How to handle grouped by sentences

[Jordi Cabré]

I'm interested in handle grouped sentences like:

var fs = this.backend.DigitalInputs.GroupBy(di => di.Channel).Select(g => new { Channel = g.Key, Count = g.Count() });

As far I've been able to figure out, I'm guessing "group" information in embedded into main QueryModel creating another nested QueryModel into mainQueryModel's MainFromClause:

QueryModel ->
  * MainFromClause: {from IGrouping`2 g in {value(Backend.Implementations.Lest.Linq.LivingQueryable`1[Backend.Domain.DigitalInput]) => GroupBy([di].Channel, [di])}}
    * FromExpression: {{value(Backend.Implementations.Lest.Linq.LivingQueryable`1[Backend.Domain.DigitalInput]) => GroupBy([di].Channel, [di])}}
      * QueryModel:
        * MainFromClause: {from DigitalInput di in value(Backend.Implementations.Lest.Linq.LivingQueryable`1[Backend.Domain.DigitalInput])}
        * ResultOperators: (1) -> {GroupBy([di].Channel, [di])}

My question is about which would be the best or common strategy for going through this kind of nested query models?

My first approuch was around handling GroupResultOperator, however, I realized it's never reached.

Currently, I've a Transformer and a Translafor QueryModel visitors.

In my Transformer's VisitMainFromClause is a dummy method:

public override void VisitMainFromClause(MainFromClause fromClause, QueryModel queryModel)

{
    // do nothing

    base.VisitMainFromClause(fromClause, queryModel);

}

And my Translator's VisitMainFromClause is a bit less dummy method.

Basiclly, checks out whether fromClause.ItemType is aknowledged.
However, with a groupby information in it, this fromClause.ItemType is like a Grouping`1...:

public override void VisitMainFromClause(MainFromClause fromClause, QueryModel queryModel)
{
    LQL.Expressions.Catalog.LQLEntityExpression entity = fromClause.ItemType.GetDomainEntity();
    this.lqlQueryModel.Search = entity;
}

Thanks.

[Gordon Watts]

Hi,

  My LINQ queries can get super-complex, so in order to solve this I ended up writing a recursive parsing system. So my query visitor would call an expression parser to parse an expression, and then the expression would be a SubQueryExpression, which would trigger a call back into the QueryModel.

 

  Now, for the group by operator – it is a ResultOperator – just like “Count()”, or “Sum()”. The difference is that it is a sequence ResultOperator. So I added extra code to deal with a type of result operator that was a sequence rather than just a single value.

 

  I hope this helps. You are more than welcome to look at my code, but my application is so specific. Trying to translate LINQ to procedural C++ in this conext means the final result strongly influenced the architecture I used (also, when I started this I was “hacking” and didn’t know what I was doing, so probably made some important decision without realizing it).

 

  The top level query processor. Note the code for parsing the result operators: https://github.com/gordonwatts/LINQtoROOT/blob/master/LINQToTTree/LINQToTTreeLib/QueryVisitors/QueryVisitor.cs#L69 (in particular the bit about a sequence result operator).

 

  The group by result operator: https://github.com/gordonwatts/LINQtoROOT/blob/master/LINQToTTree/LINQToTTreeLib/ResultOperators/ROGroup.cs#L49

 

It may well be understanding my code is more effort than just forging ahead with some test cases in your environment. 😉

 

Cheers,
Gordon

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