我试图用返回类型实现左外连接扩展方法IQueryable.
我写的功能如下
public static IQueryable<TResult> LeftOuterJoin2<TOuter, TInner, TKey, TResult>(
this IQueryable<TOuter> outer,
IQueryable<TInner> inner,
Func<TOuter, TKey> outerKeySelector,
Func<TInner, TKey> innerKeySelector,
Func<TOuter, TInner, TResult> resultSelector)
{
return
from outerItem in outer
join innerItem in inner on outerKeySelector(outerItem)
equals innerKeySelector(innerItem) into joinedData
from r in joinedData.DefaultIfEmpty()
select resultSelector(outerItem, r);
}
它无法生成查询.原因可能是:我用过Func<>而不是Expression<>.我也尝试过Expression<>.它给我一个在线错误outerKeySelector(outerItem),这outerKeySelector是一个被用作方法的变量
我发现了一些关于SO(比如这里)和CodeProjects的讨论,但那些适用于IEnumerable不适合的类型IQueryable.
Ton*_*ony 33
介绍
这个问题非常有趣.问题是Funcs是委托,而表达式是树,它们是完全不同的结构.当您使用当前的扩展实现时,它使用循环并在每个元素的每个步骤上执行选择器,并且它运行良好.但是当我们谈论实体框架和LINQ时,我们需要树遍历来将其转换为SQL查询.所以它比Funcs更"难"(但我还是喜欢Expressions)并且下面描述了一些问题.
当你想做左外连接时你可以使用这样的东西(取自这里:如何在JOIN扩展方法中实现左连接)
var leftJoin = p.Person.Where(n => n.FirstName.Contains("a"))
.GroupJoin(p.PersonInfo,
n => n.PersonId,
m => m.PersonId,
(n, ms) => new { n, ms = ms.DefaultIfEmpty() })
.SelectMany(z => z.ms.Select(m => new { n = z.n, m ));
这是好的,但它不是我们需要的扩展方法.我想你需要这样的东西:
using (var db = new Database1Entities("..."))
{
var my = db.A.LeftOuterJoin2(db.B, a => a.Id, b => b.IdA,
(a, b) => new { a, b, hello = "Hello World!" });
// other actions ...
}
创建此类扩展有许多难点:
Where,Select等脚步
考虑2个简单的表:( A列:Id,文本)和B(列Id,IdA,文本).
外连接可以分3步实现:
// group join as usual + use DefaultIfEmpty
var q1 = Queryable.GroupJoin(db.A, db.B, a => a.Id, b => b.IdA,
(a, b) => new { a, groupB = b.DefaultIfEmpty() });
// regroup data to associated list a -> b, it is usable already, but it's
// impossible to use resultSelector on this stage,
// beacuse of type difference (quite deep problem: some anonymous type != TOuter)
var q2 = Queryable.SelectMany(q1, x => x.groupB, (a, b) => new { a.a, b });
// second regroup to get the right types
var q3 = Queryable.SelectMany(db.A,
a => q2.Where(x => x.a == a).Select(x => x.b),
(a, b) => new {a, b});
码
好吧,我不是一个好的出纳员,这里是我的代码(对不起,我无法更好地格式化它,但它有效!):
public static IQueryable<TResult> LeftOuterJoin2<TOuter, TInner, TKey, TResult>(
this IQueryable<TOuter> outer,
IQueryable<TInner> inner,
Expression<Func<TOuter, TKey>> outerKeySelector,
Expression<Func<TInner, TKey>> innerKeySelector,
Expression<Func<TOuter, TInner, TResult>> resultSelector)
{
// generic methods
var selectManies = typeof(Queryable).GetMethods()
.Where(x => x.Name == "SelectMany" && x.GetParameters().Length == 3)
.OrderBy(x=>x.ToString().Length)
.ToList();
var selectMany = selectManies.First();
var select = typeof(Queryable).GetMethods().First(x => x.Name == "Select" && x.GetParameters().Length == 2);
var where = typeof(Queryable).GetMethods().First(x => x.Name == "Where" && x.GetParameters().Length == 2);
var groupJoin = typeof(Queryable).GetMethods().First(x => x.Name == "GroupJoin" && x.GetParameters().Length == 5);
var defaultIfEmpty = typeof(Queryable).GetMethods().First(x => x.Name == "DefaultIfEmpty" && x.GetParameters().Length == 1);
// need anonymous type here or let's use Tuple
// prepares for:
// var q2 = Queryable.GroupJoin(db.A, db.B, a => a.Id, b => b.IdA, (a, b) => new { a, groupB = b.DefaultIfEmpty() });
var tuple = typeof(Tuple<,>).MakeGenericType(
typeof(TOuter),
typeof(IQueryable<>).MakeGenericType(
typeof(TInner)
)
);
var paramOuter = Expression.Parameter(typeof(TOuter));
var paramInner = Expression.Parameter(typeof(IEnumerable<TInner>));
var groupJoinExpression = Expression.Call(
null,
groupJoin.MakeGenericMethod(typeof (TOuter), typeof (TInner), typeof (TKey), tuple),
new Expression[]
{
Expression.Constant(outer),
Expression.Constant(inner),
outerKeySelector,
innerKeySelector,
Expression.Lambda(
Expression.New(
tuple.GetConstructor(tuple.GetGenericArguments()),
new Expression[]
{
paramOuter,
Expression.Call(
null,
defaultIfEmpty.MakeGenericMethod(typeof (TInner)),
new Expression[]
{
Expression.Convert(paramInner, typeof (IQueryable<TInner>))
}
)
},
tuple.GetProperties()
),
new[] {paramOuter, paramInner}
)
}
);
// prepares for:
// var q3 = Queryable.SelectMany(q2, x => x.groupB, (a, b) => new { a.a, b });
var tuple2 = typeof (Tuple<,>).MakeGenericType(typeof (TOuter), typeof (TInner));
var paramTuple2 = Expression.Parameter(tuple);
var paramInner2 = Expression.Parameter(typeof(TInner));
var paramGroup = Expression.Parameter(tuple);
var selectMany1Result = Expression.Call(
null,
selectMany.MakeGenericMethod(tuple, typeof (TInner), tuple2),
new Expression[]
{
groupJoinExpression,
Expression.Lambda(
Expression.Convert(Expression.MakeMemberAccess(paramGroup, tuple.GetProperty("Item2")),
typeof (IEnumerable<TInner>)),
paramGroup
),
Expression.Lambda(
Expression.New(
tuple2.GetConstructor(tuple2.GetGenericArguments()),
new Expression[]
{
Expression.MakeMemberAccess(paramTuple2, paramTuple2.Type.GetProperty("Item1")),
paramInner2
},
tuple2.GetProperties()
),
new[]
{
paramTuple2,
paramInner2
}
)
}
);
// prepares for final step, combine all expressinos together and invoke:
// var q4 = Queryable.SelectMany(db.A, a => q3.Where(x => x.a == a).Select(x => x.b), (a, b) => new { a, b });
var paramTuple3 = Expression.Parameter(tuple2);
var paramTuple4 = Expression.Parameter(tuple2);
var paramOuter3 = Expression.Parameter(typeof (TOuter));
var selectManyResult2 = selectMany
.MakeGenericMethod(
typeof(TOuter),
typeof(TInner),
typeof(TResult)
)
.Invoke(
null,
new object[]
{
outer,
Expression.Lambda(
Expression.Convert(
Expression.Call(
null,
select.MakeGenericMethod(tuple2, typeof(TInner)),
new Expression[]
{
Expression.Call(
null,
where.MakeGenericMethod(tuple2),
new Expression[]
{
selectMany1Result,
Expression.Lambda(
Expression.Equal(
paramOuter3,
Expression.MakeMemberAccess(paramTuple4, paramTuple4.Type.GetProperty("Item1"))
),
paramTuple4
)
}
),
Expression.Lambda(
Expression.MakeMemberAccess(paramTuple3, paramTuple3.Type.GetProperty("Item2")),
paramTuple3
)
}
),
typeof(IEnumerable<TInner>)
),
paramOuter3
),
resultSelector
}
);
return (IQueryable<TResult>)selectManyResult2;
}
用法
再次使用:
db.A.LeftOuterJoin2(db.B, a => a.Id, b => b.IdA,
(a, b) => new { a, b, hello = "Hello World!" });
看看这个你可以想一下这个sql查询是什么？这可能是巨大的.你猜怎么着？它很小:
SELECT
1 AS [C1],
[Extent1].[Id] AS [Id],
[Extent1].[Text] AS [Text],
[Join1].[Id1] AS [Id1],
[Join1].[IdA] AS [IdA],
[Join1].[Text2] AS [Text2],
N'Hello World!' AS [C2]
FROM [A] AS [Extent1]
INNER JOIN (SELECT [Extent2].[Id] AS [Id2], [Extent2].[Text] AS [Text], [Extent3].[Id] AS [Id1], [Extent3].[IdA] AS [IdA], [Extent3].[Text2] AS [Text2]
FROM [A] AS [Extent2]
LEFT OUTER JOIN [B] AS [Extent3] ON [Extent2].[Id] = [Extent3].[IdA] ) AS [Join1] ON [Extent1].[Id] = [Join1].[Id2]
希望能帮助到你.
接受的答案是解释左外连接背后复杂性的一个很好的开始.
我发现了三个相当严重的问题,特别是在采用这种扩展方法并在更复杂的查询中使用它时(将多个左外连接与正常连接链接,然后汇总/ max/count/...)在将所选答案复制到您的生产环境,请继续阅读.
考虑链接的SO帖子中的原始示例,它代表LINQ中完成的任何左外连接:
var leftJoin = p.Person.Where(n => n.FirstName.Contains("a"))
.GroupJoin(p.PersonInfo,
n => n.PersonId,
m => m.PersonId,
(n, ms) => new { n, ms = ms })
.SelectMany(z => z.ms.DefaultIfEmpty(), (n, m) => new { n = n, m ));
使用Tuple有效,但当这被用作更复杂查询的一部分时,EF失败(不能使用构造函数).要解决这个问题,您需要动态生成新的匿名类(搜索堆栈溢出)或使用无构造函数类型.我创造了这个
internal class KeyValuePairHolder<T1, T2>
{
public T1 Item1 { get; set; }
public T2 Item2 { get; set; }
}
使用"Queryable.DefaultIfEmpty"方法.在原始方法和GroupJoin方法中,编译器选择的正确方法是"Enumerable.DefaultIfEmpty"方法.这对简单查询没有影响,但请注意接受的答案如何有一堆转换(在IQueryable和IEnumerable之间).这些演员也会在更复杂的查询中引起问题. 可以在Expression中使用"Enumerable.DefaultIfEmpty"方法,EF知道不执行它,而是将其转换为连接.
最后,这是一个更大的问题:有两个选择完成,而原始只做一个选择.你可以在代码注释中读取原因(因为类型不同(非常深的问题:一些匿名类型!= TOuter))并在SQL中看到它(从内连接中选择(左外连接b)) 这里的问题原始SelectMany方法采用在Join方法中创建的对象类型:TOuter的KeyValuePairHolder和Tinner的IEnumerable作为它的第一个参数,但传递的resultSelector表达式采用一个简单的TOUter作为它的第一个参数.您可以使用ExpressionVisitor重写传递给正确形式的表达式.
internal class ResultSelectorRewriter<TOuter, TInner, TResult> : ExpressionVisitor
{
private Expression<Func<TOuter, TInner, TResult>> resultSelector;
public Expression<Func<KeyValuePairHolder<TOuter, IEnumerable<TInner>>, TInner, TResult>> CombinedExpression { get; private set; }
private ParameterExpression OldTOuterParamExpression;
private ParameterExpression OldTInnerParamExpression;
private ParameterExpression NewTOuterParamExpression;
private ParameterExpression NewTInnerParamExpression;
public ResultSelectorRewriter(Expression<Func<TOuter, TInner, TResult>> resultSelector)
{
this.resultSelector = resultSelector;
this.OldTOuterParamExpression = resultSelector.Parameters[0];
this.OldTInnerParamExpression = resultSelector.Parameters[1];
this.NewTOuterParamExpression = Expression.Parameter(typeof(KeyValuePairHolder<TOuter, IEnumerable<TInner>>));
this.NewTInnerParamExpression = Expression.Parameter(typeof(TInner));
var newBody = this.Visit(this.resultSelector.Body);
var combinedExpression = Expression.Lambda(newBody, new ParameterExpression[] { this.NewTOuterParamExpression, this.NewTInnerParamExpression });
this.CombinedExpression = (Expression<Func<KeyValuePairHolder<TOuter, IEnumerable<TInner>>, TInner, TResult>>)combinedExpression;
}
protected override Expression VisitParameter(ParameterExpression node)
{
if (node == this.OldTInnerParamExpression)
return this.NewTInnerParamExpression;
else if (node == this.OldTOuterParamExpression)
return Expression.PropertyOrField(this.NewTOuterParamExpression, "Item1");
else
throw new InvalidOperationException("What is this sorcery?", new InvalidOperationException("Did not expect a parameter: " + node));
}
}
使用表达式visitor和KeyValuePairHolder来避免使用元组,我在下面选择的答案的更新版本修复了三个问题,更短,并产生更短的SQL:
internal class QueryReflectionMethods
{
internal static System.Reflection.MethodInfo Enumerable_Select = typeof(Enumerable).GetMethods().First(x => x.Name == "Select" && x.GetParameters().Length == 2);
internal static System.Reflection.MethodInfo Enumerable_DefaultIfEmpty = typeof(Enumerable).GetMethods().First(x => x.Name == "DefaultIfEmpty" && x.GetParameters().Length == 1);
internal static System.Reflection.MethodInfo Queryable_SelectMany = typeof(Queryable).GetMethods().Where(x => x.Name == "SelectMany" && x.GetParameters().Length == 3).OrderBy(x => x.ToString().Length).First();
internal static System.Reflection.MethodInfo Queryable_Where = typeof(Queryable).GetMethods().First(x => x.Name == "Where" && x.GetParameters().Length == 2);
internal static System.Reflection.MethodInfo Queryable_GroupJoin = typeof(Queryable).GetMethods().First(x => x.Name == "GroupJoin" && x.GetParameters().Length == 5);
internal static System.Reflection.MethodInfo Queryable_Join = typeof(Queryable).GetMethods(System.Reflection.BindingFlags.Static | System.Reflection.BindingFlags.Public).First(c => c.Name == "Join");
internal static System.Reflection.MethodInfo Queryable_Select = typeof(Queryable).GetMethods().First(x => x.Name == "Select" && x.GetParameters().Length == 2);
public static IQueryable<TResult> CreateLeftOuterJoin<TOuter, TInner, TKey, TResult>(
IQueryable<TOuter> outer,
IQueryable<TInner> inner,
Expression<Func<TOuter, TKey>> outerKeySelector,
Expression<Func<TInner, TKey>> innerKeySelector,
Expression<Func<TOuter, TInner, TResult>> resultSelector)
{
var keyValuePairHolderWithGroup = typeof(KeyValuePairHolder<,>).MakeGenericType(
typeof(TOuter),
typeof(IEnumerable<>).MakeGenericType(
typeof(TInner)
)
);
var paramOuter = Expression.Parameter(typeof(TOuter));
var paramInner = Expression.Parameter(typeof(IEnumerable<TInner>));
var groupJoin =
Queryable_GroupJoin.MakeGenericMethod(typeof(TOuter), typeof(TInner), typeof(TKey), keyValuePairHolderWithGroup)
.Invoke(
"ThisArgumentIsIgnoredForStaticMethods",
new object[]{
outer,
inner,
outerKeySelector,
innerKeySelector,
Expression.Lambda(
Expression.MemberInit(
Expression.New(keyValuePairHolderWithGroup),
Expression.Bind(
keyValuePairHolderWithGroup.GetMember("Item1").Single(),
paramOuter
),
Expression.Bind(
keyValuePairHolderWithGroup.GetMember("Item2").Single(),
paramInner
)
),
paramOuter,
paramInner
)
}
);
var paramGroup = Expression.Parameter(keyValuePairHolderWithGroup);
Expression collectionSelector = Expression.Lambda(
Expression.Call(
null,
Enumerable_DefaultIfEmpty.MakeGenericMethod(typeof(TInner)),
Expression.MakeMemberAccess(paramGroup, keyValuePairHolderWithGroup.GetProperty("Item2")))
,
paramGroup
);
Expression newResultSelector = new ResultSelectorRewriter<TOuter, TInner, TResult>(resultSelector).CombinedExpression;
var selectMany1Result =
Queryable_SelectMany.MakeGenericMethod(keyValuePairHolderWithGroup, typeof(TInner), typeof(TResult))
.Invoke(
"ThisArgumentIsIgnoredForStaticMethods", new object[]{
groupJoin,
collectionSelector,
newResultSelector
}
);
return (IQueryable<TResult>)selectMany1Result;
}
}
如前面的答案所述，当您希望将IQueryable转换为SQL时，需要使用Expression而不是Func，因此必须走Expression Tree路线。
但是，这是一种无需您自己构建表达式树即可获得相同结果的方法。诀窍是，您需要引用LinqKit（可通过NuGet获取）并在查询上调用AsExpandable（）。这将负责构建基础的表达式树（请参阅此处）。
下面的示例将GroupJoin与SelectMany和DefaultIfEmpty（）方法一起使用：
码
public static IQueryable<TResult> LeftOuterJoin<TOuter, TInner, TKey, TResult>(
this IQueryable<TOuter> outer,
IQueryable<TInner> inner,
Expression<Func<TOuter, TKey>> outerKeySelector,
Expression<Func<TInner, TKey>> innerKeySelector,
Expression<Func<TOuter, TInner, TResult>> resultSelector)
{
return outer
.AsExpandable()// Tell LinqKit to convert everything into an expression tree.
.GroupJoin(
inner,
outerKeySelector,
innerKeySelector,
(outerItem, innerItems) => new { outerItem, innerItems })
.SelectMany(
joinResult => joinResult.innerItems.DefaultIfEmpty(),
(joinResult, innerItem) =>
resultSelector.Invoke(joinResult.outerItem, innerItem));
}
样本数据
假设我们具有以下EF实体，并且users和address变量是对基础DbSet的访问：
public class User
{
public int Id { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
}
public class UserAddress
{
public int UserId { get; set; }
public string LastName { get; set; }
public string Street { get; set; }
}
IQueryable<User> users;
IQueryable<UserAddress> addresses;
用法1
让我们按用户ID加入：
var result = users.LeftOuterJoin(
addresses,
user => user.Id,
address => address.UserId,
(user, address) => new { user.Id, address.Street });
转换为（使用LinqPad）：
SELECT
[Extent1].[Id] AS [Id],
[Extent2].[Street] AS [Street]
FROM [dbo].[Users] AS [Extent1]
LEFT OUTER JOIN [dbo].[UserAddresses] AS [Extent2]
ON [Extent1].[Id] = [Extent2].[UserId]
用法2
现在，让我们使用匿名类型作为键加入多个属性：
var result = users.LeftOuterJoin(
addresses,
user => new { user.Id, user.LastName },
address => new { Id = address.UserId, address.LastName },
(user, address) => new { user.Id, address.Street });
请注意，匿名类型属性必须具有相同的名称，否则会出现语法错误。
这就是为什么我们拥有Id = address.UserId而不是address.UserId的原因。
这将被翻译为：
SELECT
[Extent1].[Id] AS [Id],
[Extent2].[Street] AS [Street]
FROM [dbo].[Users] AS [Extent1]
LEFT OUTER JOIN [dbo].[UserAddresses] AS [Extent2]
ON ([Extent1].[Id] = [Extent2].[UserId]) AND ([Extent1].[LastName] = [Extent2].[LastName])
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