mysql - ruby/sql 语法未定义方法 `query' for nil :NilClass (NoMethodError)

Question

我不断收到以下错误消息:

.rb:215:in block in collectData': undefined methodquery' for nil:NilClass (NoMethodError)

这是我的代码:

databases = {
  'aaa': Mysql2::Client.new(
      host:     '',
      username: '',
      password: '',
      database: ''
    ),
  'bbb': Mysql2::Client.new(
      host:     '',
      username: '',
      password: '',
      database: ''
    )
}

client1.query(" bla bla bla").each do |row|
  if row['mmm'] == 99
    # Look in next db
    next_db = databases[row['db']]
    row2 = next_db.query("SELECT...")

我从第一个 sql 查询 client1.query 中获取数据库的名称，例如 row['db'] = "aaa" 或 “bbb”。

对于测试，这样的东西也不起作用:

client1.query(" bla bla bla").each do |row|
  if row['mmm'] == 99
    # Look in next db
    next_db = databases['aaa']
    row2 = next_db.query("SELECT...")

像这样的东西是有效的:

client1 = Mysql2::Client.new(
    host:     '',
    username: '',
    password: '',
    database: ''
  )

client2 = Mysql2::Client.new(
    host:     '',
    username: '',
    password: '',
    database: ''
  )

client1.query(" bla bla bla").each do |row|
  if row['mmm'] == 99
    # Look in next db
    next_db = client2
    row2 = next_db.query("SELECT ")

我似乎无法理解为什么我的方法是“未定义”。

请帮忙，

谢谢!

Answer 1

因此，您拥有数据库的哈希值，并且您希望根据第一个数据库查询的结果查询下一个数据库...

基本上，您拥有的代码是正确的，只是您从不检查nil。在这种情况下，无论 row['db'] 返回什么，在 databases 哈希中都找不到匹配的数据库。我假设这是一个需要修复的异常。

所以...

# hash of databases
databases = {
   'aaa' => Mysql2::Client.new(host: '', username: '', password: '', database: ''),
   'bbb' => Mysql2::Client.new(host: '',username: '', password: '', database: '')
 }

# get the first database
client1 = databases['aaa'] 

# perform the query
client1.query("bla bla bla").each do |row|
  if row['mmm'] == 99
    # look in next db
    client2 = databases[row['db']]

    if (client2)
      row2 = client2.query("SELECT...")
    else
      raise "Database `#{row['db']}` is not defined."
    end
  end
end