c# - 无法创建类型的常量值在此上下文中仅支持原始类型或枚举类型

Question

我收到以下查询的错误

Unable to create a constant value of type API.Models.PersonProtocol. Only primitive types or enumeration types are supported in this context

下面的

ppCombined是一个PersonProtocolType的IEnumerable对象，由2个PersonProtocol列表拼接而成。

为什么会失败？我们不能在 JOIN 的 SELECT 中使用 LINQ JOIN 子句吗？

var persons = db.Favorites
    .Where(x => x.userId == userId)
    .Join(db.Person, x => x.personId, y => y.personId, (x, y) =>
        new PersonDTO
        {
            personId = y.personId,
            addressId = y.addressId,                   
            favoriteId = x.favoriteId,
            personProtocol = (ICollection<PersonProtocol>) ppCombined
                .Where(a => a.personId == x.personId)
                .Select( b => new PersonProtocol()
                 {
                     personProtocolId = b.personProtocolId,
                     activateDt = b.activateDt,
                     personId = b.personId
                 })
        });

Answer 1

这是行不通的，因为 ppCombined 是内存中对象的集合，您不能将数据库中的一组数据与内存中的另一组数据连接起来。在从数据库中检索到其他属性后，您可以尝试在内存中提取ppCombined 集合的过滤项personProtocol::

var persons = db.Favorites
    .Where(f => f.userId == userId)
    .Join(db.Person, f => f.personId, p => p.personId, (f, p) =>
        new // anonymous object
        {
            personId = p.personId,
            addressId = p.addressId,   
            favoriteId = f.favoriteId,
        })
    .AsEnumerable() // database query ends here, the rest is a query in memory
    .Select(x =>
        new PersonDTO
        {
            personId = x.personId,
            addressId = x.addressId,   
            favoriteId = x.favoriteId,
            personProtocol = ppCombined
                .Where(p => p.personId == x.personId)
                .Select(p => new PersonProtocol
                {
                    personProtocolId = p.personProtocolId,
                    activateDt = p.activateDt,
                    personId = p.personId
                })
                .ToList()
        });