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);
            else
                throw 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 ";
            else
                return "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);
            
        }
    }