halp.rb

  def self.search(query)
    query
      .split(" ")
      .collect do |query|
        query.split(":")
      end.inject(self) do |klass, (name, q)|
        association = klass.reflect_on_association(name.pluralize.to_sym)
        association_table = association.klass.arel_table

        if [:has_and_belongs_to_many, :belongs_to].include?(association.macro)
          joins(name.pluralize.to_sym).where(association_table["name"].eq(q))
        else
          all
        end
      end
  end

@soulcutter, in Rails 4, all just returns a scope. The naming is better that way, though.