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

Rails 2.3.3 and lower

From the ActiveRecord CHANGELOG^{(v1.12.0, October 16th, 2005)}:

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

Rails 2.3.4 and above

Most of the above no longer holds true, after September 10 2012:

• 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 @aaronrustad‘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.)