c# - 我如何 linq 对象分组

Question

如何对这些对象进行分组？我可以通过 LINQ 获取并绑定它。但这没有用，因为我需要反向分组。像那样 ：

主组 > 组列表 > 子组列表

也许我可以通过 LINQ 查询首先获取值来做到这一点。我不知道。

我是 LINQ 的初学者。我没有太多信息。预先感谢您的帮助。我像这样获取并绑定 mongodb 数据：

var subGroupCollection = Database.GetCollection<SubGroup>(typeof(SubGroup).Name);
var groupCollection = Database.GetCollection<Group>(typeof(Group).Name);
var mainGroupCollection = Database.GetCollection<MainGroup>(typeof(MainGroup).Name);

var query = from sg in subGroupCollection.AsQueryable()
            join mg in mainGroupCollection on sg.MainGroupId equals mg.Id into mainGroups
            join z in groupCollection on sg.GroupId equals z.Id into groups
            select new SoccerOddType
            {
                Id = sg.Id,
                IsActive = sg.IsActive,
                GroupId = sg.GroupId,
                Name = sg.Name,
                LastUpdateDate = sg.LastUpdateDate,
                CreatedDate = sg.CreatedDate,
                Order = sg.Order,
                Discount = sg.Discount,
                DiscountType = sg.DiscountType,
                MainGroupId = sg.MainGroupId,
                MainGroup = mainGroups.First(),
                Group = groups.First()
            };

从 ：

public class MainGroup
{
    public string Id { get; set; }
    public string Name { get; set; }
    public bool IsActive { get; set; }
}

public class Group
{
    public string Id { get; set; }
    public string MainGroupId { get; set; }
    [BsonIgnore] public Group MainGroup { get; set; }
    public string Name { get; set; }
    public bool IsActive { get; set; }
}

public class SubGroup
{
    public string Id { get; set; }
    public string Name { get; set; }
    public string MainGroupId { get; set; }
    public string GroupId { get; set; }
    [BsonIgnore] public Group MainGroup { get; set; }
    [BsonIgnore] public Group Group { get; set; }
    public bool IsActive { get; set; }
    public decimal Discount { get; set; }
    public EnmDiscountType DiscountType { get; set; }
}

到 ：

public class MainGroupViewModel
{
    public string Id { get; set; }
    public string Name { get; set; }
    public bool IsActive { get; set; }
    public List<GroupViewModel> Groups { get; set; }
}

public class GroupViewModel
{
    public string Id { get; set; }
    public string MainGroupId { get; set; }
    public string Name { get; set; }
    public bool IsActive { get; set; }
    public List<SubGroupViewModel> SubGroups { get; set; }
}

public class SubGroupViewModel
{
    public string Id { get; set; }
    public string Name { get; set; }
    public string MainGroupId { get; set; }
    public string GroupId { get; set; }
    public bool IsActive { get; set; }
    public decimal Discount { get; set; }
    public EnmDiscountType DiscountType { get; set; }
}

标签： c#mongodblinqmongodb-querygrouping

Answer 1

如果您有一个一对多的关系，使用外键，并且您想要项目及其子项目，例如学校及其学生、客户及其订单、主组及其子组，那么您可以使用简单的 Select得到你的结果，或使用GroupJoin

选择

var result = mainGroupCollection.Select(mainGroup => new MainGroupViewModel
{
    Id = mainGroup.Id,
    Name = mainGroup.Name,
    ...

    // Keep only the Groups of this MainGroup, using foreign key
    Groups = groupCollection
        .Where(group => group.MainGroupId == mainGroup.Id)
        .Select(group => new GroupViewModel
        {
            Id = group.Id,
            Name = group.Name,
            ...

            // Keep only the subGroups of this Group, using foreign key
            SubGroups = subGroupCollection
                .Where(subGroup => subGroup.GroupId == group.Id)
                .Select(subGroup => new SubGroupViewModel
                {
                    Id = group.Id,
                    Name = group.Name,
                    ...
                })
                .ToList(),
        })
        .ToList(),
});

虽然这种方法有效，但效率不高，因为对于 mainGroupCollection 中的每个元素，它必须枚举整个 GroupCollection，而对于每个元素，它必须枚举整个 SubGroupCollection。

如果您从数据库中查询，则取决于您使用的 DBMS，这不是一个大问题。但是，我会去 GroupJoin

团体加入

Enumerable.GroupJoin效率更高（或 IQueryable 等效项）。Enumerable 版本使用 Dictionary 来查看它是否已经找到具有此 Id 的项目，因此它不需要对每个集合进行多次枚举。

// GroupJoin the mainGroupCollection with the groupCollection:
var result = mainGroupCollection.GroupJoin(groupCollection,
    mainGroup = mainGroup.Id,         // from every mainGroup take the primary key
    group => group.MainGroupId,       // from every group take the foreign key

    // ResultSelector: for every mainGroup and its groups make one MainGroupViewModel
    (mainGroup, groupsOfThisMainGroup) => new MainGroupViewModel
    {
        Id = mainGroup.Id,
        Name = mainGroup.Name,
        ...

        // for the Groups: GroupJoin the groups of this main group with the subGroups
        Groups = groupsOfThisMainGroup.GroupJoin(subGroupCollection,
            groupOfThisMainGroup => groupOfThisMainGroup.Id,
            subGroup => subGroup.GroupId,

            // result selector
            (group, subGroupsOfThisGroup) => new GroupViewModel
            {
                Id = group.Id,
                Name = group.Name,

               SubGroups = subGroupsOfThisGroup.Select(subGroup => new SubGroupViewModel
               {
                    Id = subGroup.Id,
                    Name = subGroup.Name,
                    ...
               })
               .ToList(),
        })
        .ToList(),
    });