注意 linq to entity 连表查询

1.连表的时候 获取两个表中的字段，在其中一个表对应的实体类中添加另外一个表中需要的字段即可。类与数据库中的字段并不一定一一对应。

2. var a=... select new   类名 {}  ，两个表在同一个edmx文件中，按照一二的话，返回该类，或者返回 var b= ....select  {} ,进行迭代输出。但b是什么类型呢？IEnmurable类型的泛型集合?

3. 如果联合查询的两个表不在同一个上下文呢，即两个表在不同的edmx文件中，怎么关联呢？

  一种做法：在业务逻辑层对根据条件返回的泛型集合或指定类型对象进行筛选。第二种:是否可以新建一个实体，对应两个表，用codefirst中Fluent API,或两个表建立视图？然后。。。

4.下面是linq to sql 的第一种做法

  public static IQueryable<Project> GetAll()
        {
            var result =from a in myDcFmDataContext.FmBankDetail
                         join b in myDcFmDataContext.FmBusiness
                         on a.BankDetailID equals b.BankDetailID
                         join c in myDcFmDataContext.FmBusinessDetail
                         on b.BusinessID equals c.BusinessID
                         where c.FromTable.Trim() =="FLOW_EDM_OPR"
                         select new Project
                         {
                             BankDetailChequeNo = a.BankDetailChequeNo,
                             BankDetailDate = a.BankDetailDate,
                             BankDetailVoucher = a.BankDetailVoucher,
                             BankDetailRemark = a.BankDetailRemark,
                             BusinessDetailSum = c.BusinessDetailSum,
                             FromID = c.FromID
                         };
            foreach (var r in result)
            {
                var p = (from a in myDcProjectDataContext.FLOW_EDM_OPR
                         where r.FromID == a.ID
                         select a).FirstOrDefault();
                r.QTY = p.QTY.Value;
                r.PRICE = p.PRICE.Value;

            }

  public class Project
    {
        public Project()
        {
            //
            //TODO: 在此处添加构造函数逻辑
            //
        }
        public string BankDetailChequeNo { get; set; }
        public DateTime BankDetailDate { get; set; }
        public string BankDetailVoucher { get; set; }
        public string BankDetailRemark { get; set; }
        public decimal BusinessDetailSum { get; set; }
        public int FromID { get; set; }
        public int QTY { get; set; }
        public double PRICE { get; set; }
    }
            return result;