SimpleForm Sticky Association Input

form.html.haml

-# form.html.haml 

= simple_form_for @project do |f|
  -# normal SimpleForm association
  = f.association :company
  -# new and improved "sticky" association
  = f.sticky_association :company
  = f.input :name
  = f.submit

simple_form_sticky_association.rb

# config/initializers/simple_form_sticky_association.rb

require 'simple_form'

SimpleForm::FormBuilder.class_eval do
  def sticky_association(association, options = {}, &block)
    # adapted from https://github.com/plataformatec/simple_form/blob/master/lib/simple_form/form_builder.rb#L206
    options = options.dup
    return simple_fields_for(*[association, options.delete(:collection), options].compact, &block) if block_given?
    raise ArgumentError, "Association cannot be used in forms not associated with an object" unless @object

    reflection = find_association_reflection(association)
    raise "Association #{association.inspect} not found" unless reflection

    options[:as] ||= :select
    options[:collection] ||= fetch_association_collection(reflection, options)

    # Custom logic
    # Ensure the associated model is included in the collection
    associated_id = self.object.send(reflection.foreign_key)
    collection_items = options[:collection].to_a
    item_exists = collection_items.find_index{|x| x.id == associated_id}.present?
    if (!item_exists)
      associated_model = reflection.klass.unscoped.find(associated_id)
      collection_items.push(associated_model)
      options[:collection] = collection_items
    end
    # End custom logic

    attribute = build_association_attribute(reflection, association, options)

    input(attribute, options.merge(reflection: reflection))
  end
end