使用 LINQ 连接多个表以返回列表答案

【问题标题】：Joining multiple tables to return a list using LINQ使用 LINQ 连接多个表以返回列表
【发布时间】：2019-05-04 22:40:13
【问题描述】：

我想弄清楚一个 LINQ 语句 - 我对此比较陌生，所以请原谅我的无知。我想返回一个人列表，每个人都有一个兴趣列表。

person(p) 表通过 p.id = pi.personid 连接到 personinterest(pi) 表

personinterest 表连接到 interest(i) 表，pi.interestid 到 i.id。

    public class Persons
    {
        public int Id { get; set; }
        public string LastName { get; set; }
        public string FirstName { get; set; }
...
        public IList<Interest> PersonInterests { get; set; }
    }

    public class Interest
    {
        public string InterestName { get; set; }
    }

我要返回的类是一个人，每个人的 PersonInterests 列表中填充了 0 到多个兴趣。我在下面的 Linq 语句正在返回数据，但是每个人的记录只获得一种兴趣，并且具有多个兴趣的人的人员数据重复，如下所示 linq 语句

            var interests = _db.Interests;

            return (from p in _db.People
                    join i in _db.PersonInterests on p.Id equals i.PersonId
                    join s in _db.Interests on i.InterestId equals s.Id
                    select new Persons{
                        Id = p.Id,
                        FirstName = p.FirstName,
                        LastName = p.LastName,
                        Age = p.Age,
                        Address = p.Address,
                        City = p.City,
                        StateAbbrev = p.StateAbbrev,
                        ZipCode = p.ZipCode,
                        PersonInterests = (from r in interests where r.Id == i.InterestId select r).ToList()
                    }).ToList();

结果：

{"id":1,"lastName":"Alexander","firstName":"Carson","age":23,"address":"123 4th Street","city":"Jamestown","stateAbbrev":"NV","zipCode":"65465","personInterests":[{"id":1,"interestName":"Basketball"}],"photo":null}
{"id":1,"lastName":"Alexander","firstName":"Carson","age":23,"address":"123 4th Street","city":"Jamestown","stateAbbrev":"NV","zipCode":"65465","personInterests":[{"id":2,"interestName":"Camping"}],"photo":null},

我希望数据看起来像这样：

{"id":1,"lastName":"Alexander","firstName":"Carson","age":23,"address":"123 4th Street","city":"Jamestown","stateAbbrev":"NV","zipCode":"65465","personInterests":[{"id":1,"interestName":"Basketball"}, {"id":2,"interestName":"Camping"}],"photo":null}

我已经为此苦苦挣扎了一段时间，非常感谢任何帮助。

【问题讨论】：

您使用的是 LINQ to SQL 还是 EF 或 EF Core？

标签： linq polymorphic-associations

【解决方案1】：

这样做

(from p in _db.People
    select new {
    Id = p.Id,
    FirstName = p.FirstName,
    LastName = p.LastName,
    Age = p.Age,
    Address = p.Address,
    City = p.City,
    StateAbbrev = p.StateAbbrev,
    ZipCode = p.ZipCode,
    Photo = p.Photo,
    Interests = (from i in _db.Interests
                    where i.PersonId == p.Id
                    select i.InterestName).ToList()
}).ToList();

【讨论】：

【解决方案2】：

所以，盯着这个看几个小时后，我意识到尝试在一个查询中执行此操作是愚蠢的。我把它分开并让它工作。首先检索人员数据，然后为每个人建立他们感兴趣的列表。

    public async Task<IEnumerable<PersonResource>> GetPeople()
    {
        IEnumerable<PersonResource> people = await (from p in _db.People
              select new PersonResource
              {
                Id = p.Id,
                FirstName = p.FirstName,
                LastName = p.LastName,
                Age = p.Age,
                Address = p.Address,
                City = p.City,
                StateAbbrev = p.StateAbbrev,
                ZipCode = p.ZipCode,
                Photo = p.Photo,
                Interests = new List<string>()
              }).ToListAsync();

        foreach (PersonResource person in people)
        {
            person.Interests = (from iint in _db.Interests
                          join n in _db.PersonInterests on iint.Id equals n.InterestId
                          where n.PersonId == person.Id
                          select iint.InterestName).ToList();
        }

        return people;



        // return Mapper.Map<List<Person>, List<PersonResource>>(people);


    }

【讨论】：