Django Max函数未返回最大值

Question

I need the maximum value of a particular field in my database, increment it by 1 and that becomes my new order number.

The way I do that is:

def generate_order_number(self):
    q_number = Order.objects.all().aggregate(Max('number'))['number__max']
    if q_number:
        q_number += 1
    else:
        q_number = 1

    return q_number

This code works perfectly fine until order number 10. However, when order number 11 is attempted, the aggregate query returns max value as 9 and thus, the function returns 10 as the new order number - which fails the entire process (number is unique in the database).

Why would this be happening?

I am using Django/PostgreSQL combination. My database entry for number is:

number = models.BigIntegerField(
    _("Quotation number"), db_index=True, unique=True)

Also, when I try to directly manipulate the database for order number, the error I get is:

ProgrammingError at /admin/order/order/21/
operator does not exist: character varying = integer
LINE 1: ..._order" WHERE ("order_order"."number" = 11 AND N...
                                                         ^
HINT:  No operator matches the given name and argument type(s). You might need to add explicit type casts.

How do I fix this?

Answer 1

The PostgreSQL serial datatype internally creates both a smallint / int / bigint column and a sequence. Effectively making it equivalent to a normal int column with a default set to some sequence.

For example, this:

CREATE TABLE tablename (
    colname SERIAL
);

Internally results in this:

CREATE SEQUENCE tablename_colname_seq;
CREATE TABLE tablename (
    colname integer NOT NULL DEFAULT nextval('tablename_colname_seq')
);
ALTER SEQUENCE tablename_colname_seq OWNED BY tablename.colname;

This is where your question comes in, while a plain CREATE SEQUENCE just adds a simple sequence that increments by one and will never go down. Using setval you can reset the sequence current value to reuse old numbers.

Do note that this can result in duplicate key constraints since the primary key will still be unique so if you do get duplicate key constraints, simply reset the sequence to a range that isn't in use yet ( SELECT MAX(colname)+1 FROM tablename should give you a correct startpoint).