What is causing this ActiveRecord::ReadOnlyRecord error?

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.

From stackoverflow

• 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.)
  : the :include is helpful in reducing the # of queries done, I didn't know about that; but I tried to fix it by changing the Tableau/Deckcards association to a has_many: through, and now I'm getting a 'could not find association' msg; I may have to post another question for that

  vladr : @codeman, yes, the :include will reduce the number of queries *and* will bring the included table into your condition scope (a sort of implicit join without Rails marking your records as read-only, which it does as soon as it sniffs anything SQL-ish in your find, including :join/:select clauses IIRC

  vladr : For 'has_many :a, through => :b' to work, the B association must be declared as well, e.g. 'has_many :b; has_many :a, :through => :b', I hope this is your case?

  : yes, I didn't have the 'has_many :b' association; once I got that it all worked great - Thanks!

  BigCanOfTuna : This might have changed in recent releases, but you can simply add :readonly => false as part of the find method attributes.

  Lee : This answer is also applicable if you have a has_and_belongs_to_many association with a custom :join_table specified.

• 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]

• 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.

  Olly : I don't believe this is the case, with 2.3.4 at least

• 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.