Question

I’m in OPA for some days now and I really start to like it. I’m attending the first year of computer science and we make some database class the next year-
The little I know about Databases are from php, I have used MySQL with php and SQLlite with c++. But this type of database is a bit different from what I’ve seen.

I have followed the guide about database in OPA http://doc.opalang.org/manual/Hello–database but I have a question:

In the guide we declare a new Database:

type user_status = {regular} or {premium} or {admin}
type user_id = int
type user = { user_id id, string name, int age, user_status status }
database users {
   user /all[{id}]
   /all[_]/status = { regular }
}

We learn how to read this database and make some query to this database with Maps, but how do I add a new element? I was testing a bit:

/users/all[{id:0}]/name<-getusername;

but id should be auto increment, from the little I know.

Thanks everyone for the help =D
I really want to get in OPA, the little I have make is really impressive!

Answer 1

mongoDB and auto-increment

With mongoDB (the default Opa database) there is no auto-increment (like in SQL), for scalability reason.
But if you really need one, you can use a counter to create this feature yourself:

database users {
   user /all[{id}]
   int  /fresh_key
   /all[_]/status = { regular }
}

And increment the key each time you use it: /users/fresh_key++

Random fresh key

You can also generate a random id, for example with something like Random.string(6)
Read this thread to learn more about this technique: http://lists.owasp.org/pipermail/opa/2012-April/001052.html

User defined unique key

But if you are dealing with users, maybe you already have a unique key: what about using “login” or “email” as the unique key?