如何从LINQ中包含的实体中进行选择，并获得类似于SQL Join的平面列表？

Question

I have two classes:

public class Topic
{
    public Topic()
    {
        this.SubTopics = new HashSet<SubTopic>();
    }
    public int TopicId { get; set; }
    public string Name { get; set; }
    public virtual ICollection<SubTopic> SubTopics { get; set; }
}

public class SubTopic
    public int SubTopicId { get; set; }
    public int Number { get; set; }
    public int TopicId { get; set; }
    public string Name { get; set; }
    public virtual Topic Topic { get; set; }
}

What I would like to do is to get a Data Transfer Object output from LINQ that will show me. I do want to see the TopicId repeated if there is more than one SubTopic inside that topic:

TopicId  Name     SubTopicId  Name
1        Topic1   1           SubTopic1
1        Topic1   2           SubTopic2
1        Topic1   3           SubTopic3
2        Topic2   4           SubTopic4

I tried to code a Linq statement like this:

            var r = context.Topics
                .Select ( s => new {
                        id = s.TopicId,
                        name = s.Name,
                        sid = s.SubTopics.Select( st => st.SubTopicId),
                        sidname = s.SubTopics.Select ( st => st.Name)
                    }).
            ToList();

But this does not really work as it returns sid and sidname as lists.

How will it be possible for me to get a flat output showing what I need?

Answer 1

You need SelectMany to expand a nested collection, along these lines

var r = context.Topics.SelectMany(t => t.SubTopics
 .Select(st => new 
    {
        TopicID = t.TopicId, 
        TopicName = t.Name, 
        SubTopicID = st.SubTopicId,
        SubTopicName = st.Name
    }));

Answer 2

try this :

var r = context.Topics
            .Select ( s => new {
                    id = s.TopicId,
                    name = s.Name,
                    sid = s.SubTopics.Where(st=>st.TopicId==s.TopicId).Select( st => st.SubTopicId ),
                    sidname = s.SubTopics..Where(st=>st.TopicId==s.TopicId).Select ( st => st.Name)
                }).
        ToList();

Hope it will help

Answer 3

@Sweko provided an answer that satisfies the exact output that you requested. However, this can be even simpler if you just return the subtopic intact. It may run a bit quicker as well, since you don't need to create a new object for each element in the result.

Lastly, it looks like you wanted your result set ordered. For completeness, I've added those clauses as well.

var r = context.Topics
           .SelectMany( topic => topic.SubTopics )
              .OrderBy(sub => sub.TopicId)
              .ThenBy(sub => sub.SubTopicId);