Smark.Data 功能强大又灵活的Expression

    Expression对象是Smark.Data的核心对象，虽然Expression是个条件对象，但它所具备的功能是你想象不到的：），以下把Expression的功能列出来。

public RESULT Avg<RESULT, Entity>(string field) where Entity : Smark.Data.Mappings.DataObject;
        public RESULT Avg<RESULT, Entity>(string field, bool DISTINCT) where Entity : Smark.Data.Mappings.DataObject;
        public RESULT Avg<RESULT, Entity>(string field, IConnectinContext cc) where Entity : Smark.Data.Mappings.DataObject;
        public RESULT Avg<RESULT, Entity>(string field, bool DISTINCT, IConnectinContext cc) where Entity : Smark.Data.Mappings.DataObject;
        public int Count<T>() where T : Smark.Data.Mappings.DataObject;
        public int Count<T>(IConnectinContext cc) where T : Smark.Data.Mappings.DataObject;
        public int Delete<T>() where T : Smark.Data.Mappings.DataObject;
        public int Delete<T>(IConnectinContext cc) where T : Smark.Data.Mappings.DataObject;
        public int Edit<T>(Action<T> handler) where T : Smark.Data.Mappings.DataObject, new();
        public int Edit<T>(params Field[] fields) where T : Smark.Data.Mappings.DataObject, new();
        public int Edit<T>(IConnectinContext cc, Action<T> handler) where T : Smark.Data.Mappings.DataObject, new();
        public int Edit<T>(IConnectinContext cc, params Field[] fields) where T : Smark.Data.Mappings.DataObject, new();
        public IList<T> List<T>() where T : Smark.Data.Mappings.DataObject, new();
        public IList<RESULT> List<T, RESULT>()
            where T : Smark.Data.Mappings.DataObject, new()
            where RESULT : new();
        public IList<RESULT> List<T, RESULT>(params string[] orderby)
            where T : Smark.Data.Mappings.DataObject, new()
            where RESULT : new();
        public IList<T> List<T>(params string[] orderby) where T : Smark.Data.Mappings.DataObject, new();
        public IList<T> List<T>(Region region) where T : Smark.Data.Mappings.DataObject, new();
        public IList<RESULT> List<T, RESULT>(Region region)
            where T : Smark.Data.Mappings.DataObject, new()
            where RESULT : new();
        public IList<RESULT> List<T, RESULT>(IConnectinContext cc, params string[] orderby)
            where T : Smark.Data.Mappings.DataObject, new()
            where RESULT : new();
        public IList<T> List<T>(IConnectinContext cc, params string[] orderby) where T : Smark.Data.Mappings.DataObject, new();
        public IList<RESULT> List<T, RESULT>(IConnectinContext cc, Region region)
            where T : Smark.Data.Mappings.DataObject, new()
            where RESULT : new();
        public IList<T> List<T>(IConnectinContext cc, Region region) where T : Smark.Data.Mappings.DataObject, new();
        public IList<T> List<T>(Region region, params string[] orderby) where T : Smark.Data.Mappings.DataObject, new();
        public IList<RESULT> List<T, RESULT>(Region region, params string[] orderby)
            where T : Smark.Data.Mappings.DataObject, new()
            where RESULT : new();
        public IList<T> List<T>(IConnectinContext cc, Region region, params string[] orderby) where T : Smark.Data.Mappings.DataObject, new();
        public IList<RESULT> List<T, RESULT>(IConnectinContext cc, Region region, params string[] orderby)
            where T : Smark.Data.Mappings.DataObject, new()
            where RESULT : new();
        public T ListFirst<T>() where T : Smark.Data.Mappings.DataObject, new();
        public RESULT ListFirst<T, RESULT>()
            where T : Smark.Data.Mappings.DataObject, new()
            where RESULT : new();
        public RESULT ListFirst<T, RESULT>(IConnectinContext cc)
            where T : Smark.Data.Mappings.DataObject, new()
            where RESULT : new();
        public T ListFirst<T>(IConnectinContext cc) where T : Smark.Data.Mappings.DataObject, new();
        public T ListFirst<T>(params string[] orderby) where T : Smark.Data.Mappings.DataObject, new();
        public RESULT ListFirst<T, RESULT>(params string[] orderby)
            where T : Smark.Data.Mappings.DataObject, new()
            where RESULT : new();
        public T ListFirst<T>(IConnectinContext cc, params string[] orderby) where T : Smark.Data.Mappings.DataObject, new();
        public RESULT ListFirst<T, RESULT>(IConnectinContext cc, params string[] orderby)
            where T : Smark.Data.Mappings.DataObject, new()
            where RESULT : new();
        public RESULT Max<RESULT, Entity>(string field) where Entity : Smark.Data.Mappings.DataObject;
        public RESULT Max<RESULT, Entity>(string field, bool DISTINCT) where Entity : Smark.Data.Mappings.DataObject;
        public RESULT Max<RESULT, Entity>(string field, IConnectinContext cc) where Entity : Smark.Data.Mappings.DataObject;
        public RESULT Max<RESULT, Entity>(string field, bool DISTINCT, IConnectinContext cc) where Entity : Smark.Data.Mappings.DataObject;
        public RESULT Min<RESULT, Entity>(string field) where Entity : Smark.Data.Mappings.DataObject;
        public RESULT Min<RESULT, Entity>(string field, bool DISTINCT) where Entity : Smark.Data.Mappings.DataObject;
        public RESULT Min<RESULT, Entity>(string field, IConnectinContext cc) where Entity : Smark.Data.Mappings.DataObject;
        public RESULT Min<RESULT, Entity>(string field, bool DISTINCT, IConnectinContext cc) where Entity : Smark.Data.Mappings.DataObject;
        public RESULT Sum<RESULT, Entity>(string field) where Entity : Smark.Data.Mappings.DataObject;
        public RESULT Sum<RESULT, Entity>(string field, bool DISTINCT) where Entity : Smark.Data.Mappings.DataObject;
        public RESULT Sum<RESULT, Entity>(string field, IConnectinContext cc) where Entity : Smark.Data.Mappings.DataObject;
        public RESULT Sum<RESULT, Entity>(string field, bool DISTINCT, IConnectinContext cc) where Entity : Smark.Data.Mappings.DataObject;

    以上就是Expression所具备的功能，它的工作可以完成数据查询，统计汇总，修改和删除等操作。当你用Smark.Data进行数据访问的时候相信大部分都是在和这个Expression打交道。下面详细地介绍它的每一个功能。

数据查询

当我们new一个Expression出来的时候就可以进行相关操作，只是该操作并不带上条件针对全表操作。

var employees = exp.List<Employees>();

获取所有雇员，不过实现查询就没这么简单，有可能加上条件获取某页数据加上分页等。

Expression exp = new Expression();
                if (ProductName != null)
                {
                    exp &= Modules.Product.productName.Like(ProductName + "%");
                    UrlParams.Add("productname", ProductName);
                }
                if (PriceFrom != null)
                {
                    exp &= Modules.Product.unitPrice >= PriceFrom;
                    UrlParams.Add("pricefrom", PriceFrom.ToString());
                }
                if (PriceTo != null)
                {
                    exp &= Modules.Product.unitPrice <= PriceTo;
                    UrlParams.Add("priceTo", PriceTo.ToString());
                }
                DataPage.PageSize = 10;
                DataPage.RecordCount = exp.Count<Modules.Product>();
                Records = exp.List<Modules.Product>(new Region(DataPage.PageIndex, DataPage.PageSize),OrderField);

以上是一个比较常用的查询，根据情况添加相应的查询条件，统计相关条件的记录数并获取对应页的记录数。

数据删除

删除一般会执行SQL或在组件中删除对象，但在Smark.Data中Expression可以轻松完成相应的工作。

Expression exp = new Expression();
        exp.Delete<Employee>();

 删除Employee所有对象，实际情况更多的是基于条件的删除如下:

(Modules.Employee.city == "gz").Delete<Modules.Employee>();

这样我们就可以把某个城市的employee删除了

数据更新

Expression exp = new Expression();
    exp.Edit<Modules.Employee>(o => { o.City = "bbq"; });
    (Modules.Employee.city == "gz").Edit<Modules.Employee>(o => { o.City = "bbq"; });

基于同样的方式就能进行数据编辑

Expression能处理怎样的条件？

Expression所提供的条件组合非常灵活，重载了|和&使条件编写可以更好地接近实际SQL的方式，对于一此运算的多样性也利用得很好.

qual = (Qualification.sellerID == qinfo.SellerID & Qualification.storeID == qinfo.StoreID).ListFirst<Qualification>();

Expression exp = new Expression();
                if (EmployeeID != null)
                    exp &= Modules.Order.employeeID.At() == EmployeeID;
                if (CustomerID != null)
                    exp &= Modules.Order.customerID.At() == CustomerID;
                if (OrderDateFrom != null)
                    exp &= Modules.Order.orderDate >= OrderDateFrom;
                if (OrderDateTo != null)
                    exp &= Modules.Order.orderDate <= OrderDateTo;
                if (RequiredDateFrom != null)
                    exp &= Modules.Order.requiredDate >= RequiredDateFrom;
                if (RequiredDateTo != null)
                    exp &= Modules.Order.requiredDate <= RequiredDateTo;

(Modules.Employee.employeeID == new int[] {2,4,5 }).Delete<Modules.Employee>();

(Modules.Employee.employeeID != new int[] {2,4,5 }).Delete<Modules.Employee>();

Modules.Order.orderDate["1997-1-1", "1997-2-1"].List<Modules.Order>();

(Modules.Order.employeeID== Modules.Employee.employeeID[Modules.Employee.city==new []{"gz","sz"}]).List<Modules.Order>();

Expression对象是非常灵活，但它的作用建是立在实体描述<T>上，Smark.Data的实体描述也非常灵活除了描述表外，还能描述关联查询，汇总统计结合Expression对象可以满足大部分实际应用的需要。在下一章节会讲解实体对象的描述。

Smark.Data是基于 Apache License 2.0 (Apache)协议的开源组件，详情可以到http://www.ikende.com/了解