class ActiveAdmin::Inputs::FilterSelectInput

def collection

Provides an efficient default lookup query if the attribute is a DB column. 
def collection
  unless Rails::VERSION::MAJOR == 3 && Rails::VERSION::MINOR < 2
    return pluck_column if !options[:collection] && column_for(method)
  end
  super
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

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

def input_name

That doesn't fit our scenario, so we override it here.
When it's a HABTM or has_many association, Formtastic builds "object_ids".
Otherwise:

If MetaSearch already responds to the given method, just use it. 
def input_name
  return method if @object.respond_to? method
  name = method.to_s
  name.concat '_id' if reflection
  name.concat 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
  @object.base.reorder("#{method} asc").uniq.pluck method
end