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27-char字符串时的追溯：

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27个字符串变成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

此：

session.execute(s_email_lookup_by_email, (email_address))

并没有真正将1元组传递给execute方法。

在python中，要创建一个1元组，你需要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，这使得(email_address)等同于email_address而没有括号。

在Python中准备Cassandra SELECT语句

1 个答案: