指定的 LINQ 表达式包含对与不同上下文关联的查询的引用
先贴代码

var pb = from d in pr.Purchases join b in br.Books

                                           on d.bookID equals b.bookID

                                           where d.pUserID == userID && d.sta != "2"

                                           orderby d.pTime descending

                                           select new PurchaseBook {

                                               pID = d.pID, 

                                               bookID = d.bookID,

                                               bookName = d.bookName,

                                               pUserID = d.pUserID,

                                               pUserName = d.pUserName,

                                               sta = d.sta,

                                               pTime = d.pTime,

                                               ZT = d.ZT,

                                               sUserID = d.sUserID,

                                               sUserName = d.sUserName,

                                               price = d.price,

                                               number = d.number,

                                               author = b.author,

                                               pub=b.pub

                                           };

br.Books和pr.Purchases是同一个数据库中的两张表，分别放到了两个类中，连接数据的时候是分别建立连接