无法创建类型的常量值此上下文仅支持基元类型或枚举类型

Question

我在下面的查询中收到此错误

无法创建类型的常量值API.Models.PersonProtocol.在此上下文中仅支持基元类型或枚举类型

ppCombined下面是一个IEnumerable对象PersonProtocolType,由2个PersonProtocol列表的concat构成.

为什么这会失败？我们不能使用LINQ JOIN条款里面SELECT的JOIN？

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

这不起作用,因为它ppCombined是内存中对象的集合,您无法使用内存中的另一组数据将数据库中的一组数据连接起来.你可以尝试,而不是提取经过滤项personProtocol中的ppCombined集合中的内存后,您检索从数据库中其他属性:

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()
        });