Question

我正在利用KallAlchemy来充实一个数据库,我经常需要检查一个矿石在处理之前是否在数据库中存在。这可能是一个非常规的问题,但我发现自己经常遇到这样的情况:

my_object = session.query(SomeObject).filter(some_fiter).first()
if my_object: # Mostly in databases...
    # Juchee it exists
    # process
else:
    # It does not exist. :-(
    my_object = SomeObject()
    # process

下面是:

if my_object = session.query(someObject).blabla.first():
    # if my_object is None this scope is left alone
    # if my_object is not None I can work with my_object here...

我知道,这一辛迪加是错误的,但我想解释一下,我就是这样说的。任何类似方式都会使我感到欣慰。

是否对这一模式采取了一种leg强的做法? 这一问题的目的不仅在于图西语,而且针对每一种类似情况。

披露我的眼睛,击中“你的问题”,等待聪明的人和心脏的原教旨主义者要求某些可能不适当的东西:>;

Answer 1

你们想要执行一个效率高的Exist que

(ret, ), = Session.query(exists().where(SomeObject.field==value))

Mike Bayer explain it in his blog post:
http://techspot.zzzeek.org/2008/09/09/selecting-booleans/

如果你不希望获得教育,你可以使用cal:

ret = Session.query(exists().where(SomeObject.field==value)).scalar()

Answer 2

This has been asked a long time ago but for future visitors a more concise way to check is

 if session.query(model).filter(some_filter).count():
     # do stuff

Answer 3

wrap it on a function (shamelessly stolen from django get_or_create, this doesnt return a tuple though)

get_or_create(model, **kwargs):
    try:
        # basically check the obj from the db, this syntax might be wrong
        object = session.query(model).filter(**kwargs).first()
        return object
    except DoesNotExistException: # or whatever error/exception it is on SQLA
        object = model()
        # do it here if you want to save the obj to the db
        return object

that s it. to use it:

obj = get_or_create(SomeObject, filters)

change the **kwargs to a simple argument (like some_filters) if you want

try to wrap something you often use (wrap them to functions or classes)

thats only pseudo code, there might be syntax error.

EDIT: 强调

Answer 4

我知道这并不是所有一步,但这是可以接受的吗?

my_object = session.query(SomeObject).filter(some_filter).first()
if my_object is None:
    my_object = SomeObject()
#process

Answer 5

from sqlalchemy.orm.util import has_identity

my_object = session.query(SomeObject).get(id) or SomeObject()
# Processing...

# Check if the object exists in the database
if not has_identity(my_object):
    session.add(my_object)

session.commit()

如果需要,可改为过滤器+首先()

Answer 6

if DBSession.query(ObjectType).filter(ObjectType.some_parametter == "This").first() is None:

这是检查是否存在记录的有效一线方法。这样做是有效的,因为它只贪 gr了第一物体,而且由于先()返回,它可以一行。没有相应的记录。希望帮助!

Answer 7

你们可以这样做:

sth = session.query.filter_by().first()
if sth is None:
    ....
else:
    ....

我对此进行了测试。它运作良好。

Answer 8

这里有一些建议。如何使用诺德基金会例外?

try:
    existing = dbsession.query(SomeObject).filter_by(value=value).one()
    return existing
except sqlalchemy.orm.exc.NoResultFound:
    obj = SomeObject()

Answer 9

这里的职能是检查是否存在使用Kalchemy的物体。

def exists(obj, **kwargs):
    """" if obj filtered by kwargs exist return it otherwise return None
        obj : is the sql alchemy model object which existence is being checked here.
        **kwargs : (username = user_name, email=user_email)
    """
    db_obj = obj.query.filter_by(**kwargs).first()
    if db_obj is not None:
        return True
    else:
        return False

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