如何在需要计数和项目时避免编写重复查询

Question

我构建了一个连接大约六个表的 LINQ 查询。问题是，出于分页目的，我想首先计算将返回多少项目。因此，我运行遇到的问题是必须编写两次完全相同的查询：一次获取项目计数，另一次构建我的项目集合。

示例：

using (var context = new DbContext())
{
    var items = from i in context.Table1
                join a in context.TableA on i.SomeProperty equals a.SomeProperty
                join b in context.TableB on i.SomeOtherProperty equals b.SomeProperty
                join c in context.TableC on i.AnotherProperty equals c.SomeProperty
                etc.
                etc.
                select i;

    count = items.Count();
}   

return count;

.
.
.

using (var context = new DbContext())
{
    var items = from i in context.Table1
                join a in context.TableA on i.SomeProperty equals a.SomeProperty
                join b in context.TableB on i.SomeOtherProperty equals b.SomeProperty
                join c in context.TableC on i.AnotherProperty equals c.SomeProperty
                etc.
                etc.
                select new
                {
                    DynamicProp1 = i.SomeProperty,
                    DyanmicProp2 = a.SomeProperty,
                    DyanmicProp3 = b.SomePropery,
                    etc.
                    etc.
                }
    ... do some stuff with 'items'...
}

我想不出任何方法来避免这种重复查询。我需要访问所有连接的表才能构建我的集合。如果有任何提示或建议，我将不胜感激。

Answer 1

您可以创建方法来获取上下文和 return 具有所有需要实体的项目的 IQueryable：

class Holder
{
  TableAItem A{get;set;}
  TableBItem B{get;set;}
  ...
}

IQueryable<Holder> GetQuery(DbContext context)
{
  return from i in context.Table1
         join a in context.TableA on i.SomeProperty equals a.SomeProperty
         join b in context.TableB on i.SomeOtherProperty equals b.SomeProperty
         join c in context.TableC on i.AnotherProperty equals c.SomeProperty
         ...
         select new Holder
         {
           A = i,
           B = b
           ....
         };
}

using (var context = new DbContext())
{
    var items = GetQuery(context);

    count = items.Count();
}   

return count;


using (var context = new DbContext())
{
    var items = from r in GetQuery(context)
                select new
                {
                    DynamicProp1 = r.a.SomeProperty,
                    DyanmicProp2 = r.a.SomeProperty,
                    DyanmicProp3 = r.b.SomePropery,
                    etc.
                    etc.
                }
    ... do some stuff with 'items'...
}

Answer 2

请记住，进行查询不会执行查询，这称为延迟执行。那么为什么不进行查询，然后将其作为 IQueryable<> 对象传递。例如，考虑以下代码：

只是 return 字符串最后一个字符的简单方法，但它也会写出它在做什么：

public char GetLastChar(string input)
{
    Console.WriteLine("GetLastChar from {0}", input);
    return input.Last();
}

现在这段代码使用的方法是：

var listOfStuff = new List<string> { "string1", "string2", "string3" };

Console.WriteLine("Making the query");

var results = from s in listOfStuff
              select GetLastChar(s);

Console.WriteLine("Before getting count");
var count = results.Count();
Console.WriteLine("Now enumerating the query");

foreach(var s in results)
{
    Console.WriteLine(s);
}

您将看到如下输出：

Making the query
Before getting count
GetLastChar from string1
GetLastChar from string2
GetLastChar from string3
3
Now enumerating the query
GetLastChar from string1
1
GetLastChar from string2
2
GetLastChar from string3
3

如何在需要计数和项目时避免编写重复查询

How to avoid writing duplicate queries when needing counts and then items

entity-framework

linq-to-entities