如何更新数量？ Django的

Question

我真的很难更新机票的数量。我的网站（项目）模仿了一个销售音乐活动门票的电子商务网站。

当用户购买门票时，门票数量应该更新。示例：100个可用票据和5个已购买，db应该说剩余95个。但如果门票低于0，则会出现警告“此活动不再有门票”。

有人可以帮我实现吗？这是我需要做的最后一点，然后我就完成了!!干杯。代码：

当我说需要更新时，Venue类的容量就是我所指的。

的 Models.py

class Genre(models.Model):
    genre = models.CharField(max_length=30, default=None, unique=True)

    def __str__(self):
        return self.genre

    class Meta:
        ordering = [ "genre" ]
        verbose_name_plural = "genres"

class CardChoices(models.Model):
    payment_type = models.CharField(max_length=30, default=None, unique=True)

    def __str__(self):
        return self.payment_type

    class Meta:
        ordering = [ "payment_type" ]
        verbose_name_plural = "payment types"
### Start
class Venue(models.Model):
    """The 'Venue' model represents a collection of different venues."""
    name = models.CharField(max_length=30, default=None)
    address = models.CharField(max_length=255, default=None)
    capacity = models.PositiveIntegerField(default=0)
    website = models.URLField(max_length=50, null=True)
    phone_number = models.CharField(validators=[
        RegexValidator(regex='^\d{11}$', message='Length has to be 11',
            code='Invalid number')], blank=True, null=True, max_length=11)
    #phone_number = models.IntegerField(max_length=11, unique=True, validators=[RegexValidator(regex='^\d{10}$', message='Length has to be 11', code='Invalid number')], default=None)
    description = models.TextField()
    slug = models.SlugField(unique=True, default=None)

    # Returns the name of a category
    def __str__(self):
        return self.name

    # Call the slugify method and update the slug field when the name is changed
    def save(self, *args, **kwargs):
        self.slug = slugify(self.name)
        super(Venue, self).save(*args, **kwargs)

    class Meta:
        # Orders the categories using the name field in ascending order
        ordering = [ "name" ]
        verbose_name_plural = "venues"    

class Artist(models.Model):
    """The 'Artist' model represents a collection of different artists."""
    name = models.CharField(max_length=30, default=None)
    description = models.CharField(max_length=255)
    image = models.ImageField(upload_to='artist/images')
    genre = models.ForeignKey('Genre', related_name='genres', default=None)
    slug = models.SlugField(unique=True, default=None)

    def __str__(self):
        return self.name

    def save(self, *args, **kwargs):
        self.slug = slugify(self.name)
        super(Artist, self).save(*args, **kwargs)

    class Meta:
        ordering = [ "name" ]
        verbose_name_plural = "artists"

class Event(models.Model):
    """The 'Event' model represents a collection of different events."""
    artists = models.ManyToManyField(Artist)
    venue = models.ForeignKey(Venue)
    name = models.CharField(max_length=255, default=None)
    quantity = models.IntegerField()
    price = models.IntegerField() # Needs to be repeated so that django-carton can convert the value (dirty hack needs fixing)
    #price = PriceField('Price', currency='GBP', max_digits=10, decimal_places=2)
    date = models.DateField()
    start_time = models.TimeField()
    end_time = models.TimeField()
    curfew_time = models.TimeField()
    description = models.TextField()
    slug = models.SlugField(unique=True, default=None)

    def __str__(self):
        return self.name

    def save(self, *args, **kwargs):
        self.slug = slugify(self.name)
        super(Event, self).save(*args, **kwargs)

    class Meta:
        ordering = [ "name" ]
        verbose_name_plural = "events"

class UserAccount(models.Model): # needs to be associated with INDIVIDUAL purchases, a user can hold more than one purchase
    user = models.OneToOneField(User) # Links user account with a Django User model instance
    #event = models.ForeignKey(Event, blank=True)
    name = models.CharField(max_length=30)    
    address = models.CharField(max_length=255)
    phone_number = models.IntegerField(max_length=11, unique=True, validators=[RegexValidator(regex='^\d{10}$', message='Length has to be 11', code='Invalid number')])
    email = models.EmailField(validators=[validate_email])

    def __str__(self):
        return self.user.email

class Purchase(models.Model): # needs to be associated with a user account
    cash_only = models.BooleanField('Cash only', default=False)
    payment_type = models.ForeignKey('CardChoices', related_name='Payment Types')
    card_name = models.CharField(max_length=26, default=None, validators=[alphanumeric_RE]) #done
    card_number = models.CharField(max_length=19, default=None) # OVERWRITTEN
    security_code = models.IntegerField(max_length=3, default=None) # OVERWRITTEN
    expiry_date = models.DateField(default=datetime.now) # OVERWRITTEN
    date = models.DateField(editable=True, auto_now_add=True, default=datetime.now)
    delivery_option = models.BooleanField('Is Collecting Ticket', default=True)
    reference_number = models.CharField(max_length=LENGTH, unique=True, default=None)
    temp_session_key = models.CharField(null=True, editable=False, max_length=255, default=None)

def __str__(self):
    return self.card_name

def save(self, *args, **kwargs): # Thanks to workmajj for providing the logic!
    """
    Upon saving, generate a code by randomly picking LENGTH number of
    characters from CHARSET and concatenating them. If code has already
    been used, repeat until a unique code is found, or fail after trying
    MAX_TRIES number of times. (This will work reliably for even modest
    values of LENGTH and MAX_TRIES, but do check for the exception.)
    Discussion of method: http://stackoverflow.com/questions/2076838/
    """
    loop_num = 0
    unique = False
    while not unique:
        if loop_num < MAX_TRIES:
            new_code = ''
            for i in range(LENGTH):
                new_code += CHARSET[randrange(0, len(CHARSET))]
            if not Purchase.objects.filter(reference_number=new_code):
                self.reference_number = new_code
                unique = True
            loop_num += 1
        else:
            raise ValueError("Couldn't generate a unique code.")
    super(Purchase, self).save(*args, **kwargs)

class Meta:
    ordering = [ "date" ]

Forms.py

class ContactForm(forms.Form):
    """Form for the contact page containing relevant fields and appropriate attributes."""
    name = forms.CharField(required=True, max_length=100, label='Name')
    email = forms.EmailField(required=True, label='E-mail Address', validators=[validate_email])
    subject = forms.CharField(required=True, label='Subject')
    query = forms.CharField(required=True, widget=forms.Textarea)

class PurchaseForm(forms.ModelForm):
    # Custom fields overwrite the ones in the Purchase model
    payment_type = forms.ModelChoiceField(queryset=CardChoices.objects.all())
    card_name = forms.CharField(widget=forms.TextInput(attrs={'placeholder': "Card Holder's Name"}))
    card_number = CreditCardField(required=True, widget=forms.TextInput(attrs={'placeholder': 'Credit / Debit card number'}))
    security_code = VerificationValueField(required=True)
    expiry_date = ExpiryDateField(required=True)
    DELIVERY_CHOICES = (
        (True, 'Collect from Venue'),
        (False, 'Print Ticket'),
    )
    delivery_option = forms.ChoiceField(choices=DELIVERY_CHOICES, widget=forms.RadioSelect())
    email_receipt = forms.BooleanField(required=False, label='Tick this box to receive an e-mail receipt')
    email = forms.EmailField(required=False, label='E-mail address', validators=[validate_email])

    class Meta:
        model = Purchase
        fields = ("payment_type", "card_name", "card_number", "security_code", "expiry_date", "delivery_option", "email_receipt", "email")

class UserForm(forms.ModelForm):
    password = forms.CharField(widget=forms.PasswordInput())

    class Meta:
        model = User
        fields = ("username", "email", "password")

class UserProfileForm(forms.ModelForm):
    class Meta:
        model = UserAccount
        fields = ("name", "address", "phone_number")

Answer 1

在表单验证后的视图中，您应该执行以下操作：

try:
   with transaction.atomic(): 
      x=get_number_of_tickets   
      venue=Venue.objects.get(id=5)   
      venue.capacity -= x  
      venue.update()   
      Purchase.save() 
except IntegrityError:
      raise Error

检查django事务文档以获取更多信息，因为它更完整： https://docs.djangoproject.com/en/1.8/topics/db/transactions/

Answer 2

你想避免竞争条件。交易只是其中的第一步。

您在交易中要做的第一件事就是获取您正在更改的行的锁定。这可以防止您在写入数据库时​​读取过时的值。这可以通过update语句完成，该语句将锁定行以进行进一步更新，直到提交或回滚事务：

with transaction.atomic():
    Event.objects.filter(id=5).update(quantity=F('quantity') - x)

获得锁定并完成更新后，检查数据完整性是否仍然完好（即容量不低于0）。如果是，则继续，否则，通过引发异常来回滚事务：

event = Event.objects.get(id=5)
if event.capacity < 0:
    raise ValueError("Capacity exceeded")

如果剩下足够的位置，此时您将退出atomic()块，提交事务，其他请求可以获取锁并尝试更新值。如果没有足够的容量，事务将被回滚，而其他请求永远不会知道值已经改变，因为他们一直在等待锁定。

现在，把它们放在一起：

from django.db import transaction
from django.db.models import F

def get_tickets(event_id, num_tickets):
    with transaction.atomic():
        Event.objects.filter(id=event_id).update(quantity=F('quantity') - num_tickets)
        if Event.objects.get(id=venue_id).quantity < 0:
            raise ValueError("Capacity exceeded: less than %s tickets left." % num_tickets)