group member count optimization

db/migrate/20190722003541_add_member_count_to_groups.rb

@@ -0,0 +1,10 @@
+class AddMemberCountToGroups < ActiveRecord::Migration[5.2]
+  def up
+    add_column :groups, :member_count, :integer
+    change_column_default :groups, :member_count, 0
+  end
+
+  def down
+    remove_column :groups, :member_count
+  end
+end