Using LINQ to SELECT the SUM() of a subquery - linq

Question

I am trying to learn how to use LINQ to perform a query that yields the same result as this:
SELECT (
SELECT SUM(point)
FROM communitymemberpointfeature
WHERE communitymemberpointfeature.communitymemberid = communitymember.id
) AS points, communitymember.*
FROM communitymember
After browsing around the Internet, I constructed the following statement:
var list = (from pointFeature in communityMemberPointFeatureList
join member in communityMemberList on pointFeature.CommunityMemberId equals member.Id
group pointFeature by new { pointFeature.CommunityMemberId }
into grouping
select new
{
grouping,
points = grouping.Sum(row => row.Point)
}).ToList();
But this yielded a result like
[
{
points:7200,
grouping:[
{Id:1,Point:5000,FeatureId:1,CommunityMemberId:1},
{Id:2,Point:2200,FeatureId:1,CommunityMemberId:1},
],
}
...
]
What I really want is a result set like:
[
{points:7200,CommunityMemberId:1,firstname:'john',lastname:'blah' ....},
...
]
Can someone tell me what I did wrong?

Answer 1

Edit after comment added to the end
I can imagine you have problems translating your SQL into LINQ. When trying to write LINQ statements it is usually a lot easier to start from your requirements, instead of starting from a SQL statement.
It seems to me that you have a table with CommunityMembers. Every CommunityMember has a primary key in property Id.
Furthermore, every CommunityMember has zero or more CommunityMemberPointFeatures, namely those CommunityMemberPointFeatures with a foreign key CommunityMemberId that equals the primary key of the CommunityMember that it belongs to.
For example: CommunityMember [14] has all CommunityMemberPointFeatures that have a value CommunityMemberId equal to 14.
Requirement
If I look at your SQL, it seems to me that you want to query all CommunityMembers, each with the sum of property Point of all CommunityMemberPointFeatures of this CommunityMember.
Whenever you want to query "items with their zero or more subitems", like "Schools with their Students", "Customers with their Orders", "CommunityMembers with their PointFeatures", consider using GroupJoin.
A GroupJoin is in fact a Left Outer Join, followed by a GroupBy to make Groups of the Left item with all its Right items.
var result = dbContext.CommunityMembers // GroupJoin CommunityMembers
.GroupJoin(CommunityMemberPointFeatures, // With CommunityMemberPointFeatures
communityMember => communityMember.Id, // from every CommunityMember take the Id
pointFeature => pointFeature.CommunityMemberId, // from every CommunityMemberPointFeature
// take the CommunityMemberId
// Parameter ResultSelector: take every CommunityMember, with all its matching
// CommunityMemberPointFeatures to make one new object:
(communityMember, pointFeaturesOfThisCommunityMember) => new
{
// Select the communityMember properties that you plan to use:
Id = communityMember.Id,
Name = communityMember.Name,
...
// From the point features of this CommunityMember you only want the sum
// or property Point:
Points = pointFeaturesOfThisCommunityMember
.Select(pointFeature => pointFeature.Point)
.Sum(),
// However, if you want more fields, you can use:
PointFeatures = pointFeaturesOfThisCommunityMember.Select(pointFeature => new
{
Id = pointFeature.Id,
Name = pointFeature.Name,
...
// not needed, you know the value:
// CommunityMemberId = pointFeature.CommunityMemberId,
})
.ToList(),
});
Edit after comment
If you want, you can omit Selecting the values that you plan to use.
// Parameter ResultSelector:
(communityMember, pointFeaturesOfThisCommunityMember) => new
{
CommunityMember = communityMember,
PointFeatures = pointFeaturesOfThisCommunityMember.ToList(),
),
However, I would strongly advise against this. If CommunityMember [14] has a thousand PointFeatures, then every PointFeature will have a foreign key with a value 14. So you are transporting this value 14 1001 times. What a waste of processing power, not to mention all the other fields you plan not to use.
Besides: if you do this you violate against information hiding: whenever your tables changes internally, the result of this function changes. Is that what you want?