我如何改变这个查询，使它返回所有u.u usergroups?
from u in usergroups
from p in u.UsergroupPrices
select new UsergroupPricesList
{
UsergroupID = u.UsergroupID,
UsergroupName = u.UsergroupName,
Price = p.Price
};
改编自MSDN，如何使用EF 4左连接
var query = from u in usergroups
join p in UsergroupPrices on u.UsergroupID equals p.UsergroupID into gj
from x in gj.DefaultIfEmpty()
select new {
UsergroupID = u.UsergroupID,
UsergroupName = u.UsergroupName,
Price = (x == null ? String.Empty : x.Price)
};
这可能有点过分，但我写了一个扩展方法，所以您可以使用Join语法(至少在方法调用符号中)执行LeftJoin:
persons.LeftJoin(
phoneNumbers,
person => person.Id,
phoneNumber => phoneNumber.PersonId,
(person, phoneNumber) => new
{
Person = person,
PhoneNumber = phoneNumber?.Number
}
);
我的代码只是添加一个GroupJoin和SelectMany调用到当前的表达式树。然而，它看起来相当复杂，因为我必须自己构建表达式，并修改用户在resultSelector参数中指定的表达式树，以保持整个树可被LINQ-to-Entities翻译。
public static class LeftJoinExtension
{
public static IQueryable<TResult> LeftJoin<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)
{
MethodInfo groupJoin = typeof (Queryable).GetMethods()
.Single(m => m.ToString() == "System.Linq.IQueryable`1[TResult] GroupJoin[TOuter,TInner,TKey,TResult](System.Linq.IQueryable`1[TOuter], System.Collections.Generic.IEnumerable`1[TInner], System.Linq.Expressions.Expression`1[System.Func`2[TOuter,TKey]], System.Linq.Expressions.Expression`1[System.Func`2[TInner,TKey]], System.Linq.Expressions.Expression`1[System.Func`3[TOuter,System.Collections.Generic.IEnumerable`1[TInner],TResult]])")
.MakeGenericMethod(typeof (TOuter), typeof (TInner), typeof (TKey), typeof (LeftJoinIntermediate<TOuter, TInner>));
MethodInfo selectMany = typeof (Queryable).GetMethods()
.Single(m => m.ToString() == "System.Linq.IQueryable`1[TResult] SelectMany[TSource,TCollection,TResult](System.Linq.IQueryable`1[TSource], System.Linq.Expressions.Expression`1[System.Func`2[TSource,System.Collections.Generic.IEnumerable`1[TCollection]]], System.Linq.Expressions.Expression`1[System.Func`3[TSource,TCollection,TResult]])")
.MakeGenericMethod(typeof (LeftJoinIntermediate<TOuter, TInner>), typeof (TInner), typeof (TResult));
var groupJoinResultSelector = (Expression<Func<TOuter, IEnumerable<TInner>, LeftJoinIntermediate<TOuter, TInner>>>)
((oneOuter, manyInners) => new LeftJoinIntermediate<TOuter, TInner> {OneOuter = oneOuter, ManyInners = manyInners});
MethodCallExpression exprGroupJoin = Expression.Call(groupJoin, outer.Expression, inner.Expression, outerKeySelector, innerKeySelector, groupJoinResultSelector);
var selectManyCollectionSelector = (Expression<Func<LeftJoinIntermediate<TOuter, TInner>, IEnumerable<TInner>>>)
(t => t.ManyInners.DefaultIfEmpty());
ParameterExpression paramUser = resultSelector.Parameters.First();
ParameterExpression paramNew = Expression.Parameter(typeof (LeftJoinIntermediate<TOuter, TInner>), "t");
MemberExpression propExpr = Expression.Property(paramNew, "OneOuter");
LambdaExpression selectManyResultSelector = Expression.Lambda(new Replacer(paramUser, propExpr).Visit(resultSelector.Body), paramNew, resultSelector.Parameters.Skip(1).First());
MethodCallExpression exprSelectMany = Expression.Call(selectMany, exprGroupJoin, selectManyCollectionSelector, selectManyResultSelector);
return outer.Provider.CreateQuery<TResult>(exprSelectMany);
}
private class LeftJoinIntermediate<TOuter, TInner>
{
public TOuter OneOuter { get; set; }
public IEnumerable<TInner> ManyInners { get; set; }
}
private class Replacer : ExpressionVisitor
{
private readonly ParameterExpression _oldParam;
private readonly Expression _replacement;
public Replacer(ParameterExpression oldParam, Expression replacement)
{
_oldParam = oldParam;
_replacement = replacement;
}
public override Expression Visit(Expression exp)
{
if (exp == _oldParam)
{
return _replacement;
}
return base.Visit(exp);
}
}
}
请让您的生活更轻松(不要使用join into group):
var query = from ug in UserGroups
from ugp in UserGroupPrices.Where(x => x.UserGroupId == ug.Id).DefaultIfEmpty()
select new
{
UserGroupID = ug.UserGroupID,
UserGroupName = ug.UserGroupName,
Price = ugp != null ? ugp.Price : 0 //this is to handle nulls as even when Price is non-nullable prop it may come as null from SQL (result of Left Outer Join)
};
如果您喜欢方法调用符号，您可以使用SelectMany和DefaultIfEmpty组合来强制左连接。至少在实体框架6击中SQL服务器。例如:
using(var ctx = new MyDatabaseContext())
{
var data = ctx
.MyTable1
.SelectMany(a => ctx.MyTable2
.Where(b => b.Id2 == a.Id1)
.DefaultIfEmpty()
.Select(b => new
{
a.Id1,
a.Col1,
Col2 = b == null ? (int?) null : b.Col2,
}));
}
(注意MyTable2.Col2是int类型的列)。生成的SQL如下所示:
SELECT
[Extent1].[Id1] AS [Id1],
[Extent1].[Col1] AS [Col1],
CASE WHEN ([Extent2].[Col2] IS NULL) THEN CAST(NULL AS int) ELSE CAST( [Extent2].[Col2] AS int) END AS [Col2]
FROM [dbo].[MyTable1] AS [Extent1]
LEFT OUTER JOIN [dbo].[MyTable2] AS [Extent2] ON [Extent2].[Id2] = [Extent1].[Id1]
对于2个及以上的左连接(左连接creatatoruser和initiatorUser)
IQueryable<CreateRequestModel> queryResult = from r in authContext.Requests
join candidateUser in authContext.AuthUsers
on r.CandidateId equals candidateUser.Id
join creatorUser in authContext.AuthUsers
on r.CreatorId equals creatorUser.Id into gj
from x in gj.DefaultIfEmpty()
join initiatorUser in authContext.AuthUsers
on r.InitiatorId equals initiatorUser.Id into init
from x1 in init.DefaultIfEmpty()
where candidateUser.UserName.Equals(candidateUsername)
select new CreateRequestModel
{
UserName = candidateUser.UserName,
CreatorId = (x == null ? String.Empty : x.UserName),
InitiatorId = (x1 == null ? String.Empty : x1.UserName),
CandidateId = candidateUser.UserName
};
我可以通过在主模型上调用DefaultIfEmpty()来做到这一点。这允许我在惰性加载的实体上左连接，对我来说似乎更容易读:
var complaints = db.Complaints.DefaultIfEmpty()
.Where(x => x.DateStage1Complete == null || x.DateStage2Complete == null)
.OrderBy(x => x.DateEntered)
.Select(x => new
{
ComplaintID = x.ComplaintID,
CustomerName = x.Customer.Name,
CustomerAddress = x.Customer.Address,
MemberName = x.Member != null ? x.Member.Name: string.Empty,
AllocationName = x.Allocation != null ? x.Allocation.Name: string.Empty,
CategoryName = x.Category != null ? x.Category.Ssl_Name : string.Empty,
Stage1Start = x.Stage1StartDate,
Stage1Expiry = x.Stage1_ExpiryDate,
Stage2Start = x.Stage2StartDate,
Stage2Expiry = x.Stage2_ExpiryDate
});
如果UserGroups与UserGroupPrices表有一对多的关系，那么在EF中，一旦这种关系在代码中定义:
//In UserGroups Model
public List<UserGroupPrices> UserGrpPriceList {get;set;}
//In UserGroupPrices model
public UserGroups UserGrps {get;set;}
您可以简单地通过以下方式提取左连接结果集:
var list = db.UserGroupDbSet.ToList();
假设左表的DbSet是UserGroupDbSet，它将包括UserGrpPriceList，这是来自右表的所有相关记录的列表。