1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
|
"""Illustrates using **write only relationships** for simpler handling
of ORM collections under asyncio.
"""
from __future__ import annotations
import asyncio
import datetime
from typing import Optional
from sqlalchemy import ForeignKey
from sqlalchemy import func
from sqlalchemy.ext.asyncio import async_sessionmaker
from sqlalchemy.ext.asyncio import AsyncAttrs
from sqlalchemy.ext.asyncio import create_async_engine
from sqlalchemy.future import select
from sqlalchemy.orm import DeclarativeBase
from sqlalchemy.orm import Mapped
from sqlalchemy.orm import mapped_column
from sqlalchemy.orm import relationship
from sqlalchemy.orm import WriteOnlyMapped
class Base(AsyncAttrs, DeclarativeBase):
pass
class A(Base):
__tablename__ = "a"
id: Mapped[int] = mapped_column(primary_key=True)
data: Mapped[Optional[str]]
create_date: Mapped[datetime.datetime] = mapped_column(
server_default=func.now()
)
# collection relationships are declared with WriteOnlyMapped. There
# is no separate collection type
bs: WriteOnlyMapped[B] = relationship()
class B(Base):
__tablename__ = "b"
id: Mapped[int] = mapped_column(primary_key=True)
a_id: Mapped[int] = mapped_column(ForeignKey("a.id"))
data: Mapped[Optional[str]]
async def async_main():
"""Main program function."""
engine = create_async_engine(
"postgresql+asyncpg://scott:tiger@localhost/test",
echo=True,
)
async with engine.begin() as conn:
await conn.run_sync(Base.metadata.drop_all)
async with engine.begin() as conn:
await conn.run_sync(Base.metadata.create_all)
async_session = async_sessionmaker(engine, expire_on_commit=False)
async with async_session() as session:
async with session.begin():
# WriteOnlyMapped may be populated using any iterable,
# e.g. lists, sets, etc.
session.add_all(
[
A(bs=[B(), B()], data="a1"),
A(bs=[B()], data="a2"),
A(bs=[B(), B()], data="a3"),
]
)
stmt = select(A)
result = await session.scalars(stmt)
for a1 in result:
print(a1)
print(f"created at: {a1.create_date}")
# to iterate a collection, emit a SELECT statement
for b1 in await session.scalars(a1.bs.select()):
print(b1)
result = await session.stream(stmt)
async for a1 in result.scalars():
print(a1)
# similar using "streaming" (server side cursors)
async for b1 in (await session.stream(a1.bs.select())).scalars():
print(b1)
await session.commit()
result = await session.scalars(select(A).order_by(A.id))
a1 = result.first()
a1.data = "new data"
asyncio.run(async_main())
|
