Django Python unique =真表达式

Question

I want to save Entries in my database such that I can delete them later. To identify them, i put the key attribute in my class Entry in models.py as shown below:

class Entry(models.Model):
    name = models.CharField(max_length=200)
    key = models.IntegerField(unique=True,default=0)

Every time I start the server, I will find the biggest key that is in Entry.objects.all() .

Every time I create the first new Entry after starting the server, I want to take the key I found in the very beginning, increase it by 1 and then set this as the key for my new Entry . For every subsequent Entry , I will just take the key of the previous element, increase it by 1 and. set it as the key for my new Entry

However, before I do this, I want to know what Django considers as unique. For example, if i added three entries initially with keys 1 , 2 and 3 , and then I delete the last element with key 3 . If I then restart the server, the biggest key I will find is 2 and the next Entry I will add will have key 2+1 which is equal to 3 again.

Is this key unique? It was entered before but I deleted that element right? So is uniqueness determined by whatever I entered in the past or just depending on the elements currently in the database?

Also, does Django keep a track of all the instances of Entry that I added, so that it would somehow know if I added an Entry with key 3 before?

Note that my data is stored in a file called db.sqlite3

Thank you.

Answer 1

Seems like you are looking for something that already exists; models have an id field by default which is unique and monotonic (newer entries have bigger id).

Answer 2

尝试在您的键字段中添加primary_key=True 。

Answer 3

None of this has anything to do with Django at all. This is pure database stuff: unique constraints are handled exclusively by the database, in your case SQLite, although the functionality is exactly the same for other DBs as well.

Of course, a unique constraint only takes into account rows that actually exist: what would be the point otherwise?

There are other things to bear in mind with your proposal as well. In particular, there is the issue of race conditions: if you do a query to get the maximum key value, and then create a new row with key+1, you run the risk of another user in the meantime adding their own row at key+1. Much better, as Iris and ryanageles suggest, to use the built-in primary key which is already automatically defined as the id field.