add more tests to models

spec/models/domain_block_spec.rb

@@ -1,5 +1,23 @@
 require 'rails_helper'
 
 RSpec.describe DomainBlock, type: :model do
+  describe 'validations' do
+    it 'has a valid fabricator' do
+      domain_block = Fabricate.build(:domain_block)
+      expect(domain_block).to be_valid
+    end
 
+    it 'is invalid without a domain' do
+      domain_block = Fabricate.build(:domain_block, domain: nil)
+      domain_block.valid?
+      expect(domain_block).to model_have_error_on_field(:domain)
+    end
+
+    it 'is invalid if the domain already exists' do
+      domain_block_1 = Fabricate(:domain_block, domain: 'dalek.com')
+      domain_block_2 = Fabricate.build(:domain_block, domain: 'dalek.com')
+      domain_block_2.valid?
+      expect(domain_block_2).to model_have_error_on_field(:domain)
+    end
+  end
 end