Enterprise roles must be unique

app/models/enterprise_role.rb

@@ -2,5 +2,8 @@ class EnterpriseRole < ActiveRecord::Base
   belongs_to :user, :class_name => Spree.user_class
   belongs_to :enterprise
 
+  validates_presence_of :user_id, :enterprise_id
+  validates_uniqueness_of :enterprise_id, scope: :user_id, message: "^That role is already present."
+
   scope :by_user_email, joins(:user).order('spree_users.email ASC')
 end

db/migrate/20140815053659_add_unique_and_fk_constraints_to_enterprise_roles.rb

@@ -0,0 +1,8 @@
+class AddUniqueAndFkConstraintsToEnterpriseRoles < ActiveRecord::Migration
+  def change
+    add_index :enterprise_roles, [:user_id, :enterprise_id], unique: true
+
+    add_foreign_key :enterprise_roles, :spree_users, column: :user_id
+    add_foreign_key :enterprise_roles, :enterprises, column: :enterprise_id
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended to check this file into your version control system.
 
-ActiveRecord::Schema.define(:version => 20140723023713) do
+ActiveRecord::Schema.define(:version => 20140815053659) do
 
   create_table "adjustment_metadata", :force => true do |t|
     t.integer "adjustment_id"
@@ -222,6 +222,7 @@ ActiveRecord::Schema.define(:version => 20140723023713) do
   end
 
   add_index "enterprise_roles", ["enterprise_id"], :name => "index_enterprise_roles_on_enterprise_id"
+  add_index "enterprise_roles", ["user_id", "enterprise_id"], :name => "index_enterprise_roles_on_user_id_and_enterprise_id", :unique => true
   add_index "enterprise_roles", ["user_id"], :name => "index_enterprise_roles_on_user_id"
 
   create_table "enterprises", :force => true do |t|

spec/features/admin/enterprise_roles_spec.rb

@@ -45,6 +45,23 @@ feature %q{
       page.should have_relationship u, e
       EnterpriseRole.where(user_id: u, enterprise_id: e).should be_present
     end
+
+    scenario "attempting to create a relationship with invalid data" do
+      u = create(:user, email: 'u@example.com')
+      e = create(:enterprise, name: 'One')
+      create(:enterprise_role, user: u, enterprise: e)
+
+      expect do
+        # When I attempt to create a duplicate relationship
+        visit admin_enterprise_roles_path
+        select 'u@example.com', from: 'enterprise_role_user_id'
+        select 'One', from: 'enterprise_role_enterprise_id'
+        click_button 'Create'
+
+        # Then I should see an error message
+        page.should have_content "That role is already present."
+      end.to change(EnterpriseRole, :count).by(0)
+    end
   end