Thinking In Design Pattern——Query Object模式

什么是Query Object模式

Query Object的架构设计

Query Object在服务层的应用

测试

Query Object模式

Query Object：可以在领域服务层构造查询然后传给资源库使用，并使用某种查询翻译器将对象查询（Query）翻译成底层数据库持久化框架可以理解的查询（即翻译成一条Sql 语句）。而Query Object即可以理解为表示数据库查询的对象。且可以构造任意查询，然后传给Repository。Query Object模式的主要好处是它完全将底层的数据库查询语言抽象出来。

如果没有某种查询机制，我们的持久化层可能会这样定义方法：

    public interface IOrderRepository{IEnumerable<Order> FindAll(Query query);IEnumerable<Order> FindAllVipCustomer();IEnumerable<Order> FindOrderBy(Guid customerId);IEnumerable<Order> FindAllCustomersWithOutOrderId();}

很明显，可以看出持久化层很不简洁，Repository将充满大量检索方法，而我们希望我们的持久化层尽量简洁些，根据传入参数能够动态的翻译成数据库查询语言，就像下面写的这样：

public interface IOrderRepository{       IEnumerable<Order> FindBy(Query query);IEnumerable<Order> FindBy(Query query, int index, int count);         }

这个Query就是核心——一个表示数据库查询的对象，好处是显而易见的：完全将底层的数据库查询语言抽象出来，因此将数据持久化和检索的基础设施关注点从业务层中分离出来。

Query Object模式的架构

添加一个枚举，CriteriaOperator：

public enum CriteriaOperator{Equal,//=LessThanOrEqual,// <=NotApplicable//≠// TODO: 省略了其他的操作符，可继续添加}

接着添加Criterion类，表示构成查询的过滤器部分：指定一个实体属性（OR Mapping）、要比较的值以及比较方式：

 public class Criterion{private string _propertyName;//实体属性private object _value;//进行比较的值private CriteriaOperator _criteriaOperator;//何种比较方式public Criterion(string propertyName, object value, CriteriaOperator criteriaOperator){_propertyName = propertyName;_value = value;_criteriaOperator = criteriaOperator;}public string PropertyName {get { return _propertyName; }}public object Value{get { return _value; }}public CriteriaOperator criteriaOperator{get { return _criteriaOperator; }}/// <summary>/// Lambda表达式树：创建一个过滤器/// </summary>/// <typeparam name="T"></typeparam>/// <param name="expression"></param>/// <param name="value"></param>/// <param name="criteriaOperator"></param>/// <returns></returns>public static Criterion Create<T>(Expression<Func<T, object>> expression, Object value, CriteriaOperator criteriaOperator){string propertyName = PropertyNameHelper.ResolvePropertyName<T>(expression);Criterion myCriterion = new Criterion(propertyName, value, criteriaOperator);return myCriterion;}}

为了避免在构建查询时出现令人畏惧的魔幻字符串，我们创建一个辅助方法，使用表达式参数。

public static class PropertyNameHelper{public static string ResolvePropertyName<T>(Expression<Func<T, object>> expression){var expr = expression.Body as MemberExpression;if (expr==null){var u = expression.Body as UnaryExpression;expr = u.Operand as MemberExpression;}return expr.ToString().Substring(expr.ToString().IndexOf(".")+1);}}

这样就可以像查询中添加一个新的查询条件：

query.Add(Criterion.Create<Order>(c=>c.CustomerId,customerId,CriteriaOperator.Equal));

而不是使用魔幻字符串：

  query.Add(new Criterion("CustomerId", customerId, CriteriaOperator.Equal));

下面要创建表示查询的排序属性：

 public class OrderByClause{public string PropertyName { get; set; }public bool Desc { get; set; }}

接着，创建另一个枚举，确定如何各个Criterion进行评估：

public enum QueryOperator{And,Or            }

有时候的复杂非常难以创建，在这些情况下，可以使用指向数据库视图或存储过程的命名查询，添加一个QueryName来存放查询列表：

 public enum QueryName{       Dynamic = 0,//动态创建RetrieveOrdersUsingAComplexQuery = 1//使用已经创建好了的存储过程、视图、特别是查询比较复杂时使用存储过程}

最后，添加Query类，将Query Object模式组合在一起：

    public class Query{private QueryName _name;private IList<Criterion> _criteria;public Query(): this(QueryName.Dynamic, new List<Criterion>()){ }public Query(QueryName name, IList<Criterion> criteria){ _name = name;_criteria = criteria;}public QueryName Name{get { return _name; }}/// <summary>/// 判断该查询是否已经动态生成或与Repository中某个预先建立的查询相关/// </summary>/// <returns></returns>public bool IsNamedQuery(){return Name != QueryName.Dynamic;}public IEnumerable<Criterion> Criteria{get {return _criteria ;}}          public void Add(Criterion criterion){if (!IsNamedQuery())//　动态查询
                _criteria.Add(criterion);elsethrow new ApplicationException("You cannot add additional criteria to named queries");}public QueryOperator QueryOperator { get; set; }public OrderByClause OrderByProperty { get; set; }}

最后创建一个工厂类，提供已存在的查询：

 public static class NamedQueryFactory{public static Query CreateRetrieveOrdersUsingAComplexQuery(Guid CustomerId){IList<Criterion> criteria = new List<Criterion>();Query query = new Query(QueryName.RetrieveOrdersUsingAComplexQuery, criteria);criteria.Add(new Criterion ("CustomerId", CustomerId, CriteriaOperator.NotApplicable));return query;}}

Query Object在服务层的运用

建立领域模型和领域服务类：

 public class Order{public Guid Id { get; set; }public bool HasShipped { get; set; }public DateTime OrderDate { get; set; }public Guid CustomerId { get; set; }}

添加Repository接口：

  public interface IOrderRepository{       IEnumerable<Order> FindBy(Query query);IEnumerable<Order> FindBy(Query query, int index, int count);         }

建立领域服务层：

    public class OrderService{private IOrderRepository _orderRepository;public OrderService(IOrderRepository orderRepository){_orderRepository = orderRepository;}public IEnumerable<Order> FindAllCustomersOrdersBy(Guid customerId){IEnumerable<Order> customerOrders = new List<Order>();Query query = new Query();//推介使用这种query.Add(Criterion.Create<Order>(c=>c.CustomerId,customerId,CriteriaOperator.Equal));//输入魔幻字符串，容易出错query.Add(new Criterion("CustomerId", customerId, CriteriaOperator.Equal));query.OrderByProperty = new OrderByClause { PropertyName = "CustomerId", Desc = true };customerOrders = _orderRepository.FindBy(query); return customerOrders;}public IEnumerable<Order> FindAllCustomersOrdersWithInOrderDateBy(Guid customerId, DateTime orderDate){IEnumerable<Order> customerOrders = new List<Order>();Query query = new Query();query.Add(new Criterion("CustomerId", customerId, CriteriaOperator.Equal));query.QueryOperator = QueryOperator.And; query.Add(new Criterion("OrderDate", orderDate, CriteriaOperator.LessThanOrEqual));query.OrderByProperty = new OrderByClause { PropertyName = "OrderDate", Desc = true };customerOrders = _orderRepository.FindBy(query);return customerOrders;}public IEnumerable<Order> FindAllCustomersOrdersUsingAComplexQueryWith(Guid customerId){IEnumerable<Order> customerOrders = new List<Order>();Query query = NamedQueryFactory.CreateRetrieveOrdersUsingAComplexQuery(customerId);customerOrders = _orderRepository.FindBy(query);return customerOrders;}}

OrderService类包含3个方法，他们将创建的查询传递给Repository。FindAllCustomersOrdersBy和FindAllCustomersOrdersWithInOrderDateBy方法通过Criterion和OrderByClaus添加来创建动态查询。FindAllCustomersOrdersUsingAComplexQueryWith是命名查询，使用NamedQueryFactory来创建要传给Repository的Query Object。

最后创建一个翻译器：QueryTranslator，将查询对象翻译成一条可在数据库上运行的Sql命令：

public static class OrderQueryTranslator{private static string baseSelectQuery = "SELECT * FROM Orders ";public static void TranslateInto(this Query query, SqlCommand command){if (query.IsNamedQuery()){command.CommandType = CommandType.StoredProcedure;command.CommandText = query.Name.ToString();foreach (Criterion criterion in query.Criteria){command.Parameters.Add(new SqlParameter("@" + criterion.PropertyName, criterion.Value));}}else{StringBuilder sqlQuery = new StringBuilder();sqlQuery.Append(baseSelectQuery);bool _isNotfirstFilterClause = false;if (query.Criteria.Count() > 0)sqlQuery.Append("WHERE ");   foreach (Criterion criterion in query.Criteria){if (_isNotfirstFilterClause)sqlQuery.Append(GetQueryOperator(query));                                            sqlQuery.Append(AddFilterClauseFrom(criterion));command.Parameters.Add(new SqlParameter("@" + criterion.PropertyName, criterion.Value));_isNotfirstFilterClause = true;}sqlQuery.Append(GenerateOrderByClauseFrom(query.OrderByProperty));command.CommandType = CommandType.Text; command.CommandText = sqlQuery.ToString();}}private static string GenerateOrderByClauseFrom(OrderByClause orderByClause){return String.Format("ORDER BY {0} {1}",FindTableColumnFor(orderByClause.PropertyName), orderByClause.Desc ? "DESC" : "ASC");          }private static string GetQueryOperator(Query query){if (query.QueryOperator == QueryOperator.And)return "AND ";elsereturn "OR ";}private static string AddFilterClauseFrom(Criterion criterion){return string.Format("{0} {1} @{2} ", FindTableColumnFor(criterion.PropertyName), FindSQLOperatorFor(criterion.criteriaOperator), criterion.PropertyName);}private static string FindSQLOperatorFor(CriteriaOperator criteriaOperator){switch (criteriaOperator){ case CriteriaOperator.Equal:return "=";case CriteriaOperator.LessThanOrEqual:return "<=";default:throw new ApplicationException("No operator defined.");}}private static string FindTableColumnFor(string propertyName){switch (propertyName){case "CustomerId":return "CustomerId";case "OrderDate":return "OrderDate";default:throw new ApplicationException("No column defined for this property.");}}}

建立简单仓储对象：

 public class OrderRepository : IOrderRepository {        private string _connectionString;public OrderRepository(string connectionString){_connectionString = connectionString;}public IEnumerable<Order> FindBy(Query query){// Move to method below with Index and count
IList<Order> orders = new List<Order>();using (SqlConnection connection =new SqlConnection(_connectionString)){SqlCommand command = connection.CreateCommand();query.TranslateInto(command);              connection.Open();using (SqlDataReader reader = command.ExecuteReader()){while (reader.Read()){orders.Add(new Order{CustomerId = new Guid(reader["CustomerId"].ToString()),OrderDate = DateTime.Parse(reader["OrderDate"].ToString()),Id = new Guid(reader["Id"].ToString())                            });}}                }return orders;}public IEnumerable<Order> FindBy(Query query, int index, int count){throw new NotImplementedException();            }       }

测试

 [TestFixture]public class SQLQueryTranslatorTests{[Test]public void The_Translator_Should_Produce_Valid_SQL_From_A_Query_Object(){int customerId = 9;string expectedSQL = "SELECT * FROM Orders WHERE CustomerId = @CustomerId ORDER BY CustomerId DESC";Query query = new Query();query.Add(new Criterion("CustomerId", customerId, CriteriaOperator.Equal));//query.Add(Criterion.Create<Order>(c => c.CustomerId, customerId, CriteriaOperator.Equal));query.OrderByProperty = new OrderByClause { PropertyName = "CustomerId", Desc = true };SqlCommand command = new SqlCommand();query.TranslateInto(command);Assert.AreEqual(expectedSQL, command.CommandText);}}

转载于:https://www.cnblogs.com/OceanEyes/archive/2012/11/14/think-in-design-pattern-query-object.html

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Thinking In Design Pattern——Query Object模式

Query Object模式

Query Object模式的架构

Query Object在服务层的运用

测试

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