Hello,
My opinion on the problem:
A database is ACID, so there will be no problem if every information
are stored in the database.
There should be a "available_quantity" property on a item.
This will allow you to have more control of the stock too.
If a customer put an item in his basket this checks the
available quantity of the item and update it. So you
succeed to sell only available item.
Then the release item problem:
If the user didnt buy the item before X minutes, you need
to forbid his basket and you put again his items inside
the available_quantity.
Don't know if it helped you?
Thanks,
PS:
May I suppose to you that the get_quantity function and set_quantity
should be in the manager of the model?
something like:
```
class ShopItemManager(Manager):
def get_quantity(self, name):
return self.get_queryset().get(name=name).quantity
class ShopItem(Model):
...
objects = ShopItemManager()
```
This allow you to tied up your function to the model.
On 02/18/2015 06:01 AM, Abraham Varricatt wrote:
Hello,
I'm trying to make an app where folks can order X quantity of an item.
The condition is that the order should only be made if inventory
exists. Assume that we have stock of Y items. This means that only if
Y >= X should we allow the sale to go through. And once we accept an
order, the inventory should be updated so that Y = Y - X.
The way I've currently implemented this is with a pair of
getter/setter methods. I have a model 'ShopItem' which has two fields
called 'quantity' and 'name' (this is unique). I've made a utility
class (outside model source file) where I've made the following 2
functions;
def get_quantity(name):
stuff_left = 0
try:
item = ShopItem.objects.get(name=name)
stuff_left = item.quantity
except ShopItem.DoesNotExist:
pass # return zero
return stuff_left
def set_quantity(name, stuff_left):
item = ShopItem.objects.get(name=name)
item.quantity = stuff_left
item.save()
Elsewhere in my project, if I need to display the remaining quantity
of an item in a view, I'll pass the result from get_quantity(). At
sale time here is how I update things;
def customer_buy(name, number):
temp = get_quantity(name)
if (temp >= number):
temp = temp - number
set_quantity(name, temp)
// do other things
else:
// sale failed, do something else
I tested this on my system and things appear to work. But I'm
concerned about race conditions. What if two different customers came
in to buy the same item? As in; if I only have 7 items in stock, but
one customer came to buy 6nos and another 5nos. There is a chance (as
per my understanding) that both orders will be accepted - even worse,
my inventory will not accurately reflect the updated situation.
Any ideas on how this issue can be resolved? I've come across this
- https://docs.djangoproject.com/en/1.6/ref/models/instances/#updating-attributes-based-on-existing-fields
, but not sure how to apply it to the current scenario.
Learning django,
Abraham V.
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