如何用sqlalchemy編寫自己的聚合函數？

Question

如何用SQLAlchemy編寫自己的聚合函數？ 作為一個簡單的例子，我想使用numpy來計算方差。 使用sqlite它看起來像這樣：

import sqlite3 as sqlite
import numpy as np

class self_written_SQLvar(object):
  def __init__(self):
    import numpy as np
    self.values = []
  def step(self, value):
    self.values.append(value)
  def finalize(self):
    return np.array(self.values).var()

cxn = sqlite.connect(':memory:')
cur = cxn.cursor()
cxn.create_aggregate("self_written_SQLvar", 1, self_written_SQLvar)
# Now - how to use it:
cur.execute("CREATE TABLE 'mytable' ('numbers' INTEGER)")
cur.execute("INSERT INTO 'mytable' VALUES (1)") 
cur.execute("INSERT INTO 'mytable' VALUES (2)") 
cur.execute("INSERT INTO 'mytable' VALUES (3)") 
cur.execute("INSERT INTO 'mytable' VALUES (4)")
a = cur.execute("SELECT avg(numbers), self_written_SQLvar(numbers) FROM mytable")
print a.fetchall()
>>> [(2.5, 1.25)]

Answer 1

新聚合函數的創建依賴於后端，必須直接使用下划線連接的API完成。 SQLAlchemy沒有提供創建它們的工具。

但是在創建之后，您通常可以在SQLAlchemy中使用它們。

例：

import sqlalchemy
from sqlalchemy import Column, Table, create_engine, MetaData, Integer
from sqlalchemy import func, select
from sqlalchemy.pool import StaticPool
from random import randrange
import numpy
import sqlite3

class NumpyVarAggregate(object):
  def __init__(self):
    self.values = []
  def step(self, value):
    self.values.append(value)
  def finalize(self):
    return numpy.array(self.values).var()

def sqlite_memory_engine_creator():
    con = sqlite3.connect(':memory:')
    con.create_aggregate("np_var", 1, NumpyVarAggregate)
    return con

e = create_engine('sqlite://', echo=True, poolclass=StaticPool,
                  creator=sqlite_memory_engine_creator)
m = MetaData(bind=e)
t = Table('mytable', m, 
            Column('id', Integer, primary_key=True),
            Column('number', Integer)
          )
m.create_all()

現在進行測試：

# insert 30 random-valued rows
t.insert().execute([{'number': randrange(100)} for x in xrange(30)])

for row in select([func.avg(t.c.number), func.np_var(t.c.number)]).execute():
    print 'RESULT ROW: ', row

打印（打開SQLAlchemy語句echo）：

2009-06-15 14:55:34,171 INFO sqlalchemy.engine.base.Engine.0x...d20c PRAGMA 
table_info("mytable")
2009-06-15 14:55:34,174 INFO sqlalchemy.engine.base.Engine.0x...d20c ()
2009-06-15 14:55:34,175 INFO sqlalchemy.engine.base.Engine.0x...d20c 
CREATE TABLE mytable (
    id INTEGER NOT NULL, 
    number INTEGER, 
    PRIMARY KEY (id)
)
2009-06-15 14:55:34,175 INFO sqlalchemy.engine.base.Engine.0x...d20c ()
2009-06-15 14:55:34,176 INFO sqlalchemy.engine.base.Engine.0x...d20c COMMIT
2009-06-15 14:55:34,177 INFO sqlalchemy.engine.base.Engine.0x...d20c INSERT
INTO mytable (number) VALUES (?)
2009-06-15 14:55:34,177 INFO sqlalchemy.engine.base.Engine.0x...d20c [[98], 
[94], [7], [1], [79], [77], [51], [28], [85], [26], [34], [68], [15], [43], 
[52], [97], [64], [82], [11], [71], [27], [75], [60], [85], [42], [40], 
[76], [12], [81], [69]]
2009-06-15 14:55:34,178 INFO sqlalchemy.engine.base.Engine.0x...d20c COMMIT
2009-06-15 14:55:34,180 INFO sqlalchemy.engine.base.Engine.0x...d20c SELECT
avg(mytable.number) AS avg_1, np_var(mytable.number) AS np_var_1 FROM mytable
2009-06-15 14:55:34,180 INFO sqlalchemy.engine.base.Engine.0x...d20c []
RESULT ROW: (55.0, 831.0)

請注意，我沒有使用SQLAlchemy的ORM（只使用了SQLAlchemy的sql表達式部分），但您也可以使用ORM。

Answer 2

首先你必須從sqlalchemy導入func

你可以寫

func.avg（ '字段名'）

或func.avg（'fieldname'）。label（'user_deined'）

或者你可以通過mre信息

http://www.sqlalchemy.org/docs/05/ormtutorial.html#using-subqueries