Provide select2 input class for simple_form (simple_form).
Based on auto_select2 gem.
Add this line to your application's Gemfile:
gem 'simple_form_auto_select2'
f.association :project, as: :select2
f.input :user_id, as: :ajax_select2, placeholder: t(:select_user_from_list),
default_class_name: :user, default_text_column: [:id, :firstname, :lastname]
More info see in documentation of auto_select2 gem and in auto_select2_tag_example project