对我有趣的问题。不幸的是,Rails没有提供自动增加列的方法,所以我们必须尽量少用自动化。我在使用PostgreSQL作为我的数据库的Rails 3.0.7尝试这样做,它的工作原理,并希望这将是有用的:
为token_number创建序列现在PGSql Documentation
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
,因为没有被设置token_number的可能性通过用户手动,我们只需要在没有设置的情况下生成token_number。 Read about Callbacks here。有了我们,
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
一个示例运行。请注意,对于第一个令牌,我不设置令牌编号,它会生成令牌编号序列,第二个令牌编号。
ruby-1.9.2-p0 > token = Token.new
=> #<Token id: nil, name: nil, token_number: nil, created_at: nil, updated_at: nil>
ruby-1.9.2-p0 > 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
ruby-1.9.2-p0 > token = Token.new
=> #<Token id: nil, name: nil, token_number: nil, created_at: nil, updated_at: nil>
ruby-1.9.2-p0 > token.token_number = 3000
=> 3000
ruby-1.9.2-p0 > 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
ruby-1.9.2-p0 >
是否有可能以某种方式使用默认值与'nextval'组合以避免使用回调? – freemanoid 2014-09-17 10:26:39