sqlalchemy什么时候执行查询？

As I've just start learning to use sqlalchemy recently, the result of the following code make me confused about when sqlalchemy execute the query:

由于我刚开始学习使用sqlalchemy,以下代码的结果让我对sqlalchemy执行查询时感到困惑:

query = db.session.query(MyTable)
query = query.filter(...)    
query = query.limit(...)   
query = query.offset(...)    
records = query     #records=query.all()
for r in records:
    #do something

note the line

注意这条线

records = query #records=query.all()

Seems that it brings the same correct result(stored in variable "records") when using "query" and "query.all()", I wonder when was the query executed?

当使用“query”和“query.all()”时,它似乎带来了相同的正确结果(存储在变量“records”中),我想知道查询是什么时候执行的?

If it is executed during the first line "db.session.query(MyTable)", the result set may be large at this point; if during the fifth line "records = query", how could that happen as there's no function call at all?

如果它在第一行“db.session.query(MyTable)”期间执行,则此时结果集可能很大;如果在第五行“记录=查询”期间,那怎么可能发生,因为根本没有函数调用?

1 个解决方案

#1

In your example, the query gets executed upon for r in records. Accessing the query object via iterator triggers the execution. (Normally, only then will it be compiled into a SELECT statement)

在您的示例中,对记录中的r执行查询。通过迭代器访问查询对象会触发执行。 (通常,只有这样才能编译成SELECT语句)

Up until this time, the query will be built (via filter, limit etc).

在此之前,将构建查询(通过过滤器,限制等)。

Please read also the ORM Tutorial on querying

请阅读有关查询的ORM教程

#1