何时使用 SQLAlchemy .get() 与 .filter(Foo.ID == primary_key_id).first() [英] When to use SQLAlchemy .get() vs .filter(Foo.ID == primary_key_id).first()

问题描述

只是想知道我什么时候想使用一个和另一个.它们有何不同?

Just curious about when I would want to use one vs the other. How are they different?

我们已经设置了我们的系统，我们可以做到这一点:

We have our system set up such that we can do this:

my_user = User.query().filter(User.ID == 5).first()

或

my_user = User.query().get(5)

推荐答案

这两行是一回事.只有引发的异常不同.实际上，get() 是在 one() 之上实现的.如果您的 filter() 返回的不仅仅是结果，那将会有所不同，但这在您的情况下确实是不可能的.

Those two lines are the same thing. Only exceptions raised differ. In fact, get() is implemented on top of one(). There would be a difference if your filter() returned more than a result, but this is indeed not possible in your case.

顺便说一下，SQL 没有 GET 操作，它只有 SELECT(带有可选的 LIMIT).

By the way, SQL does not have a GET operation, it only has SELECT (with optional LIMIT).

sqlalchemy/:

sqlalchemy/orm/query.py:

def get(self, ident):
    ...
    return self._get_impl(ident, loading.load_on_ident)

sqlalchemy/orm//a>:

sqlalchemy/orm/loading.py:

def load_on_ident(query, key,
                  refresh_state=None, lockmode=None,
                  only_load_props=None):
    ...
    try:
        return q.one()
    except orm_exc.NoResultFound:
        return None

q.one() 依次调用 q.one_or_none().

q.one() in turn calls q.one_or_none().

现在比较first() 使用 one_or_none():

def first(self):
    ...
    ret = list(self[0:1])
    if len(ret) > 0:
        return ret[0]
    else:
        return None


def one_or_none(self):
    ...
    ret = list(self)

    l = len(ret)
    if l == 1:
        return ret[0]
    elif l == 0:
        return None
    else:
        raise orm_exc.MultipleResultsFound(
            "Multiple rows were found for one_or_none()")

因此，first() 执行带有 LIMIT 的 SELECT，one_or_none() 执行无限制的 SELECT.但是，正如我们已经说过的，无论有没有 LIMIT，查询的结果都不会改变，因此两者是等价的.

Therefore, first() executes a SELECT with a LIMIT, one_or_none() executes an unlimited SELECT. But, as we already said, either with or without LIMIT the result of the query cannot change, therefore the two are equivalent.

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