How to implement “autoincrement” on Google AppEngine

Question

I have to label something in a \"strong monotone increasing\" fashion. Be it Invoice Numbers, shipping label numbers or the like.

1. A number MUST NOT BE used twi

Answer 1

I implemented something very simplistic for my blog, which increments an IntegerProperty, iden rather than the Key ID.

I define max_iden() to find the maximum iden integer currently being used. This function scans through all existing blog posts.

def max_iden():
    max_entity = Post.gql("order by iden desc").get()
    if max_entity:
        return max_entity.iden
    return 1000    # If this is the very first entry, start at number 1000

Then, when creating a new blog post, I assign it an iden property of max_iden() + 1

new_iden = max_iden() + 1
p = Post(parent=blog_key(), header=header, body=body, iden=new_iden)
p.put()

I wonder if you might also want to add some sort of verification function after this, i.e. to ensure the max_iden() has now incremented, before moving onto the next invoice.

Altogether: fragile, inefficient code.