call_and_output.txt

# Call and output
prod = Product.create(:hive_id => 1214241241414241, :name => "whatever")
res = prod.min_max_avg

# Error

Failure/Error: res = prod.min_max_avg
    keys must be strings or symbols
    # /home/michael/.rvm/gems/ruby-1.8.7-p249/gems/bson-1.0.3/lib/../lib/bson/bson_c.rb:24:in `serialize'
    # /home/michael/.rvm/gems/ruby-1.8.7-p249/gems/bson-1.0.3/lib/../lib/bson/bson_c.rb:24:in `serialize'
    # /home/michael/.rvm/gems/ruby-1.8.7-p249/gems/mongo-1.0.3/lib/../lib/mongo/cursor.rb:364:in `construct_query_message'
    # /home/michael/.rvm/gems/ruby-1.8.7-p249/gems/mongo-1.0.3/lib/../lib/mongo/cursor.rb:346:in `send_initial_query'
    # /home/michael/.rvm/gems/ruby-1.8.7-p249/gems/mongo-1.0.3/lib/../lib/mongo/cursor.rb:324:in `refill_via_get_more'
    # /home/michael/.rvm/gems/ruby-1.8.7-p249/gems/mongo-1.0.3/lib/../lib/mongo/cursor.rb:319:in `num_remaining'
    # /home/michael/.rvm/gems/ruby-1.8.7-p249/gems/mongo-1.0.3/lib/../lib/mongo/cursor.rb:92:in `has_next?'
    # /home/michael/.rvm/gems/ruby-1.8.7-p249/gems/mongo-1.0.3/lib/../lib/mongo/cursor.rb:205:in `to_a'
    # /home/michael/.rvm/gems/ruby-1.8.7-p249/gems/plucky-0.3.2/lib/plucky/query.rb:75:in `all'
    # /home/michael/.rvm/gems/ruby-1.8.7-p249/gems/mongo_mapper-0.8.2/lib/mongo_mapper/plugins/querying/decorator.rb:25:in `all'
    # ./app/models/series.rb:21:in `aggregatable_prices'
    # ./app/models/product.rb:33:in `min_max_avg'
    # /home/michael/.rvm/gems/ruby-1.8.7-p249@global/gems/activesupport-3.0.0.beta4/lib/active_support/core_ext/object/returning.rb:39:in `returning'
    # ./app/models/product.rb:32:in `min_max_avg'
    # ./spec/models/product_spec.rb:72

product.rb

class Product
  include MongoMapper::Document

  key :hive_id, String, :required => true, :index => true
  key :name,    String, :required => true
  timestamps!

  validates_uniqueness_of :hive_id

  many :prices
  # Returns an array with the min, max and average for the range defined under
  # the Prices aggregatable scope
  def min_max_avg
    @min_max_avg ||= returning({}) do |res|
      # Chokes HERE on aggregatable_prices
      totals = Series.aggregatable_prices(:amount, :qty).inject({:min => nil, :max => nil, :sum => 0.0, :qty => 0}) {|h,price|
        # yadda yadda yadda.
      }
    end
  end
end

series.rb

class Series
  include MongoMapper::Document

  AGGREGATION_INTERVAL = 5.days

  key :started_on, Time, :required => true
  key :ended_on,   Time

  many :prices

  # This scope is what we use for calculating min, max and average
  scope :aggregatable, lambda {where(:started_on.gte => AGGREGATION_INTERVAL.ago)}

  class << self
    def aggregatable_prices(*select_cols)
      finder_options = select_cols.empty? ? {} : {:fields => select_cols}
      aggregatable.all.collect {|s| s.prices.all(finder_options)}.flatten
    end
  end
end