Update Admin user datatables to use AJAX

app/datatables/user_datatable.rb

@@ -0,0 +1,69 @@
+# frozen_string_literal: true
+
+class UserDatatable < AjaxDatatablesRails::Base
+  def_delegator :@view, :show_roles
+  def_delegator :@view, :admin_user_path
+  def_delegator :@view, :edit_admin_user_path
+
+  def view_columns
+    # Declare strings in this format: ModelName.column_name
+    # or in aliased_join_table.column_name format
+    @view_columns ||= {
+      id:           { source: 'User.id', cond: :eq },
+      confirmed_at: { source: 'User.confirmed_at', searchable: false },
+      email:        { source: 'User.email' },
+      name:         { source: 'User.name' },
+      attended:     { source: 'attended_count', searchable: false },
+      roles:        { source: 'Role.name' },
+      view_url:     { source: 'User.id', searchable: false, orderable: false },
+      edit_url:     { source: 'User.id', searchable: false, orderable: false }
+    }
+  end
+
+  private
+
+  def data
+    records.map do |record|
+      {
+        id:           record.id,
+        confirmed_at: record.confirmed_at,
+        email:        record.email,
+        name:         record.name,
+        attended:     record.attended_count,
+        roles:        record.roles.any? ? show_roles(record.get_roles) : 'None',
+        view_url:     admin_user_path(record),
+        edit_url:     edit_admin_user_path(record),
+        DT_RowId:     record.id
+      }
+    end
+  end
+
+  # rubocop:disable Naming/AccessorMethodName
+  def get_raw_records
+    User.left_outer_joins(:registrations, :roles).distinct
+      .select('users.*, COUNT(CASE WHEN registrations.attended = "t" THEN 1 END) AS attended_count').group('users.id')
+  end
+  # rubocop:enable Naming/AccessorMethodName
+
+  def records_total_count
+    fetch_records.unscope(:group).count(:all)
+  end
+
+  def records_filtered_count
+    filter_records(fetch_records).unscope(:group).count(:all)
+  end
+
+  # ==== These methods represent the basic operations to perform on records
+  # and feel free to override them
+
+  # def filter_records(records)
+  # end
+
+  # def sort_records(records)
+  # end
+
+  # def paginate_records(records)
+  # end
+
+  # ==== Insert 'presenter'-like methods below if necessary
+end