Performing joins on the query containing nested query with FirstOrDefault fails

Hi,

performing joins on the query containing nested query with FirstOrDefault fails.

Exception message:
Object reference not set to an instance of an object.

Stack trace:
   at LinqToDB.Linq.Builder.SelectContext.<>c.<ConvertToIndexInternal>b__49_2(ValueTuple`4 context, Int32 n, IBuildContext ctx, Expression ex, Int32 l, Expression _)
   at LinqToDB.Linq.Builder.SelectContext.ProcessMemberAccess[T,TContext](TContext context, Expression expression, MemberExpression levelExpression, Int32 level, Func`7 action)
   at LinqToDB.Linq.Builder.SelectContext.ConvertToIndexInternal(Expression expression, Int32 level, ConvertFlags flags)
   at LinqToDB.Linq.Builder.SelectContext.ConvertToIndex(Expression expression, Int32 level, ConvertFlags flags)
   at LinqToDB.Linq.Builder.SubQueryContext.ConvertToSql(Expression expression, Int32 level, ConvertFlags flags)
   at LinqToDB.Linq.Builder.ExpressionContext.ConvertToSql(Expression expression, Int32 level, ConvertFlags flags)
   at LinqToDB.Linq.Builder.ExpressionBuilder.ConvertToSql(IBuildContext context, Expression expression, Boolean unwrap, ColumnDescriptor columnDescriptor, Boolean isPureExpression)
   at LinqToDB.Linq.Builder.JoinBuilder.BuildJoin(ExpressionBuilder builder, SqlSearchCondition condition, IBuildContext outerKeyContext, Expression outerKeySelector, IBuildContext innerKeyContext, Expression innerKeySelector)
   at LinqToDB.Linq.Builder.JoinBuilder.BuildMethodCall(ExpressionBuilder builder, MethodCallExpression methodCall, BuildInfo buildInfo)
   at LinqToDB.Linq.Builder.MethodCallBuilder.BuildSequence(ExpressionBuilder builder, BuildInfo buildInfo)
   at LinqToDB.Linq.Builder.ExpressionBuilder.BuildSequence(BuildInfo buildInfo)
   at LinqToDB.Linq.Builder.FirstSingleBuilder.BuildMethodCall(ExpressionBuilder builder, MethodCallExpression methodCall, BuildInfo buildInfo)
   at LinqToDB.Linq.Builder.MethodCallBuilder.BuildSequence(ExpressionBuilder builder, BuildInfo buildInfo)
   at LinqToDB.Linq.Builder.ExpressionBuilder.BuildSequence(BuildInfo buildInfo)
   at LinqToDB.Linq.Builder.ExpressionBuilder.Build[T]()
   at LinqToDB.Linq.Query`1.CreateQuery(ExpressionTreeOptimizationContext optimizationContext, ParametersContext parametersContext, IDataContext dataContext, Expression expr)
   at LinqToDB.Linq.Query`1.GetQuery(IDataContext dataContext, Expression& expr, Boolean& dependsOnParameters)
   at LinqToDB.Linq.ExpressionQuery`1.GetQuery(Expression& expression, Boolean cache, Boolean& dependsOnParameters)
   at LinqToDB.Linq.ExpressionQuery`1.System.Linq.IQueryProvider.Execute[TResult](Expression expression)
   at System.Linq.Queryable.FirstOrDefault[TSource](IQueryable`1 source)
   at Linq2dbTest.Program.Main(String[] args) in Program.cs:line 34

Steps to reproduce

using LinqToDB;
using LinqToDB.Mapping;
using System.Linq;

namespace Linq2dbTest
{
    public static class Program
    {
        public static void Main()
        {
            var builder = MappingSchema.Default.GetFluentMappingBuilder();

            builder.Entity<Entity1>();

            builder.Entity<Entity2>();

            builder.Entity<Entity3>();

            using (var con = new Db())
            {
                var subquery = con.GetTable<Entity2>();

                var query = con.GetTable<Entity1>()
                    .Select(entity1 => new
                    {
                        Entity1 = entity1,
                        Entity2 = subquery.FirstOrDefault(entity2 => entity2.Id == entity1.Id)
                    })
                    .GroupJoin(con.GetTable<Entity3>(),
                        x => x.Entity2.Id,
                        x => x.Id,
                        (x, y) => x);

                var result = query.FirstOrDefault();
            }
        }

        public class Db : LinqToDB.Data.DataConnection
        {
            public Db() : base(ProviderName.SqlServer2017,
                "data source=localhost;initial catalog=xxx;integrated security=True;MultipleActiveResultSets=True")
            {
            }
        }

        public class Entity1
        {
            public int Id { get; set; }
        }

        public class Entity2
        {
            public int Id { get; set; }
        }

        public class Entity3
        {
            public int Id { get; set; }
        }
    }
}

Environment details

linq2db version: 3.5.1 Database Server: SQL Server 2019 Database Provider: SQL Operating system: W10 .NET Framework: 4.8

[sdanyliv]

Sorry for delay, try this one. Query syntax is more preferable here:

var query = 
    from e1 in con.GetTable<Entity1>()
    from e2 in subquery.Where(e2 => e2.Id == e1.Id)
        .Take(1)
        .DefaultIfEmpty()
    from e3 in con.GetTable<Entity3>().Where(e3 => e3.Id == e2.Id)
    select e3;

I’m working on fixing FirstOrDefault in queries, not easy task.

[stomasovic]

Hi,

We succeeded using Where.Take.DefaultIfEmpty instead of FirstOrDefault var query = con.GetTable<Entity1>() .Select(entity1 => new { Entity1 = entity1, Entity2 = subquery.FirstOrDefault(entity2 => entity2.Id == entity1.Id) }) use following instead: var query = con.GetTable<Entity1>() .Select(entity1 => new { Entity1 = entity1, Entity2 = subquery.Where(entity2 => entity2.Id == entity1.Id).Take(1).DefaultIfEmpty() })