实体框架左连接

adapted from MSDN, how to left join using EF 4

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)
};

It might be a bit of an overkill, but I wrote an extension method, so you can do a LeftJoin using the Join syntax (at least in method call notation):

persons.LeftJoin(
phoneNumbers,
person => person.Id,
phoneNumber => phoneNumber.PersonId,
(person, phoneNumber) => new
{
Person = person,
PhoneNumber = phoneNumber?.Number
}
);

My code does nothing more than adding a GroupJoin and a SelectMany call to the current expression tree. Nevertheless, it looks pretty complicated because I have to build the expressions myself and modify the expression tree specified by the user in the resultSelector parameter to keep the whole tree translatable by LINQ-to-Entities.

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);
}
}
}

I was able to do this by calling the DefaultIfEmpty() on the main model. This allowed me to left join on lazy loaded entities, seems more readable to me:

        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
});

Please make your life easier (don't use 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)
};

If UserGroups has a one to many relationship with UserGroupPrices table, then in EF, once the relationship is defined in code like:

//In UserGroups Model
public List<UserGroupPrices> UserGrpPriceList {get;set;}


//In UserGroupPrices model
public UserGroups UserGrps {get;set;}

You can pull the left joined result set by simply this:

var list = db.UserGroupDbSet.ToList();

assuming your DbSet for the left table is UserGroupDbSet, which will include the UserGrpPriceList, which is a list of all associated records from the right table.

If you prefer method call notation, you can force a left join using SelectMany combined with DefaultIfEmpty. At least on Entity Framework 6 hitting SQL Server. For example:

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,
}));
}

(Note that MyTable2.Col2 is a column of type int). The generated SQL will look like this:

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]

For 2 and more left joins (left joining creatorUser and 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
};