How To Count Associated Entities using Where In Entity Framework

问题

I have this:

        var queryResult = (from post in posts
                    select new
                               {
                                   post,
                                   post.Author,
                                   post.Tags,
                                   post.Categories,
                                   Count = post.Comments.Count()
                               }).ToList();

But I need something like this:

        var queryResult = (from post in posts
                    select new
                               {
                                   post,
                                   post.Author,
                                   post.Tags,
                                   post.Categories,
                                   Count = post.Comments.Where(x=>x.IsPublic).Count()
                               }).ToList();

But post.Comments is a ICollection

回答1:

How about using Enumerable.Cast<T>() like this ?

var queryResult = (from post in posts
                    select new
                               {
                                   post,
                                   post.Author,
                                   post.Tags,
                                   post.Categories,
                                   Count = post.Comments.Cast<Comment>()
                                                        .Where(x=>x.IsPublic).Count()
                               }).ToList();

assuming post.Comments is of type Comment

回答2:

Try:

var queryResult = (from post in context.Posts
                select new
                           {
                               post,
                               post.Author,
                               post.Tags,
                               post.Categories,
                               Count = context.Comments.Where(c => c.Post == post).Where(c => IsPublic == 1).Count()
                           }).ToList();

回答3:

This works:

        var queryResult = (from post in posts
                           join comment in comments.Where(x=> x.IsPublic) on post.Id equals comment.Post.Id into g
                    select new
                               {
                                   post,
                                   post.Author,
                                   post.Tags,
                                   post.Categories,
                                   Count = g.Count()
                               })

But in all solutions we have this problem How to use Include and Anonymous Type in same query in Entity Framework?

回答4:

I'd written it as LukLed did but to make it work I'll use the PK of Post entity:

var queryResult = (from post in context.Posts
                select new
                           {
                               post,
                               post.Author,
                               post.Tags,
                               post.Categories,
                               Count = context.Comments.Where(c => c.Post.Id == post.Id && c.IsPublic == 1).Count()
                           }).ToList();

Or Post.Id could just be written as PostId if forign keys are exposed through the association which I believe it would be more efficient.

来源：https://stackoverflow.com/questions/5024199/how-to-count-associated-entities-using-where-in-entity-framework