blocks|key|2934711|text|这基本上就是做这件事的方式，AFAIK没有现成的捷径。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2934712|当然，您可以对其进行泛化：|2934713|def+get_or_create(session,+model,+defaults=None,+**kwargs):
++++instance+=+session.query(model).filter_by(**kwargs).one_or_none()
++++if+instance:
++++++++return+instance,+False
++++else:
++++++++params+=+{k:+v+for+k,+v+in+kwargs.items()+if+not+isinstance(v,+ClauseElement)}
++++++++params.update(defaults+or+{})
++++++++instance+=+model(**params)
++++++++try:
++++++++++++session.add(instance)
++++++++++++session.commit()
++++++++except+Exception:++#+The+actual+exception+depends+on+the+specific+database+so+we+catch+all+exceptions.+This+is+similar+to+the+official+documentation:+https://docs.sqlalchemy.org/en/latest/orm/session_transaction.html
++++++++++++session.rollback()
++++++++++++instance+=+session.query(model).filter_by(**kwargs).one()
++++++++++++return+instance,+False
++++++++else:
++++++++++++return+instance,+True|code-block|syntax|javascript|2934714|2934715|2020更新(仅限Python+3.9%2B+)|2934716|这是一个带有Python3.9的the+new+dict+union+operator+(%7C=)的更干净的版本|offset|length|2934717|def+get_or_create(session,+model,+defaults=None,+**kwargs):
++++instance+=+session.query(Model).filter_by(**kwargs).one_or_none()
++++if+instance:
++++++++return+instance,+False
++++else:
++++++++kwargs+%7C=+defaults+or+{}
++++++++instance+=+model(**params)
++++++++try:
++++++++++++session.add(instance)
++++++++++++session.commit()
++++++++except+Exception:++#+The+actual+exception+depends+on+the+specific+database+so+we+catch+all+exceptions.+This+is+similar+to+the+official+documentation:+https://docs.sqlalchemy.org/en/latest/orm/session_transaction.html
++++++++++++session.rollback()
++++++++++++instance+=+session.query(model).filter_by(**kwargs).one()
++++++++++++return+instance,+False
++++++++else:
++++++++++++return+instance,+True|2934718|注意：|2934719|类似于Django版本，这将捕获重复的键约束和类似的错误。如果您的get或create不能保证返回单个结果，它仍然可能导致争用条件。|2934720|为了缓解这个问题，您需要在session.commit()之后添加另一个one_or_none()样式的fetch。除非您还使用with_for_update()或可序列化事务模式，否则对于竞争条件，这仍然不是100%25的保证。|style|CODE|2934721|entityMap|0|LINK|mutability|MUTABLE|url|https://www.python.org/dev/peps/pep-0584/^0|0|0|0|0|0|G|W|0|0|0|0|0|D|G|10|D|1S|H|0^^$0|@$1|2|3|4|5|6|7|17|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|18|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|19|8|@]|9|@]|A|$G|H]]|$1|I|3|-4|5|6|7|1A|8|@]|9|@]|A|$]]|$1|J|3|K|5|6|7|1B|8|@]|9|@]|A|$]]|$1|L|3|M|5|6|7|1C|8|@]|9|@$N|1D|O|1E|1|1F]]|A|$]]|$1|P|3|Q|5|F|7|1G|8|@]|9|@]|A|$G|H]]|$1|R|3|S|5|6|7|1H|8|@]|9|@]|A|$]]|$1|T|3|U|5|6|7|1I|8|@]|9|@]|A|$]]|$1|V|3|W|5|6|7|1J|8|@$N|1K|O|1L|X|Y]|$N|1M|O|1N|X|Y]|$N|1O|O|1P|X|Y]]|9|@]|A|$]]|$1|Z|3|-4|5|6|7|1Q|8|@]|9|@]|A|$]]]|10|$11|$5|12|13|14|A|$15|16]]]]

That's basically the way to do it, there is no shortcut readily available AFAIK.
You could generalize it ofcourse:
<pre><code>def get_or_create(session, model, defaults=None, **kwargs):
 instance = session.query(model).filter_by(**kwargs).one_or_none()
 if instance:
 return instance, False
 else:
 params = {k: v for k, v in kwargs.items() if not isinstance(v, ClauseElement)}
 params.update(defaults or {})
 instance = model(**params)
 try:
 session.add(instance)
 session.commit()
 except Exception: # The actual exception depends on the specific database so we catch all exceptions. This is similar to the official documentation: https://docs.sqlalchemy.org/en/latest/orm/session_transaction.html
 session.rollback()
 instance = session.query(model).filter_by(**kwargs).one()
 return instance, False
 else:
 return instance, True
</code></pre>
<hr />
<h2>2020 update (Python 3.9+ ONLY)</h2>
Here is a cleaner version with Python 3.9's <a href="https://www.python.org/dev/peps/pep-0584/" rel="nofollow noreferrer">the new dict union operator (|=)</a>
<pre><code>def get_or_create(session, model, defaults=None, **kwargs):
 instance = session.query(Model).filter_by(**kwargs).one_or_none()
 if instance:
 return instance, False
 else:
 kwargs |= defaults or {}
 instance = model(**params)
 try:
 session.add(instance)
 session.commit()
 except Exception: # The actual exception depends on the specific database so we catch all exceptions. This is similar to the official documentation: https://docs.sqlalchemy.org/en/latest/orm/session_transaction.html
 session.rollback()
 instance = session.query(model).filter_by(**kwargs).one()
 return instance, False
 else:
 return instance, True
</code></pre>
<h2>Note:</h2>
Similar to the Django version this will catch duplicate key constraints and similar errors. If your get or create is not guaranteed to return a single result it can still result in race conditions.
To alleviate some of that issue you would need to add another <code>one_or_none()</code> style fetch right after the <code>session.commit()</code>. This still is no 100% guarantee against race conditions unless you also use a <code>with_for_update()</code> or serializable transaction mode.

blocks|key|2934752|text|遵循@WoLpH的解决方案，这是对我有效的代码(简单版本)：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2934753|def+get_or_create(session,+model,+**kwargs):
++++instance+=+session.query(model).filter_by(**kwargs).first()
++++if+instance:
++++++++return+instance
++++else:
++++++++instance+=+model(**kwargs)
++++++++session.add(instance)
++++++++session.commit()
++++++++return+instance|code-block|syntax|javascript|2934754|有了这个，我就能够get_or_create我的模型中的任何对象了。|2934755|假设我的模型对象是：|2934756|class+Country(Base):
++++__tablename__+=+'countries'
++++id+=+Column(Integer,+primary_key=True)
++++name+=+Column(String,+unique=True)|2934757|为了获取或创建我的对象，我写道：|2934758|myCountry+=+get_or_create(session,+Country,+name=countryName)|2934759|entityMap^0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|V|8|@]|9|@]|A|$]]|$1|K|3|L|5|D|7|W|8|@]|9|@]|A|$E|F]]|$1|M|3|N|5|6|7|X|8|@]|9|@]|A|$]]|$1|O|3|P|5|D|7|Y|8|@]|9|@]|A|$E|F]]|$1|Q|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|R|$]]

Following the solution of @WoLpH, this is the code that worked for me (simple version):

<pre><code>def get_or_create(session, model, **kwargs):
 instance = session.query(model).filter_by(**kwargs).first()
 if instance:
 return instance
 else:
 instance = model(**kwargs)
 session.add(instance)
 session.commit()
 return instance
</code></pre>

With this, I'm able to get_or_create any object of my model. 

Suppose my model object is :

<pre><code>class Country(Base):
 __tablename__ = 'countries'
 id = Column(Integer, primary_key=True)
 name = Column(String, unique=True)
</code></pre>

To get or create my object I write :

<pre><code>myCountry = get_or_create(session, Country, name=countryName)
</code></pre>

blocks|key|1027233|text|This+SQLALchemy+recipe把这项工作做得很好很优雅。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1027234|要做的第一件事是定义一个函数，该函数被赋予一个要使用的会话，并将一个字典与跟踪当前唯一键的Session()关联起来。|1027235|def+_unique(session,+cls,+hashfunc,+queryfunc,+constructor,+arg,+kw):
++++cache+=+getattr(session,+'_unique_cache',+None)
++++if+cache+is+None:
++++++++session._unique_cache+=+cache+=+{}

++++key+=+(cls,+hashfunc(*arg,+**kw))
++++if+key+in+cache:
++++++++return+cache[key]
++++else:
++++++++with+session.no_autoflush:
++++++++++++q+=+session.query(cls)
++++++++++++q+=+queryfunc(q,+*arg,+**kw)
++++++++++++obj+=+q.first()
++++++++++++if+not+obj:
++++++++++++++++obj+=+constructor(*arg,+**kw)
++++++++++++++++session.add(obj)
++++++++cache[key]+=+obj
++++++++return+obj|code-block|syntax|javascript|1027236|使用此函数的一个示例是在mixin中：|1027237|class+UniqueMixin(object):
++++@classmethod
++++def+unique_hash(cls,+*arg,+**kw):
++++++++raise+NotImplementedError()

++++@classmethod
++++def+unique_filter(cls,+query,+*arg,+**kw):
++++++++raise+NotImplementedError()

++++@classmethod
++++def+as_unique(cls,+session,+*arg,+**kw):
++++++++return+_unique(
++++++++++++++++++++session,
++++++++++++++++++++cls,
++++++++++++++++++++cls.unique_hash,
++++++++++++++++++++cls.unique_filter,
++++++++++++++++++++cls,
++++++++++++++++++++arg,+kw
++++++++++++)|1027238|最后创建唯一的get_or_create模型：|1027239|from+sqlalchemy+import+Column,+Integer,+String,+create_engine
from+sqlalchemy.orm+import+sessionmaker
from+sqlalchemy.ext.declarative+import+declarative_base

Base+=+declarative_base()

engine+=+create_engine('sqlite://',+echo=True)

Session+=+sessionmaker(bind=engine)

class+Widget(UniqueMixin,+Base):
++++__tablename__+=+'widget'

++++id+=+Column(Integer,+primary_key=True)
++++name+=+Column(String,+unique=True,+nullable=False)

++++@classmethod
++++def+unique_hash(cls,+name):
++++++++return+name

++++@classmethod
++++def+unique_filter(cls,+query,+name):
++++++++return+query.filter(Widget.name+==+name)

Base.metadata.create_all(engine)

session+=+Session()

w1,+w2,+w3+=+Widget.as_unique(session,+name='w1'),+\
++++++++++++++++Widget.as_unique(session,+name='w2'),+\
++++++++++++++++Widget.as_unique(session,+name='w3')
w1b+=+Widget.as_unique(session,+name='w1')

assert+w1+is+w1b
assert+w2+is+not+w3
assert+w2+is+not+w1

session.commit()|1027240|食谱更深入这个想法，并提供了不同的方法，但我已经使用了这个方法，并取得了巨大的成功。|1027241|entityMap|0|LINK|mutability|MUTABLE|url|http://www.sqlalchemy.org/trac/wiki/UsageRecipes/UniqueObject^0|0|M|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|12|8|@]|9|@$A|13|B|14|1|15]]|C|$]]|$1|D|3|E|5|6|7|16|8|@]|9|@]|C|$]]|$1|F|3|G|5|H|7|17|8|@]|9|@]|C|$I|J]]|$1|K|3|L|5|6|7|18|8|@]|9|@]|C|$]]|$1|M|3|N|5|H|7|19|8|@]|9|@]|C|$I|J]]|$1|O|3|P|5|6|7|1A|8|@]|9|@]|C|$]]|$1|Q|3|R|5|H|7|1B|8|@]|9|@]|C|$I|J]]|$1|S|3|T|5|6|7|1C|8|@]|9|@]|C|$]]|$1|U|3|-4|5|6|7|1D|8|@]|9|@]|C|$]]]|V|$W|$5|X|Y|Z|C|$10|11]]]]

<a href="http://www.sqlalchemy.org/trac/wiki/UsageRecipes/UniqueObject" rel="nofollow noreferrer" title="this SQLAlchemy recipe">This SQLALchemy recipe</a> does the job nice and elegant.

The first thing to do is to define a function that is given a Session to work with, and associates a dictionary with the Session() which keeps track of current unique keys.

<pre><code>def _unique(session, cls, hashfunc, queryfunc, constructor, arg, kw):
 cache = getattr(session, '_unique_cache', None)
 if cache is None:
 session._unique_cache = cache = {}

 key = (cls, hashfunc(*arg, **kw))
 if key in cache:
 return cache[key]
 else:
 with session.no_autoflush:
 q = session.query(cls)
 q = queryfunc(q, *arg, **kw)
 obj = q.first()
 if not obj:
 obj = constructor(*arg, **kw)
 session.add(obj)
 cache[key] = obj
 return obj
</code></pre>

An example of utilizing this function would be in a mixin:

<pre><code>class UniqueMixin(object):
 @classmethod
 def unique_hash(cls, *arg, **kw):
 raise NotImplementedError()

 @classmethod
 def unique_filter(cls, query, *arg, **kw):
 raise NotImplementedError()

 @classmethod
 def as_unique(cls, session, *arg, **kw):
 return _unique(
 session,
 cls,
 cls.unique_hash,
 cls.unique_filter,
 cls,
 arg, kw
 )
</code></pre>

And finally creating the unique get_or_create model:

<pre><code>from sqlalchemy import Column, Integer, String, create_engine
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

engine = create_engine('sqlite://', echo=True)

Session = sessionmaker(bind=engine)

class Widget(UniqueMixin, Base):
 __tablename__ = 'widget'

 id = Column(Integer, primary_key=True)
 name = Column(String, unique=True, nullable=False)

 @classmethod
 def unique_hash(cls, name):
 return name

 @classmethod
 def unique_filter(cls, query, name):
 return query.filter(Widget.name == name)

Base.metadata.create_all(engine)

session = Session()

w1, w2, w3 = Widget.as_unique(session, name='w1'), \
 Widget.as_unique(session, name='w2'), \
 Widget.as_unique(session, name='w3')
w1b = Widget.as_unique(session, name='w1')

assert w1 is w1b
assert w2 is not w3
assert w2 is not w1

session.commit()
</code></pre>

The recipe goes deeper into the idea and provides different approaches but I've used this one with great success.

blocks|key|1030883|text|我一直在尝试解决这个问题，最终得到了一个相当健壮的解决方案：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1030884|def+get_one_or_create(session,
++++++++++++++++++++++model,
++++++++++++++++++++++create_method='',
++++++++++++++++++++++create_method_kwargs=None,
++++++++++++++++++++++**kwargs):
++++try:
++++++++return+session.query(model).filter_by(**kwargs).one(),+False
++++except+NoResultFound:
++++++++kwargs.update(create_method_kwargs+or+{})
++++++++created+=+getattr(model,+create_method,+model)(**kwargs)
++++++++try:
++++++++++++session.add(created)
++++++++++++session.flush()
++++++++++++return+created,+True
++++++++except+IntegrityError:
++++++++++++session.rollback()
++++++++++++return+session.query(model).filter_by(**kwargs).one(),+False|code-block|syntax|javascript|1030885|我只是写了一个关于所有细节的fairly+expansive+blog+post，但是有一些关于我为什么使用这个的相当多的想法。|offset|length|1030886|1030887|它解压成一个元组，告诉你对象是否存在。这在您的工作流程中通常很有用。|ordered-list-item|1030888|该函数使您能够使用@classmethod修饰的创建者函数(以及特定于这些函数的属性)。|style|CODE|1030889|当您有多个进程连接到数据存储时，该解决方案可防止出现竞争情况。|1030890|1030891|编辑:我已经按照this+blog+post中的解释将session.commit()更改为session.flush()。请注意，这些决策特定于所使用的数据存储(在本例中为Postgres)。|1030892|编辑2:我已经在函数中使用{}作为默认值进行了更新，因为这是典型的Python陷阱。谢谢你的the+comment，奈杰尔！如果你对这个问题很好奇，可以看看this+StackOverflow+question和this+blog+post。|1030893|entityMap|0|LINK|mutability|MUTABLE|url|https://skien.cc/blog/2014/01/15/sqlalchemy-and-race-conditions-implementing-get_one_or_create/|1|http://skien.cc/blog/2014/02/06/sqlalchemy-and-race-conditions-follow-up/|2|http://skien.cc/blog/2014/02/06/sqlalchemy-and-race-conditions-follow-up/#comment-1371570667|3|https://stackoverflow.com/questions/5712904/empty-dictionary-as-default-value-for-keyword-argument-in-python-function-dicti|4|http://pythonconquerstheuniverse.wordpress.com/category/python-gotchas/^0|0|0|E|Q|0|0|0|0|9|C|0|0|0|R|G|1A|F|8|E|1|0|1A|B|2|26|R|3|2Y|E|4|0^^$0|@$1|2|3|4|5|6|7|1F|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|1G|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|1H|8|@]|9|@$I|1I|J|1J|1|1K]]|A|$]]|$1|K|3|-4|5|6|7|1L|8|@]|9|@]|A|$]]|$1|L|3|M|5|N|7|1M|8|@]|9|@]|A|$]]|$1|O|3|P|5|N|7|1N|8|@$I|1O|J|1P|Q|R]]|9|@]|A|$]]|$1|S|3|T|5|N|7|1Q|8|@]|9|@]|A|$]]|$1|U|3|-4|5|6|7|1R|8|@]|9|@]|A|$]]|$1|V|3|W|5|6|7|1S|8|@$I|1T|J|1U|Q|R]|$I|1V|J|1W|Q|R]]|9|@$I|1X|J|1Y|1|1Z]]|A|$]]|$1|X|3|Y|5|6|7|20|8|@]|9|@$I|21|J|22|1|23]|$I|24|J|25|1|26]|$I|27|J|28|1|29]]|A|$]]|$1|Z|3|-4|5|6|7|2A|8|@]|9|@]|A|$]]]|10|$11|$5|12|13|14|A|$15|16]]|17|$5|12|13|14|A|$15|18]]|19|$5|12|13|14|A|$15|1A]]|1B|$5|12|13|14|A|$15|1C]]|1D|$5|12|13|14|A|$15|1E]]]]

I've been playing with this problem and have ended up with a fairly robust solution:

<pre class="lang-py prettyprint-override"><code>def get_one_or_create(session,
 model,
 create_method='',
 create_method_kwargs=None,
 **kwargs):
 try:
 return session.query(model).filter_by(**kwargs).one(), False
 except NoResultFound:
 kwargs.update(create_method_kwargs or {})
 created = getattr(model, create_method, model)(**kwargs)
 try:
 session.add(created)
 session.flush()
 return created, True
 except IntegrityError:
 session.rollback()
 return session.query(model).filter_by(**kwargs).one(), False
</code></pre>

I just wrote a <a href="https://skien.cc/blog/2014/01/15/sqlalchemy-and-race-conditions-implementing-get_one_or_create/" rel="noreferrer">fairly expansive blog post</a> on all the details, but a few quite ideas of why I used this.

<ol>
<li>It unpacks to a tuple that tells you if the object existed or not. This can often be useful in your workflow.</li>
<li>The function gives the ability to work with <code>@classmethod</code> decorated creator functions (and attributes specific to them).</li>
<li>The solution protects against Race Conditions when you have more than one process connected to the datastore.</li>
</ol>

EDIT: I've changed <code>session.commit()</code> to <code>session.flush()</code> as explained in <a href="http://skien.cc/blog/2014/02/06/sqlalchemy-and-race-conditions-follow-up/" rel="noreferrer">this blog post</a>. Note that these decisions are specific to the datastore used (Postgres in this case).

EDIT 2: I’ve updated using a {} as a default value in the function as this is typical Python gotcha. Thanks for <a href="http://skien.cc/blog/2014/02/06/sqlalchemy-and-race-conditions-follow-up/#comment-1371570667" rel="noreferrer">the comment</a>, Nigel! If your curious about this gotcha, check out <a href="https://stackoverflow.com/questions/5712904/empty-dictionary-as-default-value-for-keyword-argument-in-python-function-dicti">this StackOverflow question</a> and <a href="http://pythonconquerstheuniverse.wordpress.com/category/python-gotchas/" rel="noreferrer">this blog post</a>.

blocks|key|282984|text|语义上最接近的可能是：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|282985|def+get_or_create(model,+**kwargs):
++++"""SqlAlchemy+implementation+of+Django's+get_or_create.
++++"""
++++session+=+Session()
++++instance+=+session.query(model).filter_by(**kwargs).first()
++++if+instance:
++++++++return+instance,+False
++++else:
++++++++instance+=+model(**kwargs)
++++++++session.add(instance)
++++++++session.commit()
++++++++return+instance,+True|code-block|syntax|javascript|282986|不确定在sqlalchemy中依赖全局定义的Session是不是更好，但是Django版本不接受连接，所以...|offset|length|style|CODE|282987|返回的元组包含实例和指示实例是否已创建的布尔值(即，如果从数据库读取实例，则为False+)。|282988|Django的get_or_create通常用于确保全局数据可用，因此我将尽早提交。|282989|entityMap^0|0|0|M|7|0|0|7|D|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@$I|V|J|W|K|L]]|9|@]|A|$]]|$1|M|3|N|5|6|7|X|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|Y|8|@$I|Z|J|10|K|L]]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|11|8|@]|9|@]|A|$]]]|R|$]]

The closest semantically is probably:

<pre><code>def get_or_create(model, **kwargs):
 """SqlAlchemy implementation of Django's get_or_create.
 """
 session = Session()
 instance = session.query(model).filter_by(**kwargs).first()
 if instance:
 return instance, False
 else:
 instance = model(**kwargs)
 session.add(instance)
 session.commit()
 return instance, True
</code></pre>

not sure how kosher it is to rely on a globally defined <code>Session</code> in sqlalchemy, but the Django version doesn't take a connection so...

The tuple returned contains the instance and a boolean indicating if the instance was created (i.e. it's False if we read the instance from the db).

Django's <code>get_or_create</code> is often used to make sure that global data is available, so I'm committing at the earliest point possible.

blocks|key|283007|text|根据您采用的隔离级别，上面的解决方案都不起作用。我找到的最好的解决方案是以下形式的原始SQL：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|283008|INSERT+INTO+table(f1,+f2,+unique_f3)+
SELECT+'v1',+'v2',+'v3'+
WHERE+NOT+EXISTS+(SELECT+1+FROM+table+WHERE+f3+=+'v3')|code-block|syntax|javascript|283009|无论隔离级别和并行度如何，这在事务上都是安全的。|283010|注意:为了提高效率，明智的做法是为唯一列建立索引。|283011|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|P|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

Depending on the isolation level you adopted, none of the above solutions would work.
The best solution I have found is a RAW SQL in the following form:

<pre><code>INSERT INTO table(f1, f2, unique_f3) 
SELECT 'v1', 'v2', 'v3' 
WHERE NOT EXISTS (SELECT 1 FROM table WHERE f3 = 'v3')
</code></pre>

This is transactionally safe whatever the isolation level and the degree of parallelism are.

Beware: in order to make it efficient, it would be wise to have an INDEX for the unique column.

blocks|key|282007|text|erik的优秀answer的修改版本|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|282008|def+get_one_or_create(session,
++++++++++++++++++++++model,
++++++++++++++++++++++create_method='',
++++++++++++++++++++++create_method_kwargs=None,
++++++++++++++++++++++**kwargs):
++++try:
++++++++return+session.query(model).filter_by(**kwargs).one(),+True
++++except+NoResultFound:
++++++++kwargs.update(create_method_kwargs+or+{})
++++++++try:
++++++++++++with+session.begin_nested():
++++++++++++++++created+=+getattr(model,+create_method,+model)(**kwargs)
++++++++++++++++session.add(created)
++++++++++++return+created,+False
++++++++except+IntegrityError:
++++++++++++return+session.query(model).filter_by(**kwargs).one(),+True|code-block|syntax|javascript|282009|282010|使用nested+transaction仅回滚添加的新项目，而不是回滚所有内容(请参见此answer以将嵌套事务与SQLite一起使用)|unordered-list-item|282011|Move+create_method。如果创建的对象具有关系，并且通过这些关系为其分配了成员，则会自动将其添加到会话中。例如，创建一个book，它将user_id和user作为对应关系，然后在create_method中执行book.user=<user+object>会将book添加到会话中。这意味着create_method必须在with中才能从最终的回滚中受益。请注意，begin_nested会自动触发刷新。|style|CODE|282012|282013|请注意，如果使用MySQL，则必须将事务隔离级别设置为READ+COMMITTED而不是REPEATABLE+READ，这样才能正常工作。Django的get_or_create+(和here)使用相同的策略，参见Django+documentation。|282014|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/a/21146492/678022|1|http://docs.sqlalchemy.org/en/latest/orm/session_api.html#sqlalchemy.orm.session.Session.begin_nested|2|https://stackoverflow.com/a/36457404/67802|3|https://github.com/django/django/blob/master/django/db/models/query.py#L454|4|https://github.com/django/django/blob/master/django/db/models/query.py#L491|5|https://docs.djangoproject.com/el/1.10/ref/models/querysets/#get-or-create^0|7|6|0|0|0|0|2|I|1|18|6|2|0|5|D|1V|4|22|7|2A|4|2O|D|34|N|3T|4|48|D|4O|4|59|C|0|0|R|E|18|F|24|D|3|2K|4|4|36|D|5|0^^$0|@$1|2|3|4|5|6|7|1B|8|@]|9|@$A|1C|B|1D|1|1E]]|C|$]]|$1|D|3|E|5|F|7|1F|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|1G|8|@]|9|@]|C|$]]|$1|J|3|K|5|L|7|1H|8|@]|9|@$A|1I|B|1J|1|1K]|$A|1L|B|1M|1|1N]]|C|$]]|$1|M|3|N|5|L|7|1O|8|@$A|1P|B|1Q|O|P]|$A|1R|B|1S|O|P]|$A|1T|B|1U|O|P]|$A|1V|B|1W|O|P]|$A|1X|B|1Y|O|P]|$A|1Z|B|20|O|P]|$A|21|B|22|O|P]|$A|23|B|24|O|P]|$A|25|B|26|O|P]|$A|27|B|28|O|P]]|9|@]|C|$]]|$1|Q|3|-4|5|6|7|29|8|@]|9|@]|C|$]]|$1|R|3|S|5|6|7|2A|8|@$A|2B|B|2C|O|P]|$A|2D|B|2E|O|P]]|9|@$A|2F|B|2G|1|2H]|$A|2I|B|2J|1|2K]|$A|2L|B|2M|1|2N]]|C|$]]|$1|T|3|-4|5|6|7|2O|8|@]|9|@]|C|$]]]|U|$V|$5|W|X|Y|C|$Z|10]]|11|$5|W|X|Y|C|$Z|12]]|13|$5|W|X|Y|C|$Z|14]]|15|$5|W|X|Y|C|$Z|16]]|17|$5|W|X|Y|C|$Z|18]]|19|$5|W|X|Y|C|$Z|1A]]]]

A modified version of erik's excellent <a href="https://stackoverflow.com/a/21146492/678022">answer</a>

<pre><code>def get_one_or_create(session,
 model,
 create_method='',
 create_method_kwargs=None,
 **kwargs):
 try:
 return session.query(model).filter_by(**kwargs).one(), True
 except NoResultFound:
 kwargs.update(create_method_kwargs or {})
 try:
 with session.begin_nested():
 created = getattr(model, create_method, model)(**kwargs)
 session.add(created)
 return created, False
 except IntegrityError:
 return session.query(model).filter_by(**kwargs).one(), True
</code></pre>

<ul>
<li>Use a <a href="http://docs.sqlalchemy.org/en/latest/orm/session_api.html#sqlalchemy.orm.session.Session.begin_nested" rel="noreferrer">nested transaction</a> to only roll back the addition of the new item instead of rolling back everything (See this <a href="https://stackoverflow.com/a/36457404/67802">answer</a> to use nested transactions with SQLite)</li>
<li>Move <code>create_method</code>. If the created object has relations and it is assigned members through those relations, it is automatically added to the session. E.g. create a <code>book</code>, which has <code>user_id</code> and <code>user</code> as corresponding relationship, then doing <code>book.user=&lt;user object&gt;</code> inside of <code>create_method</code> will add <code>book</code> to the session. This means that <code>create_method</code> must be inside <code>with</code> to benefit from an eventual rollback. Note that <code>begin_nested</code> automatically triggers a flush.</li>
</ul>

Note that if using MySQL, the transaction isolation level must be set to <code>READ COMMITTED</code> rather than <code>REPEATABLE READ</code> for this to work. Django's <a href="https://github.com/django/django/blob/master/django/db/models/query.py#L454" rel="noreferrer">get_or_create</a> (and <a href="https://github.com/django/django/blob/master/django/db/models/query.py#L491" rel="noreferrer">here</a>) uses the same stratagem, see also the Django <a href="https://docs.djangoproject.com/el/1.10/ref/models/querysets/#get-or-create" rel="noreferrer">documentation</a>.

blocks|key|2935020|text|我稍微简化了@Kevin。避免在if/else语句中包装整个函数的解决方案。这样就只有一个return了，我觉得它更干净：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2935021|def+get_or_create(session,+model,+**kwargs):
++++instance+=+session.query(model).filter_by(**kwargs).first()

++++if+not+instance:
++++++++instance+=+model(**kwargs)
++++++++session.add(instance)

++++return+instance|code-block|syntax|javascript|2935022|entityMap^0|G|2|J|4|19|6|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]|$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|T|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|U|8|@]|D|@]|E|$]]]|L|$]]

I slightly simplified @Kevin. solution to avoid wrapping the whole function in an <code>if</code>/<code>else</code> statement. This way there's only one <code>return</code>, which I find cleaner:

<pre><code>def get_or_create(session, model, **kwargs):
 instance = session.query(model).filter_by(**kwargs).first()

 if not instance:
 instance = model(**kwargs)
 session.add(instance)

 return instance
</code></pre>

blocks|key|1526615|text|有一个Python包包含@erik的解决方案以及一个版本的update_or_create()。https://github.com/enricobarzetti/sqlalchemy_get_or_create|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1526616|entityMap|0|LINK|mutability|MUTABLE|url|https://github.com/enricobarzetti/sqlalchemy_get_or_create^0|T|I|1C|1M|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@$9|O|A|P|B|C]]|D|@$9|Q|A|R|1|S]]|E|$]]|$1|F|3|-4|5|6|7|T|8|@]|D|@]|E|$]]]|G|$H|$5|I|J|K|E|$L|M]]]]

There is a Python package that has @erik's solution as well as a version of <code>update_or_create()</code>. <a href="https://github.com/enricobarzetti/sqlalchemy_get_or_create" rel="nofollow noreferrer">https://github.com/enricobarzetti/sqlalchemy_get_or_create</a>

blocks|key|1031026|text|我经常遇到的一个问题是，当一个字段有一个最大长度(比如STRING(40))，而你想执行一个长度很大的字符串的get+or+create时，上面的解决方案就会失败。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1031027|在上述解决方案的基础上，以下是我的方法：|1031028|from+sqlalchemy+import+Column,+String

def+get_or_create(self,+add=True,+flush=True,+commit=False,+**kwargs):
++++"""

++++Get+the+an+entity+based+on+the+kwargs+or+create+an+entity+with+those+kwargs.

++++Params:
++++++++add:+(default+True)+should+the+instance+be+added+to+the+session?
++++++++flush:+(default+True)+flush+the+instance+to+the+session?
++++++++commit:+(default+False)+commit+the+session?
++++++++kwargs:+key,+value+pairs+of+parameters+to+lookup/create.

++++Ex:+SocialPlatform.get_or_create(**{'name':'facebook'})
++++++++returns+-->+existing+record+or,+will+create+a+new+record

++++---------

++++NOTE:+I+like+to+add+this+as+a+classmethod+in+the+base+class+of+my+tables,+so+that
++++all+data+models+inherit+the+base+class+-->+functionality+is+transmitted+across
++++all+orm+defined+models.

++++"""


++++#+Truncate+values+if+necessary
++++for+key,+value+in+kwargs.items():

++++++++#+Only+use+strings
++++++++if+not+isinstance(value,+str):
++++++++++++continue

++++++++#+Only+use+if+it's+a+column
++++++++my_col+=+getattr(self.__table__.columns,+key)

++++++++if+not+isinstance(my_col,+Column):
++++++++++++continue

++++++++#+Skip+non+strings+again+here
++++++++if+not+isinstance(my_col.type,+String):
++++++++++++continue

++++++++#+Get+the+max+length
++++++++max_len+=+my_col.type.length

++++++++if+value+and+max_len+and+len(value)+>+max_len:

++++++++++++#+Update+the+value
++++++++++++value+=+value[:max_len]
++++++++++++kwargs[key]+=+value

++++#+-------------------------------------------------

++++#+Make+the+query...
++++instance+=+session.query(self).filter_by(**kwargs).first()

++++if+instance:
++++++++return+instance

++++else:
++++++++#+Max+length+isn't+accounted+for+here.
++++++++#+The+assumption+is+that+auto-truncation+will+happen+on+the+child-model
++++++++#+Or+directtly+in+the+db
++++++++instance+=+self(**kwargs)

++++#+You'll+usually+want+to+add+to+the+session
++++if+add:
++++++++session.add(instance)

++++#+Navigate+these+with+caution
++++if+add+and+commit:
++++++++try:
++++++++++++session.commit()
++++++++except+IntegrityError:
++++++++++++session.rollback()

++++elif+add+and+flush:
++++++++session.flush()


++++return+instance|code-block|syntax|javascript|1031029|entityMap^0|R|A|1J|D|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]|$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|T|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|U|8|@]|D|@]|E|$K|L]]|$1|M|3|-4|5|6|7|V|8|@]|D|@]|E|$]]]|N|$]]

One problem I regularly encounter is when a field has a max length (say, <code>STRING(40)</code>) and you'd like to perform a <code>get or create</code> with a string of large length, the above solutions will fail.
Building off of the above solutions, here's my approach:
<pre><code>from sqlalchemy import Column, String

def get_or_create(self, add=True, flush=True, commit=False, **kwargs):
 &quot;&quot;&quot;

 Get the an entity based on the kwargs or create an entity with those kwargs.

 Params:
 add: (default True) should the instance be added to the session?
 flush: (default True) flush the instance to the session?
 commit: (default False) commit the session?
 kwargs: key, value pairs of parameters to lookup/create.

 Ex: SocialPlatform.get_or_create(**{'name':'facebook'})
 returns --&gt; existing record or, will create a new record

 ---------

 NOTE: I like to add this as a classmethod in the base class of my tables, so that
 all data models inherit the base class --&gt; functionality is transmitted across
 all orm defined models.

 &quot;&quot;&quot;


 # Truncate values if necessary
 for key, value in kwargs.items():

 # Only use strings
 if not isinstance(value, str):
 continue

 # Only use if it's a column
 my_col = getattr(self.__table__.columns, key)

 if not isinstance(my_col, Column):
 continue

 # Skip non strings again here
 if not isinstance(my_col.type, String):
 continue

 # Get the max length
 max_len = my_col.type.length

 if value and max_len and len(value) &gt; max_len:

 # Update the value
 value = value[:max_len]
 kwargs[key] = value

 # -------------------------------------------------

 # Make the query...
 instance = session.query(self).filter_by(**kwargs).first()

 if instance:
 return instance

 else:
 # Max length isn't accounted for here.
 # The assumption is that auto-truncation will happen on the child-model
 # Or directtly in the db
 instance = self(**kwargs)

 # You'll usually want to add to the session
 if add:
 session.add(instance)

 # Navigate these with caution
 if add and commit:
 try:
 session.commit()
 except IntegrityError:
 session.rollback()

 elif add and flush:
 session.flush()


 return instance
</code></pre>

I want to get an object from the database if it already exists (based on provided parameters) or create it if it does not.

Django's <a href="https://docs.djangoproject.com/en/1.10/ref/models/querysets/#get-or-create" rel="noreferrer"><code>get_or_create</code></a> (or <a href="https://github.com/django/django/blob/master/django/db/models/query.py#L462" rel="noreferrer">source</a>) does this. Is there an equivalent shortcut in SQLAlchemy?

I'm currently writing it out explicitly like this:

<pre><code>def get_or_create_instrument(session, serial_number):
 instrument = session.query(Instrument).filter_by(serial_number=serial_number).first()
 if instrument:
 return instrument
 else:
 instrument = Instrument(serial_number)
 session.add(instrument)
 return instrument
</code></pre>

Does SQLAlchemy have an equivalent of Django's get_or_create?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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如果对象已经存在(基于提供的参数)，我希望从数据库中获取该对象，如果不存在，则创建该对象。Django的 (或)可以做到这一点。在SQLAlchemy中有没有类似的快捷方式？我现在明确地写出来，就像这样：def get_or_create_instrument(session, serial_number):    i...

问SQLAlchemy有没有和姜戈的get_or_create等价物？
EN

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问SQLAlchemy有没有和姜戈的get_or_create等价物？EN