On Jul 18, 2013, at 6:52 PM, Victor Reichert <[email protected]> wrote:
> HI All,
>
> I'm working on my first SQL Alchemy project. I'm thinking I'm going to
> define a commit method for all the objects I want persist, I'm thinking
> something like:
>
> def commit(self):
> session = Session() #Session is a global sessionmaker
> session.add(self)
> session.commit()
> session.close()
>
> Is that a good practice?
This is an antipattern. You should keep the means of persistence separate
from the objects that you are persisting, and you should be thinking in terms
of use cases as far as how to structure transactions, meaning that a single
commit() should address all the objects that are related to a particular
operation.
A bad metaphor might be, suppose we wish to write a collection of sentences to
a file. The antipattern approach to me looks like this:
class Sentence(object):
def __init__(self, text):
self.text = text
def write(self):
handle = open(self.file, "a")
handle.write(self.text)
handle.close()
file = "myfile.txt"
for sentence in sentences:
sentence.write()
While thinking in terms of operations instead of objects looks like this:
class Sentence(object):
def __init__(self, text):
self.text = text
handle = open(self.file, "w")
for sentence in sentences:
handle.write(sentence.text)
handle.close()
besides the obvious efficiency, we don't force each sentence to deal with the
target file in isolation of all the other sentences. Dealing with the file's
lifespan is outside of the scope of the thing we're putting in the file.
By "use case", this depends a lot on what kind of application this is. If
it's a web application, you want a transaction per request. If it's a short
console script, you want a transaction for the life of the script itself.
There's a lot more written about this here:
http://docs.sqlalchemy.org/en/rel_0_8/orm/session.html#session-frequently-asked-questions
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