这个左外连接查询的正确 Linq 表达式是什么？

Question

我无法将这个他的 sql 查询转换为一个 linq 表达式，该表达式将结果作为类的 IEnumerable 返回。这是查询：Select * from Posts left outer join Ownergroups on Posts.PostId=Ownergroups.PostIDWhere Ownergroups.OwnerName = 'Group A' AND PostType = 'news'这是唯一不抛出错误的表达式，但它也只返回一个结果。        NewsViewModel vm = new NewsViewModel();       vm.NewsItems =  (from an in db.Posts.Where(g => g.PostType == "News")       from og in an.OwnerGroups.Where(g => g.OwnerName == "Group A")        select an).Distinct().OrderByDescending(bb         =>bb.PostDate).ToList();如果我尝试投影到一个新的选择，我会收到一个错误。当我尝试按 PostId 分组时，我得到了正确的结果，但无法将结果附加到我的 ViewModel；我收到一条错误消息“无法将类型 system.collections.generic 列表转换为 systems.collections.IEnumerable”我真的很感激建议。根据要求添加类：     public class Post{    public int PostId { get; set; }    public string PostType { get; set; }    [Display(Name = "Top Title")]    [MaxLength(300)]    public string Headline1 { get; set; }    [Display(Name = "Subtitle")]    [MaxLength(300)]    public string Headline2 { get; set; }    public string Headline3 { get; set; }    [Display(Name = "By Organization or Person")]    [MaxLength(250)]    public string Byline { get; set; }    [Display(Name = "Text For Your Post")]    [MaxLength(4999)]    [AllowHtml]    public string PostText1 { get; set; }    [Display(Name = "Additional Text")]    [MaxLength(4999)]    [AllowHtml]    public string PostText2 { get; set; }    public string AuthorGroup { get; set; }    [Display(Name = "Link to Video (URL)")]    [MaxLength(249)]    public string AVurl { get; set; }    [DisplayFormat(ApplyFormatInEditMode = true, DataFormatString = "{0:d}")]    public DateTime PostDate { get; set; }    [Display(Name = "Date To Archive")]    [DisplayFormat(ApplyFormatInEditMode = true, DataFormatString = "{0:d}")]    public DateTime? StopDate { get; set; }    [Display(Name = "Posted By")]    public string PostedBy { get; set; }    [Display(Name = "Last Edited")]    public DateTime LastEditDate { get; set; }    [Display(Name = "Last Edited By")]

Answer 1

以下将在所有者所在的位置进行左连接，OwnerName = 'Group A'并在PostType = 'news'可能的情况下进行连接PostId = PostId

void Main()

{

    var posts = 

        new List<Post>() 

        {

            new Post {PostId = 1, PostType = "news"},

            new Post {PostId = 2, PostType = "old"},

            new Post {PostId = 3, PostType = "news"},

        };

    var owners = 

        new List<OwnerGroup>()

        {

            new OwnerGroup {GroupId = 1, PostId = 1, OwnerName = "Group A" },

            new OwnerGroup {GroupId = 2, PostId = 1, OwnerName = "Group A" },

            new OwnerGroup {GroupId = 3, PostId = 2, OwnerName = "Group A" },

        };

    var leftJoinResult = posts

        .GroupJoin(

            owners.Where(o => o.OwnerName.Equals("Group A")), 

            r => r.PostId, rp => rp.PostId, 

            (l1, l2) => new { gjl1 = l1, gjl2 = l2 })

        .SelectMany(x => x.gjl2.DefaultIfEmpty(), (x, gjl2) => new { x.gjl1, gjl2 })

        .Where(x => x.gjl1.PostType.Equals("news") )

        // OPTIONAL: Add this line return the Post matches, not both the Post and the possible left joined OwnerGroup

        .Select(x => x.gjl1) 

        // OPTIONAL: Add this line to only get the distinct Post matches

        .GroupBy(p => p.PostId).Select(grp => grp.First());

}

public class Post

{

    public int PostId { get; set; }

    public string PostType { get; set; }

}

public class OwnerGroup

{

    public int GroupId { get;set; }

    public int PostId { get; set; }

    public String OwnerName { get; set; }

}