Aggregate functions with multiple parameters

[Alexander Turtsevich]

This is a follow up for #85 [Add ability to have custom aggregate functions with multiple parameters].

It became obvious that it was not fully implemented, specifically, I tried to make builtin STRING_AGG function work. This function takes additional argument - separator.

It turns out that System.Data.Entity.Core.Query.PlanCompiler.Normalizer.VisitCollectionAggregateFunction unconditionally drops all argument nodes except the first one (code reference).

I've tried to pass remaining child nodes to the CreateNode method and it worked just fine:

var aggExprNode = m_command.CreateNode(aggregateOp, Enumerable.Repeat(unnestVarRefNode, 1).Concat(n.Children.Skip(1)).ToList());

So this is one-line fix that enables using STRING_AGG which is super useful. I can try to create a PR if possible.

Example:

SELECT 
    [Limit1].[C1] AS [C1]
    FROM ( SELECT TOP (1) 
        (SELECT 
            STRING_AGG([Extent4].[Name], N',') AS [A1]
            FROM  [dbo].[AttorneyLanguages] AS [Extent3]
            LEFT OUTER JOIN [dbo].[Languages] AS [Extent4] ON ([Extent4].[IsDeleted] = 0) AND ([Extent3].[LanguageId] = [Extent4].[Id])
            WHERE ([Extent3].[IsDeleted] = 0) AND ([Project1].[Id] = [Extent3].[AttorneyId])) AS [C1]
        FROM ( SELECT 
            [Extent1].[Id] AS [Id], 
            (SELECT 
                COUNT(1) AS [A1]
                FROM [dbo].[AttorneyLanguages] AS [Extent2]
                WHERE ([Extent2].[IsDeleted] = 0) AND ([Extent1].[Id] = [Extent2].[AttorneyId])) AS [C1]
            FROM [dbo].[Attorneys] AS [Extent1]
            WHERE [Extent1].[IsDeleted] = 0
        )  AS [Project1]
        WHERE [Project1].[C1] > 1
    )  AS [Limit1]

[Arthur Vickers]

This issue has been closed because EF6 is no longer being actively developed. We are instead focusing on stability of the codebase, which means we will only make changes to address security issues. See the repo README for more information.

[Alexander Turtsevich]

Implemented STRING_AGG without your help, thank you MS.

[joseJositoJosete]

Hi Kemsky
My name is Jose, im from Spain and I don't speak English, I hope you understand me haha.

I need Implement STRING_AGG in EF6 and I see your comments, but I don't understand, can you write more information.

Thanks

[Alexander Turtsevich]

@joseJositoJosete, it is quite complicated, due to multiple issues with EF6.

You need EntityFramework.Functions package or implement your own convention.

   // AggregateFunctions.cs

    [Function(FunctionType.AggregateFunction, "STRING_AGG", StoreFunctionName = "STRING_AGG")]
    [return: Parameter(DbType = "nvarchar(max)", ClrType = typeof(string))]
    public static string StringAgg(this IEnumerable<string> queryable)
    {
        return string.Join(", ", queryable);
    }

// MyContext.cs

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
      // you have to skip this code when DbMigrator or migration generator is running, 
      // by adding some static field or AsyncLocal to your implementation of IDatabaseInitializer<MyContext>.
      if (...) {
            modelBuilder.Conventions.Add(new FunctionConvention(typeof(AggregateFunctions)));
      }
      // ...
}

Implement and register IDbCommandTreeInterceptor :

// DbCommandTreeInterceptorImpl.cs

public void TreeCreated(DbCommandTreeInterceptionContext interceptionContext)
    {
        if (interceptionContext.OriginalResult.DataSpace == DataSpace.SSpace && interceptionContext.Result is DbQueryCommandTree queryCommand)
        {
            var originalQuery = queryCommand.Query;

            var dbFunctionsVisitor = new DbFunctionsVisitor();

            var newQuery = originalQuery.Accept(dbFunctionsVisitor);

            interceptionContext.Result = new DbQueryCommandTree(queryCommand.MetadataWorkspace, queryCommand.DataSpace, newQuery);
        }
}

Implement visitor:

// DbFunctionsVisitor.cs

internal sealed class DbFunctionsVisitor : DefaultExpressionVisitor
{
   private static readonly string StringAggFunction = "CodeFirstDatabaseSchema.STRING_AGG";

    protected override DbFunctionAggregate VisitFunctionAggregate(DbFunctionAggregate aggregate)
    {
        if (aggregate.Function.FullName == StringAggFunction)
        {
            IList<DbExpression> list = new List<DbExpression> { aggregate.Arguments.Single(), DbExpression.FromString(", ") };

            var args = Activator.CreateInstance(DbExpressionListType, BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.CreateInstance, null, new object[] { list }, null);
            var func = (DbFunctionAggregate)Activator.CreateInstance(DbFunctionAggregateType, BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.CreateInstance, null, new object[] { aggregate.ResultType, args, aggregate.Function, false }, null);

            return base.VisitFunctionAggregate(func);
        }

        return base.VisitFunctionAggregate(aggregate);
    }
}

[joseJositoJosete]

Hi
Wou you are a good programmer and good person, everybody told me it's impossible and you did :).

I have some questions, can you help me.i implement your code and send me an error in this line

GroupBy(l => new { l.IdExpediente }).Select(a => new { a.Key.IdExpediente, dd= a.Select(s => s.Investigador.Apellido1).StringAgg() });

"The specified method 'System.String StringAgg(System.Collections.Generic.IEnumerable`1[System.String])' on the type 'ISCIII.AESEG.DAL.AggregateFunctions' cannot be translated into a LINQ to Entities store expression.'"

I think its becauese even OnModelCreating doestn execute, its because i dont use code first , i use edmx file.

In your code you wrote two types "DbFunctionAggregate" and "DbExpressionListType" I don't have these types, and I write "CollectionType" and "AggregateFunctions" , its OK

  var args = Activator.CreateInstance( typeof( CollectionType), BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.CreateInstance, null, new object[] { list }, null);
            var func = (DbFunctionAggregate)Activator.CreateInstance( typeof(AggregateFunctions), BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.CreateInstance, null, new object[] { aggregate.ResultType, args, aggregate.Function, false }, null);

Thanks

[Alexander Turtsevich]

@joseJositoJosete, I can not help you with Edmx, I have never used it.

[joseJositoJosete]

Don't worry you helped me :)