,29 @@ RSpec.describe Plan do
     end
   end
 
+  describe '#default' do
+    context 'when default plan exists' do
+      let!(:default_plan) { create(:default_plan) }
+
+      it 'returns default plan' do
+        expect(described_class.default).to eq(default_plan)
+      end
+    end
+
+    context 'when default plan does not exist' do
+      it 'creates default plan' do
+        expect { described_class.default }.to change { Plan.count }.by(1)
+      end
+
+      it 'creates plan with correct attributes' do
+        plan = described_class.default
+
+        expect(plan.name).to eq(Plan::DEFAULT)
+        expect(plan.title).to eq(Plan::DEFAULT.titleize)
+      end
+    end
+  end
+
   context 'when updating plan limits' do
     let(:plan) { described_class.default }