ActiveModel::Serializer fails w/ Array of Hashes

bracket_serializer.rb

# This is a problem with an array of Hash objects wherein each Hash object
# needs to be manipulated rather than all keys serialized.
# 
# @bracket.class # => Bracket
# @bracket.seats.class # => Array
# @bracket.seats[0].class # => Hash
# @bracket.seats[0].keys # => ['position','round', 'won']

class BracketSerializer < ActiveModel::Serializer
  attribute :final_size, key: :size
  attributes :type, :title

  # this is necessary in order to do any filtration on a per-seat basis,
  # even though seats is an array of Hashes, not an array of Seat objects
  has_many :seats, serializer: SeatSerializer

  # Returns an array of hashes in binary tree format
  # 
  # Because Hash does not respond to `read_attribute_for_serialization`,
  # ActiveModel::Serializer explodes trying to serialize the has_many
  def seats
    bracket.to_tree.to_a
  end
end

error_output

undefined method `read_attribute_for_serialization' for {:position=>256, :round=>9}:Hash

MY_PROJECT_DIR/bundler/gems/active_model_serializers-a2d73faa6335/lib/active_model/serializer.rb:493:in `block in attributes'
MY_PROJECT_DIR/bundler/gems/active_model_serializers-a2d73faa6335/lib/active_model/serializer.rb:492:in `each'
MY_PROJECT_DIR/bundler/gems/active_model_serializers-a2d73faa6335/lib/active_model/serializer.rb:492:in `attributes'
MY_PROJECT_DIR/bundler/gems/active_model_serializers-a2d73faa6335/lib/active_model/serializer.rb:397:in `serializable_hash'
MY_PROJECT_DIR/bundler/gems/active_model_serializers-a2d73faa6335/lib/active_model/serializer.rb:191:in `block in serialize'
MY_PROJECT_DIR/bundler/gems/active_model_serializers-a2d73faa6335/lib/active_model/serializer.rb:190:in `map'
MY_PROJECT_DIR/bundler/gems/active_model_serializers-a2d73faa6335/lib/active_model/serializer.rb:190:in `serialize'
MY_PROJECT_DIR/bundler/gems/active_model_serializers-a2d73faa6335/lib/active_model/serializer.rb:464:in `include!'
MY_PROJECT_DIR/bundler/gems/active_model_serializers-a2d73faa6335/lib/active_model/serializer.rb:410:in `block in include_associations!'
MY_PROJECT_DIR/bundler/gems/active_model_serializers-a2d73faa6335/lib/active_model/serializer.rb:403:in `each'
MY_PROJECT_DIR/bundler/gems/active_model_serializers-a2d73faa6335/lib/active_model/serializer.rb:403:in `include_associations!'
MY_PROJECT_DIR/bundler/gems/active_model_serializers-a2d73faa6335/lib/active_model/serializer.rb:398:in `serializable_hash'
MY_PROJECT_DIR/bundler/gems/active_model_serializers-a2d73faa6335/lib/active_model/serializer.rb:387:in `as_json'

seat_serializer.rb

# The serializer used to handle data filtration on each Hash
# in the Bracket seat array
class SeatSerializer < ActiveModel::Serializer
  attributes :position, :round
end

Hello,

I have the same problem. Did you find the mistake eventually?