How to increment a counter in LINQ

[Rasmus Svendsen]

How to increment a counter in LINQ? 

I cannot find a way to increment a counter the atomic way - like in CLI: “INCR page_view_counts['www.datastax.com'][home] BY 1”.

If the C# driver not support it yet, I hope for an implementation like this:

  public static CqlQuery<TSource> Inc<TSource, TResult>(this CqlQuery<TSource> source, Expression<Func<TSource, TResult>> memberExpression, TResult value)

  {

   // some logic

  }

The linq syntax will be simple like this:

  StatisticsTable.Where(x => x.author_id == "123").Inc(x => x.followers_count, -1).Inc(x=>x.tweets_count, 15).Update().Execute();

I use the beta version 2.0.0-beta1. Is there a way to increment a counter already?

Cheers,

Rasmus

[Jorge Bay Gondra]

Hi Rasmus,

Counter columns are implemented using an Attribute: CounterAttribute.

If you mark the column of your entity as [Counter], after you increase it / decrease it like a regular column, it will get the diff and update the value accordingly.

Cheers,

Jorge

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[Rasmus Svendsen]

Hi Jorge,

Thanks for the fast reply. I have the [Counter] attribute in my data model, but I still not get it to work.

As I understand your answer I shall create a Linq Context, but by doing so I have to get an object down on the client side made ​​the change of the counter and then call the SaveChanges(..) witch update the db, right?

I want to update the counters without having to first get the value down on the client side, like the atomic call. How to do that in the C# driver?

Cheers,

Rasmus

[Jorge Bay Gondra]

Hi Rasmus,

Without a context, as a Linq CqlQuery, it is not possible.

I agree it would be nice to have.

Best,

Jorge

[Rasmus Svendsen]

Hi Jorge,

Okay, I hope for a implementation of increase/decrease of counters in Linq CqlQuery in the near future ;-)

I have found the "Playground" project in github.

When I run the code it complete without errors, but when looking at the output at "After deletion of all tweets our \"Statistics\" table looks like" have the same tweets_counts as before the deletion, hmm...

Maybe there are a bug in the C# driver?

Cheers,

Rasmus

[Jorge Bay Gondra]

Hi Rasmus,

Yes, it was indeed a bug: when maintaining the same entities in context after executing the first batches, when unmodified in following batches, it was removed from context.

I'll push a fix right away.

Thanks for your help,

Jorge

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[Yann ROBIN]

Hi,

I've submitted a patch to support counter in Linq :

https://github.com/datastax/csharp-driver/pull/43

It's very simple, syntax would be :

query = table

    .Where(r => r.RowKey1 == 5 && r.RowKey2 == 6)

    .Select(r => new CounterTestTable1() { Value = 1 })

.Update();

And it translate to :

UPDATE "CounterTestTable1" SET "Value" = "Value" + 1 WHERE "RowKey1" = 5 AND "RowKey2" = 6

-- 

Yann