Move Identity.uniqueness_scope to a module

Moving this method to a separate module looks a bit odd, but it allows
for EE to extend the method without also having to redefine a variety of
validation rules.

app/models/identity.rb

 # frozen_string_literal: true
 
 class Identity < ActiveRecord::Base
-  def self.uniqueness_scope
-    :provider
-  end
-
   prepend EE::Identity
 
   include Sortable
@@ -13,8 +9,8 @@ class Identity < ActiveRecord::Base
   belongs_to :user
 
   validates :provider, presence: true
-  validates :extern_uid, allow_blank: true, uniqueness: { scope: uniqueness_scope, case_sensitive: false }
-  validates :user_id, uniqueness: { scope: uniqueness_scope }
+  validates :extern_uid, allow_blank: true, uniqueness: { scope: UniquenessScopes.scopes, case_sensitive: false }
+  validates :user_id, uniqueness: { scope: UniquenessScopes.scopes }
 
   before_save :ensure_normalized_extern_uid, if: :extern_uid_changed?
   after_destroy :clear_user_synced_attributes, if: :user_synced_attributes_metadata_from_provider?

app/models/identity/uniqueness_scopes.rb

+# frozen_string_literal: true
+
+class Identity < ActiveRecord::Base
+  # This module and method are defined in a separate file to allow EE to
+  # redefine the `scopes` method before it is used in the `Identity` model.
+  module UniquenessScopes
+    def self.scopes
+      [:provider]
+    end
+  end
+end
+
+Identity::UniquenessScopes.prepend(EE::Identity::UniquenessScopes)

ee/app/models/ee/identity.rb

@@ -5,17 +5,16 @@ module EE
     prepended do
       belongs_to :saml_provider
 
-      validates :secondary_extern_uid, allow_blank: true, uniqueness: { scope: uniqueness_scope, case_sensitive: false }
+      validates :secondary_extern_uid,
+        allow_blank: true,
+        uniqueness: {
+          scope: ::Identity::UniquenessScopes.scopes,
+          case_sensitive: false
+        }
 
       scope :with_secondary_extern_uid, ->(provider, secondary_extern_uid) do
         iwhere(secondary_extern_uid: normalize_uid(provider, secondary_extern_uid)).with_provider(provider)
       end
     end
-
-    class_methods do
-      def uniqueness_scope
-        [*super, :saml_provider_id]
-      end
-    end
   end
 end

ee/app/models/ee/identity/uniqueness_scopes.rb

+# frozen_string_literal: true
+
+module EE
+  module Identity
+    module UniquenessScopes
+      extend ActiveSupport::Concern
+
+      class_methods do
+        def scopes
+          [*super, :saml_provider_id]
+        end
+      end
+    end
+  end
+end