In entity framework 6 SelectMany to Join option for query throwing error The source IQueryable doesn't implement IDbAsyncEnumerable<MyModel>

Question

I have a following query throwing error as mentioned in subject

public async Task<IEnumerable<MyModel>> GetDataAsync()
{
    return await _context.baseTable.SelectData().AsNoTracking().ToListAsync();
}
    
public static IQueryable<MyModel> SelectData(this IQueryable<relet> baseTable)
{
    return baseTable
        .Where(h => h.table1 != null && h.table1.table2 != null)
        .Select(lev => new {
            table3Collection = lev.table1.table2.table3, //table3 is ICollection
            Table5Key = lev.table5 != null ? lev.table5.id : null,
            Table4Key = lev.table1.table2.table4 != null ?  lev.table1.table2.table4.id : null,
        }).ToList()
        .SelectMany(x => {
            return x.table3Collection.Select(h => new MyModel() {
                ContactKey = new Key
                {
                    Id = h.Id,
                    Type = h.Type
                },
                Table5Key = x.Table5Key,
                Table4Key = x.Table4Key
    
            });
        })
        .AsQueryable();
}

Can someone Give me idea how to use same LINQ query in group join way? Problem is occuring at .ToListAsync() line

Answer 1

You cannot create IQueryable from IEnumerable and use asynchronous extensions. Actually you just need to correct your query and remove unnecessary materializations.

public static IQueryable<MyModel> SelectData(this IQueryable<relet> baseTable)
{
    return 
        from lev in baseTable
        from t3 in lev.table1.table2.table3
        select new MyModel
        {
            ContactKey = new Key
            {
                Id = lev.Id,
                Type = lev.Type
            },
            Table5Key = (int?)lev.table5.Id,
            Table4Key = (int?)lev.table1.table2.table4.Id
        };
}

Note that from simplified query we can see that t3 is not used. It means that we just duplicate records.