Namespace can have only one chat team

app/models/chat_team.rb

 class ChatTeam < ActiveRecord::Base
   validates :team_id, presence: true
+  validates :namespace, uniqueness: true
   belongs_to :namespace
 end

spec/factories/chat_teams.rb

+FactoryGirl.define do
+  factory :chat_team, class: ChatTeam do
+    sequence :team_id do |n|
+      "abcdefghijklm#{n}"
+    end
+
+    namespace factory: :group
+  end
+end

spec/models/chat_team_spec.rb

 require 'spec_helper'
 describe ChatTeam, type: :model do
+  subject { create(:chat_team) }
+
   # Associations
   it { is_expected.to belong_to(:namespace) }
+  # Validations
+  it { is_expected.to validate_uniqueness_of(:namespace) }
+
   # Fields
   it { is_expected.to respond_to(:name) }
   it { is_expected.to respond_to(:team_id) }