Extract RepositoryOnShard concern

app/models/concerns/repository_on_shard.rb

+# frozen_string_literal: true
+
+module RepositoryOnShard
+  extend ActiveSupport::Concern
+
+  included do
+    belongs_to :shard
+    validates :shard, presence: true
+  end
+
+  def shard_name
+    shard&.name
+  end
+
+  def shard_name=(name)
+    self.shard = Shard.by_name(name)
+  end
+end

app/models/pool_repository.rb

 # frozen_string_literal: true
 
 class PoolRepository < ActiveRecord::Base
-  belongs_to :shard
-  validates :shard, presence: true
+  include RepositoryOnShard
 
   has_many :member_projects, class_name: 'Project'
 
   after_create :correct_disk_path
 
-  def shard_name
-    shard&.name
-  end
-
-  def shard_name=(name)
-    self.shard = Shard.by_name(name)
-  end
-
   private
 
   def correct_disk_path

spec/models/pool_repository_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe PoolRepository do