Rails：rake db：迁移失败`verify_readonly_attribute`

Question

此迁移在初始开发人员计算机上运行良好，但在第二个开发环境中失败。据我们所知，设置是相同的。

迁移：

class AddInventoryItemsCountToProduct < ActiveRecord::Migration 
    def change 
    add_column :products, :inventory_items_count, :integer, default: 0 

    reversible do |dir| 
     dir.up do 
     Product.all.each do |p| 
      p.update_attribute :inventory_items_count, p.inventory_items.length 
     end 
     end 
    end  
    end 
end 

没见过这个错误。任何人都知道发生了什么事？

$ rake db:migrate 
== AddInventoryItemsCountToProduct: migrating ================================ 
-- add_column(:products, :inventory_items_count, :integer, {:default=>0}) 
    -> 0.0112s 
rake aborted! 
An error has occurred, this and all later migrations canceled: 

inventory_items_count is marked as readonly/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/gems/activerecord-4.0.0/lib/active_record/persistence.rb:506:in `verify_readonly_attribute' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/gems/activerecord-4.0.0/lib/active_record/persistence.rb:217:in `update_attribute' 
/Users/meltemi/rails/input/myapp/db/migrate/20130823022914_add_inventory_items_count_to_product.rb:8:in `block (3 levels) in change' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/gems/activerecord-4.0.0/lib/active_record/relation/delegation.rb:13:in `each' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/gems/activerecord-4.0.0/lib/active_record/relation/delegation.rb:13:in `each' 
/Users/meltemi/rails/input/myapp/db/migrate/20130823022914_add_inventory_items_count_to_product.rb:7:in `block (2 levels) in change' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/gems/activerecord-4.0.0/lib/active_record/migration.rb:474:in `up' 
/Users/meltemi/rails/input/myapp/db/migrate/20130823022914_add_inventory_items_count_to_product.rb:6:in `block in change' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/gems/activerecord-4.0.0/lib/active_record/migration.rb:510:in `block in reversible' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/gems/activerecord-4.0.0/lib/active_record/migration.rb:687:in `execute_block' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/gems/activerecord-4.0.0/lib/active_record/migration.rb:510:in `reversible' 
/Users/meltemi/rails/input/myapp/db/migrate/20130823022914_add_inventory_items_count_to_product.rb:5:in `change' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/gems/activerecord-4.0.0/lib/active_record/migration.rb:571:in `exec_migration' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/gems/activerecord-4.0.0/lib/active_record/migration.rb:555:in `block (2 levels) in migrate' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/gems/activerecord-4.0.0/lib/active_record/migration.rb:554:in `block in migrate' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/gems/activerecord-4.0.0/lib/active_record/connection_adapters/abstract/connection_pool.rb:294:in `with_connection' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/gems/activerecord-4.0.0/lib/active_record/migration.rb:553:in `migrate' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/gems/activerecord-4.0.0/lib/active_record/migration.rb:709:in `migrate' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/gems/activerecord-4.0.0/lib/active_record/migration.rb:959:in `block in execute_migration_in_transaction' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/gems/activerecord-4.0.0/lib/active_record/migration.rb:1005:in `block in ddl_transaction' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/gems/activerecord-4.0.0/lib/active_record/connection_adapters/abstract/database_statements.rb:202:in `block in transaction' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/gems/activerecord-4.0.0/lib/active_record/connection_adapters/abstract/database_statements.rb:210:in `within_new_transaction' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/gems/activerecord-4.0.0/lib/active_record/connection_adapters/abstract/database_statements.rb:202:in `transaction' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/gems/activerecord-4.0.0/lib/active_record/transactions.rb:209:in `transaction' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/gems/activerecord-4.0.0/lib/active_record/migration.rb:1005:in `ddl_transaction' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/gems/activerecord-4.0.0/lib/active_record/migration.rb:958:in `execute_migration_in_transaction' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/gems/activerecord-4.0.0/lib/active_record/migration.rb:920:in `block in migrate' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/gems/activerecord-4.0.0/lib/active_record/migration.rb:916:in `each' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/gems/activerecord-4.0.0/lib/active_record/migration.rb:916:in `migrate' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/gems/activerecord-4.0.0/lib/active_record/migration.rb:764:in `up' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/gems/activerecord-4.0.0/lib/active_record/migration.rb:742:in `migrate' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/gems/activerecord-4.0.0/lib/active_record/railties/databases.rake:42:in `block (2 levels) in <top (required)>' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/bin/ruby_noexec_wrapper:14:in `eval' 
/Users/meltemi/.rvm/gems/ruby-2.0.0-p247@myapp/bin/ruby_noexec_wrapper:14:in `<main>' 
Tasks: TOP => db:migrate 
(See full trace by running task with --trace) 

Answer 1

检查这个答案：Problem with counter_cache implementation 你也可能采取第二个忠告：find_each，而不是每个