I have been playing around with my test project
I have this clean method in my model
class SomeModel(models.Model):
f1 = models.IntegerField()
f2 = models.IntegerField()
def clean(self):
if self.f1 > self.f2:
raise ValidationError({'f1': ['Should be greater than f1',]})
if self.f2 == 100:
raise ValidationError({'f2': ['That's too much',]})
I don't really know how to raise both errors and show it in the admin page because even if the two if is True, only the first if error is shown(obviously) how do I show both errors?
You could build a dict of errors and raise a ValidationError when you are done (if necessary):
class SomeModel(models.Model):
f1 = models.IntegerField()
f2 = models.IntegerField()
def clean(self):
error_dict = {}
if self.f1 > self.f2:
error_dict['f1'] = ValidationError("Should be greater than f1") # this should probably belong to f2 as well
if self.f2 == 100:
error_dict['f2'] = ValidationError("That's too much")
if error_dict:
raise ValidationError(error_dict)
I would expand the accepted answer by setting a classmethod like this:
#classmethod
def add_error(cls, errordict, error):
for key, value in error.items():
if key in errordict:
if isinstance(errordict[key], str) or isinstance(errordict[key], ValidationError):
errordict.update({key: [errordict[key], value]})
elif isinstance(errordict[key], list):
errordict[key].append(value)
else:
errordict.update(error)
return errordict
That way, if a key already exists in your error dict, it will be casted in a list. That way, all the errors will be raised, even if there are multiple errors per field.
adderror(error_dict, {'field', 'error message'})
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 get_or_create (or source) does this. Is there an equivalent shortcut in SQLAlchemy?
I'm currently writing it out explicitly like this:
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
Following the solution of #WoLpH, this is the code that worked for me (simple version):
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
With this, I'm able to get_or_create any object of my model.
Suppose my model object is :
class Country(Base):
__tablename__ = 'countries'
id = Column(Integer, primary_key=True)
name = Column(String, unique=True)
To get or create my object I write :
myCountry = get_or_create(session, Country, name=countryName)
That's basically the way to do it, there is no shortcut readily available AFAIK.
You could generalize it ofcourse:
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
2020 update (Python 3.9+ ONLY)
Here is a cleaner version with Python 3.9's the new dict union operator (|=)
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(**kwargs)
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
Note:
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 one_or_none() style fetch right after the session.commit(). This still is no 100% guarantee against race conditions unless you also use a with_for_update() or serializable transaction mode.
I've been playing with this problem and have ended up with a fairly robust solution:
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
I just wrote a fairly expansive blog post on all the details, but a few quite ideas of why I used this.
It unpacks to a tuple that tells you if the object existed or not. This can often be useful in your workflow.
The function gives the ability to work with #classmethod decorated creator functions (and attributes specific to them).
The solution protects against Race Conditions when you have more than one process connected to the datastore.
EDIT: I've changed session.commit() to session.flush() as explained in this blog post. 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 the comment, Nigel! If your curious about this gotcha, check out this StackOverflow question and this blog post.
A modified version of erik's excellent answer
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
Use a nested transaction to only roll back the addition of the new item instead of rolling back everything (See this answer to use nested transactions with SQLite)
Move create_method. 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 book, which has user_id and user as corresponding relationship, then doing book.user=<user object> inside of create_method will add book to the session. This means that create_method must be inside with to benefit from an eventual rollback. Note that begin_nested automatically triggers a flush.
Note that if using MySQL, the transaction isolation level must be set to READ COMMITTED rather than REPEATABLE READ for this to work. Django's get_or_create (and here) uses the same stratagem, see also the Django documentation.
This SQLALchemy recipe 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.
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
An example of utilizing this function would be in a mixin:
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
)
And finally creating the unique get_or_create model:
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()
The recipe goes deeper into the idea and provides different approaches but I've used this one with great success.
The closest semantically is probably:
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
not sure how kosher it is to rely on a globally defined Session 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 get_or_create is often used to make sure that global data is available, so I'm committing at the earliest point possible.
I slightly simplified #Kevin. solution to avoid wrapping the whole function in an if/else statement. This way there's only one return, which I find cleaner:
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
There is a Python package that has #erik's solution as well as a version of update_or_create(). https://github.com/enricobarzetti/sqlalchemy_get_or_create
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:
INSERT INTO table(f1, f2, unique_f3)
SELECT 'v1', 'v2', 'v3'
WHERE NOT EXISTS (SELECT 1 FROM table WHERE f3 = 'v3')
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.
One problem I regularly encounter is when a field has a max length (say, STRING(40)) and you'd like to perform a get or create with a string of large length, the above solutions will fail.
Building off of the above solutions, here's my approach:
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
The idea of the code below should be that it only fires if the field verification_pin is empty i.e. on a new record. However, it seems that every time I save the model it generates a new pin ignoring if instance.verification_pin is None statement, why, what have I missed?
#receiver(pre_save, sender=CompanyUser)
def my_callback(sender, instance, *args, **kwargs):
if instance.verification_pin is None:
instance.verification_pin = instance.generate_pin()
instance.is_active = False
instance.send_verification_pin()
Model:
class CompanyUser(User):
verification_pin = models.IntegerField(max_length=4, null=True)
objects = UserManager()
def generate_pin(self):
"""
Returns a random four digit pin.
"""
return random.randint(999, 9999)
def send_verification_pin(self):
self.email_user(
subject="Test",
message="Your pin: %s" % self.verification_pin,
from_email=settings.DEFAULT_FROM_EMAIL
)
You can set the default value for a field to a callable object.
Or, you might try using a post_save handler instead, where you can check if created is True. Also, it might be helpful to check the value in verification_pin after saving, to see if it really got set or not.
I have a need to track changes on Django model instances. I'm aware of solutions like django-reversion but they are overkill for my cause.
I had the idea to create a parameterized class decorator to fit this purpose. The arguments are the field names and a callback function. Here is the code I have at this time:
def audit_fields(fields, callback_fx):
def __init__(self, *args, **kwargs):
self.__old_init(*args, **kwargs)
self.__old_state = self.__get_state_helper()
def save(self, *args, **kwargs):
new_state = self.__get_state_helper()
for k,v in new_state.items():
if (self.__old_state[k] != v):
callback_fx(self, k, self.__old_state[k], v)
val = self.__old_save(*args, **kwargs)
self.__old_state = self.__get_state_helper()
return val
def __get_state_helper(self):
# make a list of field/values.
state_dict = dict()
for k,v in [(field.name, field.value_to_string(self)) for field in self._meta.fields if field.name in fields]:
state_dict[k] = v
return state_dict
def fx(clazz):
# Stash originals
clazz.__old_init = clazz.__init__
clazz.__old_save = clazz.save
# Override (and add helper)
clazz.__init__ = __init__
clazz.__get_state_helper = __get_state_helper
clazz.save = save
return clazz
return fx
And use it as follows (only relevant part):
#audit_fields(["status"], fx)
class Order(models.Model):
BASKET = "BASKET"
OPEN = "OPEN"
PAID = "PAID"
SHIPPED = "SHIPPED"
CANCELED = "CANCELED"
ORDER_STATES = ( (BASKET, 'BASKET'),
(OPEN, 'OPEN'),
(PAID, 'PAID'),
(SHIPPED, 'SHIPPED'),
(CANCELED, 'CANCELED') )
status = models.CharField(max_length=16, choices=ORDER_STATES, default=BASKET)
And test on the Django shell with:
from store.models import Order
o=Order()
o.status=Order.OPEN
o.save()
The error I receive then is:
TypeError: int() argument must be a string or a number, not 'Order'
The full stacktrace is here: https://gist.github.com/4020212
Thanks in advance and let me know if you would need more info!
EDIT: Question answered by randomhuman, code edited and usable as shown!
You do not need to explicitly pass a reference to self on this line:
val = self.__old_save(self, *args, **kwargs)
It is a method being called on an object reference. Passing it explicitly in this way is causing it to be seen as one of the other parameters of the save method, one which is expected to be a string or a number.
I want to implement a simple VersionedModel base model class for my app engine app. I'm looking for a pattern that does not involve explicitly choosing fields to copy.
I am trying out something like this, but it is to hacky for my taste and did not test it in the production environment yet.
class VersionedModel(BaseModel):
is_history_copy = db.BooleanProperty(default=False)
version = db.IntegerProperty()
created = db.DateTimeProperty(auto_now_add=True)
edited = db.DateTimeProperty()
user = db.UserProperty(auto_current_user=True)
def put(self, **kwargs):
if self.is_history_copy:
if self.is_saved():
raise Exception, "History copies of %s are not allowed to change" % type(self).__name__
return super(VersionedModel, self).put(**kwargs)
if self.version is None:
self.version = 1
else:
self.version = self.version +1
self.edited = datetime.now() # auto_now would also affect copies making them out of sync
history_copy = copy.copy(self)
history_copy.is_history_copy = True
history_copy._key = None
history_copy._key_name = None
history_copy._entity = None
history_copy._parent = self
def tx():
result = super(VersionedModel, self).put(**kwargs)
history_copy._parent_key = self.key()
history_copy.put()
return result
return db.run_in_transaction(tx)
Does anyone have a simpler cleaner solution for keeping history of versions for app engine models?
EDIT: Moved copy out of tx. Thx #Adam Crossland for the suggestion.
Take a look at the properties static method on Model classes. With this, you can get a list of properties, and use that to get their values, something like this:
#classmethod
def clone(cls, other, **kwargs):
"""Clones another entity."""
klass = other.__class__
properties = other.properties().items()
kwargs.update((k, p.__get__(other, klass)) for k, p in properties)
return cls(**kwargs)