What is causing this ActiveRecord::ReadOnlyRecord error?

Question

This follows this prior question, which was answered. I actually discovered I could remove a join from that query, so now the working query is

start_cards = DeckCard.find :all, :joins => [:card], :conditions => ["deck_cards.deck_id = ? and cards.start_card = ?", @game.deck.id, true]

This appears to work. However, when I try to move these DeckCards into another association, I get the ActiveRecord::ReadOnlyRecord error.

Here's the code

for player in @game.players 
  player.tableau = Tableau.new
  start_card = start_cards.pop 
  start_card.draw_pile = false
  player.tableau.deck_cards << start_card  # the error occurs on this line
end

and the relevant Models (tableau are the players cards on the table)

class Player < ActiveRecord::Base
  belongs_to :game
  belongs_to :user
  has_one :hand
  has_one :tableau
end

class Tableau < ActiveRecord::Base
  belongs_to :player
  has_many :deck_cards
end  

class DeckCard < ActiveRecord::Base
  belongs_to :card
  belongs_to :deck  
end

I am doing a similar action just after this code, adding DeckCards to the players hand, and that code is working fine. I wondered if I needed belongs_to :tableau in the DeckCard Model, but it works fine for the adding to player's hand. I do have a tableau_id and hand_id columns in the DeckCard table.

I looked up ReadOnlyRecord in the rails api, and it doesn't say much beyond the description.

Answer 1

From the ActiveRecord CHANGELOG:

Introduce read-only records. If you call object.readonly! then it will mark the object as read-only and raise ReadOnlyRecord if you call object.save. object.readonly? reports whether the object is read-only. Passing :readonly => true to any finder method will mark returned records as read-only. The :joins option now implies :readonly, so if you use this option, saving the same record will now fail. Use find_by_sql to work around.

Using find_by_sql is not really an alternative as it returns raw row/column data, not ActiveRecords. You have two options:

1. Force the instance variable @readonly to false in the record (hack)
2. Use :include => :card instead of :join => :card

Cheers, V.

Sep 2010 UPDATE

Most of the above no longer holds true. Thus, in Rails 2.3.4 and 3.0.0:

• using Record.find_by_sql is a viable option
• :readonly => true is automatically inferred only if :joins was specified without an explicit :select nor an explicit (or finder-scope-inherited) :readonly option (see the implementation of set_readonly_option! in active_record/base.rb for Rails 2.3.4, or the implementation of to_a in active_record/relation.rb and of custom_join_sql in active_record/relation/query_methods.rb for Rails 3.0.0)
• however, :readonly => true is always automatically inferred in has_and_belongs_to_many if the join table has more than the two foreign keys columns and :joins was specified without an explicit :select (i.e. user-supplied :readonly values are ignored -- see finding_with_ambiguous_select? in active_record/associations/has_and_belongs_to_many_association.rb.)
• in conclusion, unless dealing with a special join table and has_and_belongs_to_many, then @BigCanOfTuna's answer applies just fine in Rails 2.3.4 and 3.0.0.
• do not use :includes if you want to achieve an INNER JOIN (:includes implies a LEFT OUTER JOIN, which is less selective and less efficient than INNER JOIN.)

Answer 2

Instead of find_by_sql, you can specify a :select on the finder and everything's happy again...

start_cards = DeckCard.find :all, :select => 'deck_cards.*', :joins => [:card], :conditions => ["deck_cards.deck_id = ? and cards.start_card = ?", @game.deck.id, true]

Answer 3

This might have changed in recent release of Rails, but the appropriate way to solve this problem is to add :readonly => false to the find options.

Answer 4

Or in Rails 3 you can use the readonly method (replace "..." with your conditions):

( Deck.joins(:card) & Card.where('...') ).readonly(false)

See http://m.onkey.org/2010/1/22/active-record-query-interface for more info.

There are also some good Railscasts on this. See "Active Record Queries in Rails 3" and "Advanced Queries in Rails 3". I can't post the links here due to the spam filter.