在Python中准备Cassandra SELECT语句

Question

我正在尝试针对Cassandra表运行准备好的选择查询.

该表定义如下:

class EmailAddressLookup(Model, ModelOperations, JSONSerializer):
    __table_name__ = 'email_address_lookup'

    email_address = columns.Text(primary_key=True)
    user_id = columns.Integer(primary_key=True)

我的INSERT很棒.它看起来像这样:

i_email_lookup = session.prepare("""INSERT INTO email_address_lookup (user_id, email_address) VALUES (?, ?)""")

session.execute(i_email_lookup, (user_id, email_address))

但是,我的SELECT无效.它看起来像这样:

s_email_lookup_by_email = session.prepare('SELECT user_id FROM email_address_lookup WHERE "email_address"=?')

session.execute(s_email_lookup_by_email, email_address)

这是email_address是一个27-char字符串时的回溯:

Traceback (most recent call last):
  File "/home/thisguy/Documents/my_proj/api/models/case.py", line 428, in _lookup_usin_email
    matches = my_projcluster.select_email_lookup_by_email(self.email)
  File "/home/thisguy/Documents/my_proj/api/utils/decorators.py", line 14, in newfunc
    result = func(*args, **kwargs)
  File "/home/thisguy/Documents/my_proj/api/models/my_projcluster.py", line 203, in select_email_lookup_by_email
    session.execute(s_email_lookup_by_email, (email_address))
  File "/home/thisguy/.Envs/my_proj/lib/python3.4/site-packages/cassandra/cluster.py", line 1403, in execute
    future = self.execute_async(query, parameters, trace)
  File "/home/thisguy/.Envs/my_proj/lib/python3.4/site-packages/cassandra/cluster.py", line 1452, in execute_async
    future = self._create_response_future(query, parameters, trace)
  File "/home/thisguy/.Envs/my_proj/lib/python3.4/site-packages/cassandra/cluster.py", line 1464, in _create_response_future
    query = query.bind(parameters)
  File "/home/thisguy/.Envs/my_proj/lib/python3.4/site-packages/cassandra/query.py", line 390, in bind
    return BoundStatement(self).bind(values)
  File "/home/thisguy/.Envs/my_proj/lib/python3.4/site-packages/cassandra/query.py", line 486, in bind
    (len(values), len(col_meta)))
ValueError: Too many arguments provided to bind() (got 27, expected 1)

好的,所以Python试图将一个27字符的字符串变成27个args？让我们把它包装成一个元组:

session.execute(s_email_lookup_by_email, (email_address, ))

这可以防止回溯,但即使数据库中确实有匹配的电子邮件地址,查询也会返回None.

Answer 1

这个:

session.execute(s_email_lookup_by_email, (email_address))

并没有真正传递1元组的execute方法.

在python中,要创建一个1元组,您需要使用逗号跟随第一个元素:

session.execute(s_email_lookup_by_email, (email_address,))

否则括号被解释为分组运算符,并且(email_address)相当于email_address没有括号.