[英]SQLAlchemy: query custom property based on table field
I'm using SQLAlchemy declarative base to define my model. 我正在使用SQLAlchemy声明基础来定义我的模型。 I defined a property
name
that is computed from one the columns ( title
): 我定义了一个属性
name
,该name
是从列( title
)中计算出来的:
class Entry(Base):
__tablename__ = "blog_entry"
id = Column(Integer, primary_key=True)
title = Column(Unicode(255))
...
@property
def name(self):
return re.sub(r'[^a-zA-Z0-9 ]','',self.title).replace(' ','-').lower()
When trying to perform a query using name
, SQLAlchemy throws an error: 尝试使用
name
执行查询时,SQLAlchemy会抛出错误:
Session.query(Entry).filter(Entry.name == my_name).first()
>>> ArgumentError: filter() argument must be of type sqlalchemy.sql.ClauseElement or string
After investigating for a while, I found that maybe comparable_using()
could help, but I couldn't find any example that shows a comparator that references another column of the table. 经过一段时间的调查,我发现也许
comparable_using()
可能有所帮助,但我找不到任何显示比较器引用表的另一列的示例。
Is this even possible or is there a better approach? 这是可能的还是有更好的方法?
从SqlAlchemy 0.7您可以使用hybrid_property实现这一点,请参阅此处的文档: http ://www.sqlalchemy.org/docs/orm/extensions/hybrid.html
Can you imagine what SQL should be issued for your query? 你能想象应该为你的查询发出什么SQL吗? The database knows nothing about
name
, it has neither a way to calculate it, nor to use any index to speed up the search. 数据库对
name
一无所知,它既没有计算方法,也没有使用任何索引来加速搜索。
My best bet is a full scan, fetching title
for every record, calculating name
then filtering by it. 我最好的选择是全扫描,为每条记录获取
title
,计算name
然后按其过滤。 You can rawly do it by [x for x in Session.query(Entry).all() if x.name==my_name][0]
. [x for x in Session.query(Entry).all() if x.name==my_name][0]
你可以通过[x for x in Session.query(Entry).all() if x.name==my_name][0]
Session [x for x in Session.query(Entry).all() if x.name==my_name][0]
。 With a bit more of sophistication, you'll only fetch id
and title
in the filtering pass, and then fetch the full record(s) by id
. 稍微复杂一点,你只需要在过滤过程中获取
id
和title
,然后按id
获取完整的记录。
Note that a full scan is usually not nice from performance POV, unless your table is quite small. 请注意,除非您的表非常小,否则从性能POV开始,完整扫描通常并不好。
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