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Message from discussion Selecting (max) OR identity field
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Michael Kennedy  
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 More options Jun 9 2010, 2:42 pm
From: Michael Kennedy <mkennedy66996...@gmail.com>
Date: Wed, 9 Jun 2010 11:42:02 -0700
Local: Wed, Jun 9 2010 2:42 pm
Subject: Re: [NoRM MongoDB] Re: Selecting (max) OR identity field
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I've been working on a project which has the exact same requirements. Here's
how I solved it.

I have a proper internal ID using ObjectId, but for referencing from URLs
and whatnot I created as second property called PublicId. Then I just
generate a random 6 character alphanumeric string and try to get it from the
database. If one exists, then I'll just regen that id, otherwise I insert
the entry with that ID. Chances of a conflict / race condition are utterly
small.

The actual code is below.

Best regards,
Michael

Note: UniqunessMethod is a delegate I pass in which basically does a select
from mongodb where publicid == random key. But I use a delegate so I can run
this for any entity I want.

private string GetNewPublicId<TEntity>(Func<string, TEntity>
UniqunessMethod)
where TEntity : class
{
Random rand = new Random();
bool unique = false;
string id = null;

while (!unique)
{
StringBuilder sb = new StringBuilder();
 for (int i = 0; i < 6; i++)
{
int next = rand.Next(0, 36);
 if (next < 10)
{
sb.Append(next);
 }
else
{
 next -= 10;
next = 'a' + next;
sb.Append((char) next);
 }

}

id = sb.ToString();
 TEntity entity = UniqunessMethod(id);
unique = entity == null;

}

return id;
}
On Tue, Jun 8, 2010 at 8:50 AM, SaraC <sarajchi...@gmail.com> wrote:
> The problem is this application is a url shortener and requires an
> identifier that can be hashed and
> used for those purposes. I don't know of another solution while
> ensuring uniqueness.

> On Jun 8, 2:49 am, Ken Egozi <egoz...@gmail.com> wrote:
> > select MAX is definitely wrong for the reasons mentioned.
> > I would avoid Identity behaviour anyway, and not only because of the
> > sharding issue. Using Identity means that every Insert must immediately
> call
> > the database. This is not always desirable, as you might want to do more
> > stuff with the Id, but postpone the DB call for later for various
> reasons.

> > Now using the ObjectId approach has its limitations also, mainly the size
> > and the readability, as some systems like human readable ids (order
> number,
> > user id, customer service call id, etc.)
> > you can enjoy both worlds if you employ the hi-lo algorithm (or
> > a variation thereof). You get an integer id, generally sequential, fast
> to
> > generate and simple to implement.

> > The docs for Hibernate/NHibernate are all full with reasons to avoid
> > Identity and use GuidComb (their equivalence of ObjectId) or HiLo

> > On Tue, Jun 8, 2010 at 9:35 AM, Jørn Wildt <j...@fjeldgruppen.dk> wrote:
> > > Check this request for some thoughts:
> > >http://jira.mongodb.org/browse/SERVER-195

> > > Beware of concurrency issues: two reades may read MAX() = 10 and then
> > > both of them increment it to 11. You need to use MongoDB's various
> > > atomic operation to ensure this operation is safe.

> > > /Jørn

> > > On Jun 8, 8:19 am, SaraC <sarajchi...@gmail.com> wrote:
> > > > I'm new to NoSQL, I'm looking to either create an identity field
> (that
> > > > starts at 0) or build my own by selecting a max value.

> > > > I understand that the Mongo Ids are timestamps and can be used as
> > > > such. However, I need a smaller value for this column.

> > > > Thanks a bunch!

> > > --
> > > You received this message because you are subscribed to the Google
> Groups
> > > "NoRM mongodb" group.
> > > To post to this group, send email to norm-mongodb@googlegroups.com.
> > > To unsubscribe from this group, send email to
> > > norm-mongodb+unsubscribe@googlegroups.com<norm-mongodb%2Bunsubscribe@google groups.com>
> <norm-mongodb%2Bunsubscribe@google groups.com>
> > > .
> > > For more options, visit this group at
> > >http://groups.google.com/group/norm-mongodb?hl=en.

> > --
> > Ken Egozi.
> http://www.kenegozi.com/bloghttp://www.delver.comhttp://www.musicglue...כנס הקהילתי הראשון למפתחי דוטנט - בואו בהמוניכם

> --
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