在rails中生成自动增量字段

Question

我有一个模型令牌,它有一个字段token_number,我需要自动递增(从1001开始),当且仅当用户不提供它.问题是,由于用户可以选择提供此字段,因此无法完全查询数据库并请求最大的token_number.我在这个论坛上找到了一个答案,但我确信必须有一个更好的方法来执行它而不是执行SQL语句？在Ruby on Rails中自动增加非主键字段

Answer 1

对我来说有趣的问题.遗憾的是,rails并没有提供自动增加列的方法,因此我们必须采用几乎没有自动化的SQL.我在Rails 3.0.7中尝试使用PostgreSQL作为我的数据库并且它有效并且希望这将是有用的:

为token_number PGSql文档创建序列

class CreateTokens < ActiveRecord::Migration

  def self.up
    create_table :tokens do |t|
      t.string :name
      t.integer :token_number

      t.timestamps
    end

    execute "CREATE SEQUENCE tokens_token_number_seq START 1001"
  end

  def self.down
    drop_table :tokens

    execute "DROP SEQUENCE tokens_token_number_seq"
  end
end

现在,由于用户手动设置了token_number,我们只需要在未设置的情况下生成token_number.在这里阅读回调.有了这个,我们有,

class Token < ActiveRecord::Base
  # Generate the sequence no if not already provided.
  before_validation(:on => :create) do
    self.application_no = next_seq unless attribute_present?("application_no")
  end

  private
    def next_seq(column = 'application_no')
      # This returns a PGresult object [http://rubydoc.info/github/ged/ruby-pg/master/PGresult]
      result = Token.connection.execute("SELECT nextval('tokens_token_number_seq')")

      result[0]['nextval']
    end 
end

样本运行.请注意,对于第一个令牌,我没有设置token_number,它会生成token_number序列,而我正在分配第二个令牌.

token = Token.new
# => #<Token id: nil, name: nil, token_number: nil, created_at: nil, updated_at: nil> 

token.save
  SQL (0.8ms)  BEGIN
  SQL (1.7ms)  SELECT nextval('tokens_token_number_seq')
  SQL (6.6ms)   SELECT tablename
 FROM pg_tables
 WHERE schemaname = ANY (current_schemas(false))

  SQL (33.7ms)  INSERT INTO "tokens" ("name", "token_number", "created_at", "updated_at") VALUES (NULL, 1001, '2012-03-02 12:04:00.848863', '2012-03-02 12:04:00.848863') RETURNING "id"
  SQL (15.9ms)  COMMIT
# => true 

token = Token.new
# => #<Token id: nil, name: nil, token_number: nil, created_at: nil, updated_at: nil> 

token.token_number = 3000
# => 3000 

token.save
  SQL (0.8ms)  BEGIN
  SQL (1.5ms)  INSERT INTO "tokens" ("name", "token_number", "created_at", "updated_at") VALUES (NULL, 3000, '2012-03-02 12:04:22.924834', '2012-03-02 12:04:22.924834') RETURNING "id"
  SQL (19.2ms)  COMMIT
# => true 

Answer 2

另一个选择是将 nextval 设置为 postgresql 中的默认值

这使我们不需要编写回调

class CreateTokens < ActiveRecord::Migration

  def self.up
    create_table :tokens do |t|
      t.string :name
      t.integer :token_number

      t.timestamps
    end

    execute "CREATE SEQUENCE tokens_token_number_seq START 1001"
    execute "ALTER TABLE tokens ALTER COLUMN token_number SET DEFAULT NEXTVAL('tokens_token_number_seq')"
  end

  def self.down
    drop_table :tokens

    execute "DROP SEQUENCE tokens_token_number_seq"
  end
end