class ActiveAdmin::Inputs::Filters::SelectInput

def collection

Provides an efficient default lookup query if the attribute is a DB column. 
def collection
  if !options[:collection] && column
    pluck_column
  else
    super
  end
end

def include_blank

Provide the AA translation to the blank input field. 
def include_blank
  I18n.t 'active_admin.any' if super
end

def input_html_options_name 

def input_html_options_name
  "#{object_name}[#{input_name}]" # was "#{object_name}[#{association_primary_key}]"
end

def input_name 

def input_name
  return method if seems_searchable?
  searchable_method_name + (multiple? ? '_in' : '_eq')
end

def multiple_by_association?

cause the select field to be multi-select instead of a dropdown.
Would normally return true for has_many and HABTM, which would subsequently 
def multiple_by_association?
  false
end

def pluck_column 

def pluck_column
  klass.reorder("#{method} asc").distinct.pluck method
end

def searchable_method_name 

def searchable_method_name
  if searchable_has_many_through?
    "#{reflection.through_reflection.name}_#{reflection.foreign_key}"
  else
    name = method.to_s
    name.concat '_id' if reflection
    name
  end
end