我有两个表（实体框架）。我想基于一个平均值来计算平均值，但是只计算一次记录。怎么做？

Question

我有一张桌子，上面放着作者，一张桌子里放着书。它们连接在书的authorID中。我想根据类型来计算作者的平均年龄。看起来像这样：

• 书籍：ID，书名，体裁，作者ID
• 作者：作者ID，姓名，年龄

我想获得按类型分组的平均年龄。但是数据库中有同一作者的书籍。我只想在此查询中计算一次。我有这个，但是如果作者是同一个人的话，这会再次计数一次：

var query = from books in this.bookRepository.ReadAll()
            join authors in this.authorRepository.ReadAll() on books.writerId equals authors.authorId
            select new
                   {
                       books.booktype,
                       authors.age,
                       authors.authorId,
                   };

 var result = from g in query
              group g by g.booktype into groupedTypes
              select new AverageOfWritersAgeInGenreModel
                     {
                         Genre = groupedTypes.Key,
                         Age = groupedTypes.Average(x => x.age).Value,
                     };

Answer 1

您快要在那里了，您只需要从选择的值中获​​取Distinct值：

            select new
                   {
                       books.booktype,
                       authors.age,
                       authors.authorId,
                   };

我已经将代码重写为LINQ扩展：

            var authors = new List<Author>(); // for simplicity
            var books = new List<Book>(); // for simplicity

            var result = authors
                .Join(books, a => a.AuthorId, b => b.AuthorId,
                    (author, book) => new {author.AuthorId, author.Age, book.Genre})
                .Distinct() // Ged rid of duplicated authors for the same book genre
                .GroupBy(r => r.Genre)
                .Select(g => new {Genre = g.Key, Age = g.Average(x => x.Age)});