What is causing this ActiveRecord::ReadOnlyRecord error?

This follows [this][1] 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.

[1]: https://stackoverflow.com/questions/628000/rails-whats-wrong-with-this-multiple-join-with-conditions-on-the-associations "this"

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.)

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

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

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.

select('*') seems to fix this in Rails 3.2:

> Contact.select('*').joins(:slugs).where('slugs.slug' => 'the-slug').first.readonly?
=> false

Just to verify, omitting select('*') does produce a readonly record:

> Contact.joins(:slugs).where('slugs.slug' => 'the-slug').first.readonly?
=> true

Can't say I understand the rationale but at least it's a quick and clean workaround.

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]

To deactivate it...

module DeactivateImplicitReadonly
  def custom_join_sql(*args)
    result = super
    @implicit_readonly = false
    result
  end
end
ActiveRecord::Relation.send :include, DeactivateImplicitReadonly