如何使用lambdas和表达式树在Linq中进行连接？

Question

我正在尝试使用lambda表达式在Linq中进行连接...并遇到一些问题.

我有两个实体,评论和评论源.CommentSources与评论相关联.我有以下代码,它确实有效:

01 IQueryable<Data.Comment> query = ctx.DataContext.Comments;
02
03
04 if (criteria.IsDeleted == DeletedFilter.Deleted)
05    query = query.Where(row => row.DeletedBy != Guid.Empty);
06 else if (criteria.IsDeleted == DeletedFilter.NotDeleted)
07    query = query.Where(row => row.DeletedBy == Guid.Empty);
08
09 var data = query.Select(row => CommentInfo.FetchCommentInfo(row));

我需要在这个领域的评论上加入CommentSources,如果可能的话,我想使用类似的东西:

01 query = query.Join(join code goes here)

如何在表达式树中使用lambdas来完成此操作？

还有一件事......如何在Join语句中添加Where？

而不是问另一个问题......我如何在Join上做Where子句？例如,我在CommentSource上有一个名为SourceId的字段,我想过滤它.

Answer 1

你需要指定五件事(至少):

• "外部"序列(注释)(这是隐含的第一个参数)
• "内部"序列(CommentSource)
• 如何从CommentSource获取密钥
• 如何从评论到密钥
• 你想要的结果是CommentSource/Comment对

例如:

query = query.Join(ctx.DataContext.CommentSource,
                   comment => comment.CommentSourceId,
                   commentSource => commentSource.Id,
                   (comment, commentSource) 
                      => new { Comment=comment, CommentSource=commentSource });

Answer 2

这是我的最终代码:

            var query = ctx.DataContext.Comments.Join(ctx.DataContext.CommentSources,
                  c => c.CommentId, 
                  s => s.CommentId,
                  (c, s) => new {Comment = c, CommentSource = s});

            if (criteria.SourceId != null && criteria.SourceId != Guid.Empty)
                query = query.Where(row => row.CommentSource.SourceId == criteria.SourceId);

            if (criteria.IsDeleted == DeletedFilter.Deleted)
                query = query.Where(row => row.Comment.DeletedBy != Guid.Empty);
            else if (criteria.IsDeleted == DeletedFilter.NotDeleted)
                query = query.Where(row => row.Comment.DeletedBy == Guid.Empty);

            var data = query.Select(row => CommentInfo.FetchCommentInfo(row.Comment));