我在Rails / Postgres应用程序中有一列想要从Hstore更改为Array。（我将手机存储为哈希，所以可以这样做{default: 123, mobile: 1234}，但认为这是不必要的/没用的）
因此，我进行了以下迁移：
class ChangePhonesToArray < ActiveRecord::Migration
def up
new_phones = {}
Place.find_each do |p|
new_phones[p.id] = p.phones.values.map{ |v| v.gsub(%r!\D!, '') } # get rid of non-number characters while I'm at it
end
remove_column :places, :phones
add_column :places, :phones, :string, array: true, default: []
new_phones.each do |k, v|
p = Place.find(k)
p.update_attributes!(phones: v)
end
end
...
end
但是，当我这样做时，我得到了这个讨厌的数据库错误，表明那phones仍然是Hstore列！
StandardError: An error has occurred, this and all later migrations canceled:
can't cast Array to hstore/Users/sasha/.rvm/gems/ruby-2.1.2/gems/activerecord-4.1.4/lib/active_record/connection_adapters/abstract/quoting.rb:76:in `type_cast'
/Users/sasha/.rvm/gems/ruby-2.1.2/gems/activerecord-4.1.4/lib/active_record/connection_adapters/postgresql/quoting.rb:111:in `type_cast'
/Users/sasha/.rvm/gems/ruby-2.1.2/gems/activerecord-4.1.4/lib/active_record/connection_adapters/postgresql_adapter.rb:828:in `block in exec_cache'
/Users/sasha/.rvm/gems/ruby-2.1.2/gems/activerecord-4.1.4/lib/active_record/connection_adapters/postgresql_adapter.rb:827:in `map'
/Users/sasha/.rvm/gems/ruby-2.1.2/gems/activerecord-4.1.4/lib/active_record/connection_adapters/postgresql_adapter.rb:827:in `exec_cache'
/Users/sasha/.rvm/gems/ruby-2.1.2/gems/activerecord-4.1.4/lib/active_record/connection_adapters/postgresql/database_statements.rb:155:in `exec_delete'
/Users/sasha/.rvm/gems/ruby-2.1.2/gems/activerecord-4.1.4/lib/active_record/connection_adapters/abstract/database_statements.rb:101:in `update'
/Users/sasha/.rvm/gems/ruby-2.1.2/gems/activerecord-4.1.4/lib/active_record/connection_adapters/abstract/query_cache.rb:14:in `update'
我以为该列未被正确删除，因此我remove_column在第一列之后立即扔了第二列，这引发了以下错误，表明该列已被移除！（但显然并不完全）。
StandardError: An error has occurred, this and all later migrations canceled:
PG::UndefinedColumn: ERROR: column "phones" of relation "places" does not exist
: ALTER TABLE "places" DROP "phones"/Users/sasha/.rvm/gems/ruby-2.1.2/gems/rack-mini-profiler-0.9.2/lib/patches/sql_patches.rb:160:in `exec'
/Users/sasha/.rvm/gems/ruby-2.1.2/gems/rack-mini-profiler-0.9.2/lib/patches/sql_patches.rb:160:in `async_exec'
/Users/sasha/.rvm/gems/ruby-2.1.2/gems/activerecord-4.1.4/lib/active_record/connection_adapters/postgresql/database_statements.rb:128:in `block in execute'
/Users/sasha/.rvm/gems/ruby-2.1.2/gems/activerecord-4.1.4/lib/active_record/connection_adapters/abstract_adapter.rb:373:in `block in log'
/Users/sasha/.rvm/gems/ruby-2.1.2/gems/activesupport-4.1.4/lib/active_support/notifications/instrumenter.rb:20:in `instrument'
/Users/sasha/.rvm/gems/ruby-2.1.2/gems/activerecord-4.1.4/lib/active_record/connection_adapters/abstract_adapter.rb:367:in `log'
/Users/sasha/.rvm/gems/ruby-2.1.2/gems/activerecord-4.1.4/lib/active_record/connection_adapters/postgresql/database_statements.rb:127:in `execute'
知道这里发生了什么，以及如何解决/解决它吗？
仅仅因为您更改了数据库内部的表架构并不意味着您的Place类知道这些更改。ActiveRecord::Base子类仅加载一次列信息，以避免不必要地一遍又一遍地访问数据库。一旦您这样做：
Place.find_each
您Place将知道列的类型。然后，您更改背后的模式，Place并尝试编写新值，但Place不知道更改。解决此问题的常用方法是致电reset_column_information：
重置所有有关列的缓存信息，这将导致它们在下一个请求时重新加载。
此方法最常见的用法模式可能是在迁移中...
所以你只想说：
#...
remove_column :places, :phones
add_column :places, :phones, :string, array: true, default: []
Place.reset_column_information
#...
顺便说一句，不能保证hstore将保持任何顺序，因此您可能希望p.phones.values_at(:default, :mobile)而不是p.phones.values确保数组中的顺序正确。
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我来说两句