我在带有子查询的大查询中使用了Linq2db。在它里面的一个地方,我想使用字符串。连接():
...
FullPath = string.Join(" -> ", GetPathQuery(db, c.Id).Select(pi => pi.Name))
...
但我收到了一个例外:
LinqException: 'Join(" -> ", value(RI.达尔。Categories.AdminCategoryPreviewDAL)。GetPathQuery(value(RI.达尔。Categories.AdminCategoryPreviewDAL+<>c__DisplayClass4_0).db, c.Id).选择(pi => pi.名称))' 无法转换为 SQL。
我使用Postgre SQL,它具有非常适合我的concat_ws
功能。所以我尝试使用它:
[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(" -> ")
...
但我失败了,有同样的例外。
GetPathQuery 的完整源代码:
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;
}
你可以这样尝试吗,
FullPath = string.Join(" -> ", GetPathQuery(db, c.Id).Select(pi => pi.Name).ToList());
更方便查询的方法
GetPathQuery(db, c.Id).Select(pi => pi.Name)
.Aggregate(string.Empty, (results, nextString)
=> string.Format("{0} -> {1}", results, nextString));