c# - Linq2Db and string.join()

Question

I am using Linq2db in my big query with subqueries. At the one place inside it, I want to use string.Join():

...
FullPath = string.Join(" -> ", GetPathQuery(db, c.Id).Select(pi => pi.Name))
...

But I have received an exception:

LinqException: 'Join(" -> ", value(RI.DAL.Categories.AdminCategoryPreviewDAL).GetPathQuery(value(RI.DAL.Categories.AdminCategoryPreviewDAL+<>c__DisplayClass4_0).db, c.Id).Select(pi => pi.Name))' cannot be converted to SQL.

I use Postgre SQL and it has the concat_ws function which is perfect for me. So I try to use it:

[Sql.Expression("concat_ws({1}, {0})")]
public static string JoinAsString(this IQueryable<string> query, string separator)
{
    return string.Join(separator, query);
}

...
FullPath = GetPathQuery(db, c.Id).Select(pi => pi.Name).JoinAsString(" -> ")
...

But I was failed with the same exception.

---

The full source code of the GetPathQuery:

    private IQueryable<CategoryPathItemCte> GetPathQuery(IStoreDb db, Guid categoryId)
    {
        var categoryPathCte = db.GetCte<CategoryPathItemCte>(categoryHierarchy =>
        {
            return
                (
                    from c in db.Categories
                    where c.Id == categoryId
                    select new CategoryPathItemCte
                    {
                        CategoryId = c.Id,
                        ParentCategoryId = c.ParentId,
                        Name = c.Name,
                        SeoUrlName = c.SeoUrlName
                    }
                )
                .Concat
                (
                    from c in db.Categories
                    from eh in categoryHierarchy.InnerJoin(ch => ch.ParentCategoryId == c.Id)
                    select new CategoryPathItemCte
                    {
                        CategoryId = c.Id,
                        ParentCategoryId = c.ParentId,
                        Name = c.Name,
                        SeoUrlName = c.SeoUrlName
                    }
                );
        });

        return categoryPathCte;
    }

Answer 1

你可以这样试试吗

FullPath = string.Join(" -> ", GetPathQuery(db, c.Id).Select(pi => pi.Name).ToList());

更友好的查询方法

GetPathQuery(db, c.Id).Select(pi => pi.Name)
   .Aggregate(string.Empty, (results, nextString) 
               => string.Format("{0} -> {1}", results, nextString));