执行 sqlalchemy 存在查询

Question

我无法理解如何执行查询以检查 sqlalchemy 中是否已存在匹配记录。我可以在网上找到的大多数示例似乎都引用了我没有的“会话”和“查询”对象。

这是一个简短的完整程序来说明我的问题：
1. 使用“person”表建立内存sqlite db。
2.在person表中插入两条记录。
3. 检查表中是否存在特定记录。这就是它呕吐的地方。

from sqlalchemy import create_engine, Table, Column, Integer, String, MetaData
from sqlalchemy.sql.expression import exists

engine = create_engine('sqlite:///:memory:', echo=False)
metadata = MetaData()

person = Table('person', metadata,
                        Column('id', Integer, primary_key=True),
                        Column('name', String(255), nullable=False))

metadata.create_all(engine)
conn = engine.connect()

s = person.insert()
conn.execute(s, name="Alice")
conn.execute(s, name="Bob")

print("I can see the names in the table:")
s = person.select()
result = conn.execute(s)
print(result.fetchall())

print('This query looks like it should check to see if a matching record exists:')
s = person.select().where(person.c.name == "Bob")
s = exists(s)
print(s)

print("But it doesn't run...")
result = conn.execute(s)

这个程序的输出是：

I can see the names in the table:
[(1, 'Alice'), (2, 'Bob')]
This query looks like it should check to see if a matching record exists:
EXISTS (SELECT person.id, person.name 
FROM person 
WHERE person.name = :name_1)
But it doesn't run...
Traceback (most recent call last):
  File "/project_path/db_test/db_test_env/exists_example.py", line 30, in <module>
    result = conn.execute(s)
  File "/project_path/db_test/db_test_env/lib/python3.6/site-packages/sqlalchemy/engine/base.py", line 945, in execute
    return meth(self, multiparams, params)
  File "/project_path/db_test/db_test_env/lib/python3.6/site-packages/sqlalchemy/sql/elements.py", line 265, in _execute_on_connection
    raise exc.ObjectNotExecutableError(self)
sqlalchemy.exc.ObjectNotExecutableError: Not an executable object: <sqlalchemy.sql.selectable.Exists object at 0x105797438>

Answer 1

s.exists() 仅构建存在子句。要使代码正常工作，您需要做的就是为其生成一个选择。

s = exists(s).select()

这是您的完整示例：

from sqlalchemy import create_engine, Table, Column, Integer, String, MetaData
from sqlalchemy.sql.expression import exists

engine = create_engine('sqlite:///:memory:', echo=False)
metadata = MetaData()

person = Table('person', metadata,
                        Column('id', Integer, primary_key=True),
                        Column('name', String(255), nullable=False))

metadata.create_all(engine)
conn = engine.connect()

s = person.insert()
conn.execute(s, name="Alice")
conn.execute(s, name="Bob")

print("I can see the names in the table:")
s = person.select()
result = conn.execute(s)
print(result.fetchall())

print('This query looks like it should check to see if a matching record exists:')
s = person.select().where(person.c.name == "Bob")
s = exists(s).select()
print(s)

print("And it runs fine...")
result = conn.execute(s)
print(result.fetchall())

Answer 2

exists 用于 SQL 子查询。如果您有一个表 posts 包含带有 author_id 的博客文章，并映射回人员，您可以使用如下查询来查找发表博客文章的人：

select * from people where exists (select author_id from posts where author_id = people.id);

您不能将存在作为 SQL 查询中的最外层语句；它是在 SQL 布尔子句中使用的运算符。 因此，SQLAlchemy 不允许您执行该查询，因为它的格式不正确。 如果要查看一行是否存在，只需构造一个带有 where 子句的 select 语句，然后查看查询返回了多少行。

Answer 3

试试这个：

...
s = person.select().where(person.c.name == "Bob")
s = select(exists(s))
print(s)
...

Answer 4

除非有人提出更好的答案，否则这就是我想出的有效方法。让数据库计算匹配记录并将计数发送到 python 应用程序。

from sqlalchemy import select, func   # more imports not in my example code above

s = select([func.count(1)]).select_from(person).where(person.c.name == "Bob")
print(s)
record_count = conn.execute(s).scalar()
print("Matching records: ", record_count)

示例输出：

SELECT count(:count_2) AS count_1 
FROM person 
WHERE person.name = :name_1
Matching records:  1