I've been struggling with this for a while and can't find the syntax for a LINQ outer join that has multiple conditions based on date. I've been looking into the GroupJoin syntax, but that only let's you compare one field value (normally IDs).
I would like to test if the parent table has a date (e.g. "UpdateDate") that falls within multiple values defined in the child table (e.g. "StartDate" and "EndDate"). If the parent date fits the condition, pull a column or two from the child table. If not, those columns from the child table should be null (classic left join stuff).
I don't think query syntax will work because it only recognizes equijoins.
Is there a way to do this in LINQ using Lambda syntax? I've been trying to use some combination of "SelectMany" and "DefaultIfEmpty" but keep getting stuck trying to define the join.
The way to do this in linq:
var q = from a in TableA
from b in TableB.where(x => a.Date > x.StartDate && a.Date < x.EndDate).DefaultIfEmpty()
select {...}
Use parameter ResultSelector of Queryable.GroupJoin to select what you want:
var result = dbContext.Parents.GroupJoin(dbContext.Children,
// outer and inner key Selectors:
parent => parent.Id, // from every parent take the primary key
child => child.ParentId, // from every child take the foreign key to parent
// ResultSelector: take the parent and all his children to make one new object
(parent, children) => new
{
// Select only the Parent properties you actually plan to use:
Id = parent.Id,
Name = parent.Name,
...
Children = children.Select(child => new
{
// select only Child properties you plan to use:
Id = child.Id,
// No need: you know the value: ParentId = child.ParentId,
...
"If the parent date fits the condition, pull a column or two from the child table, otherwise those columns from the child table should be null "
SpecialColumnA = (parent.BirthDay.Year < 2000) ?? child.BirthDay : null,
SpecialColumnB = (parent.Name == "Kennedy" ?? child.Name : null,
});
If the conditions are the same for a lot of columns, consider to check this only once:
SpecialColumns = (parent.Birthday.Year >= 2000) ? null :
// else fill the special columns:
new
{
Name = child.Name,
SomeWeirdProperty = parent.Id + child.Id,
...
},
});
Related
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?
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?
Consider this structure with 4 tables. childinfo, motherinfo, fatherinfo and guardianinfo.
childinfo table also has motherid, fatherid and guardianid
I am trying to get all data for a child based on ID even if there is nothing in motherinfo, fatherinfo or guardianinfo tables.
My query is like this:
var joined = (from c in _context.ChildInfo.Where(c => c.ChildDob >= dobDtRange.DobStart && c.ChildDob <= dobDtRange.DobEnd)
.OrderByDescending(c => c.ChildId)
join m in _context.MotherInfo.DefaultIfEmpty() on c.MotherId equals m.MotherId into cm
from cmm in cm.DefaultIfEmpty()
join f in _context.FatherInfo.DefaultIfEmpty() on c.FatherId equals f.FatherId into cf
from cff in cf.DefaultIfEmpty()
join g in _context.Guardian.DefaultIfEmpty() on c.GuardianId equals g.GuardianId into cg
from cgg in cg.DefaultIfEmpty()
select new { c, cmm, cff, cgg })
This is not working, what am I doing wrong ?
It seems you have a One-to-Zero-or-One Relationship If your Child has no mother its foreign key MotherId is zero. Some systems have a foreign key value null instead of zero.
To fetch a Child with its Mother, or null Mother if the child has a zero value foreign key MotherId, and similar for Father, etc:
var result = dbContext.Children.Select(child => new
{
// select only the Child properties you actually plan to use
Id = child.Id,
Name = chid.Name,
...
Mother = dbContext.Mothers
.Where(mother => mother.Id == child.MotherId)
.Select(mother => new
{ // select only the mother properties you actually plan to use
Id = mother.Id,
Name = mother.Name,
...
})
.FirstOrDefault(), // will return null if there is no mother for this foreign key
Father = dbContext.Fathers
.Where(father => father.Id == child.FatherId)
.Select(father => new
{ // select only the father properties you actually plan to use
Id = father.Id,
Name = father.Name,
...
})
.FirstOrDefault(), // will return null if there is no father for this foreign key
... etc
I have two SQL tables: Movies and Tags, while Movies contains a List<Tag>
Now I want to find all movies that have at least one of the tags of a List<Tag> argument. How can I do that in LINQ?
public IEnumerable<Group<Genre, Movie>> GetMoviesGrouped(string search, List<Tag> tags)
{
var movies = from movie in Movies
where ( movie.Name.Contains(search)) && movie.Tags.???contains any element of tags???
group movie by movie.genre into g
select new Group<Genre, Movie> { Key = g.Key, Values = g };
....
}
Movies where any of the tags is contains in tags:
var movies = from movie in Movies
where ( movie.Name.Contains(search))
&& movie.Tags.Any(t => tags.Contains(t))
group movie by movie.genre into g
select new Group<Genre, Movie> { Key = g.Key, Values = g };
However since this is comparing Tag instances and not strings:
It probably won't get translated to SQL (which means you will have to hydrate the query and do the extra filtering in Linq-to-Objects), and
You may want to compare the data of the tags rather than instances (unless you have overridden Equals on Tag
something like:
where movie.Tags.Any(mt => tags.Any(t => t.ID == mt.ID)) // or whatever property(ies) of `Tag` defines equality
You can intersect both list and see if there is any element like:
&& movie.Tags.Intersect(tags).Any()
Since Tag is an object, It will compare the references. Instead you can select unique identifier from Tag and then intersect that like:
&& movie.Tags.Select(r=> r.ID).Intersect(tags.Select(t=> t.ID)).Any()
Where ID is the primary key of Tag table.
So the question is ridiculously long, so let's go to the code. What's the linq2entities equivalent of the following Sql, given entities (tables) that look like:
Parent
---
parent_id
parent_field1
Child
--
child_id
parent_id
child_field1
child_field2
The sql:
select p.*, c.*
from parent p
inner join p on
p.parent_id = child.parent_id
where
c.child_field1 = some_appropriate_value
order by
p.parent_field1
c.child_field2
L2E let's you do .include() and that seems like the appropriate place to stick the ordering and filtering for the child, but the include method doesn't accept an expression (why not!?). So, I'm guessing this can't be done right now, because that's what a lot of articles say, but they're old, and I'm wondering if it's possible with EF6.
Also, I don't have access to the context, so I need the lambda-syntax version.
I am looking for a resultant object hierarchy that looks like:
Parent1
|
+-- ChildrenOfParent1
|
Parent2
|
+-- ChildrenOfParent2
and so forth. The list would be end up being an IEnumerable. If one iterated over that list, they could get the .Children property of each parent in that list.
Ideally (and I'm dreaming here, I think), is that the overall size of the result list could be limited. For example, if there are three parents, each with 10 children, for a total of 33 (30 children + 3 parents) entities, I could limit the total list to some arbitrary value, say 13, and in this case that would limit the result set to the first parent, with all its children, and the second parent, with only one of its children (13 total entities). I'm guessing all of this would have to be done manually in code, which is disappointing because it can be done quite easily in SQL.
when you get a query from db using entityframewrok to fetch parents, parent's fields are fetched in single query. now you have a result set like this:
var parentsQuery = db.Parents.ToList();
then, if you have a foreign key on parent, entityframework creates a navigation property on parent to access to corresponding entity (for example Child table).
in this case, when you use this navigation property from parent entities which already have been fetched, to get childs, entityframework creates another connection to sql server per parent.
for example if count of parentsQueryis 15, by following query entityframework creates 15 another connection, and get 15 another query:
var Childs = parentsQuery.SelectMany(u => u.NavigationProperty_Childs).ToList();
in these cases you can use include to prevent extra connections to fetch all childs with its parent, when you are trying to get parents in single query, like this:
var ParentIncludeChildsQuery = db.Parents.Include("Childs").ToList();
then by following Query, entityframework doesn't create any connection and doesn't get any query again :
var Childs = ParentIncludeChildsQuery.SelectMany(u => u.NavigationProperty_Childs).ToList();
but, you can't create any condition and constraint using include, you can check any constraint or conditions after include using Where, Join, Contains and so forth, like this:
var Childs = ParentIncludeChildsQuery.SelectMany(u => u.NavigationProperty_Childs
.Where(t => t.child_field1 = some_appropriate_value)).ToList();
but by this query, all child have been fetched from database before
the better way to acheieve equivalent sql query is :
var query = parent.Join(child,
p => p.ID
c => c.ParentID
(p, c) => new { Parent = p, Child = c })
.Where(u => u.Child.child_field1 == some_appropriate_value)
.OrderBy(u => u.Parent.parent_field1)
.ThenBy(u => u.Child.child_field2)
.ToList();
according to your comment, this is what you want:
var query = parent.Join(child,
p => p.ID,
c => c.ParentID,
(p, c) => new { Parent = p, Child = c })
.Where(u => u.Child.child_field1 == some_appropriate_value)
.GroupBy(u => u.Parent)
.Select(u => new {
Parent = u.Key,
Childs = u.OrderBy(t => t.Child.child_field2).AsEnumerable()
})
.OrderBy(u => u.Parent.parent_field1)
.ToList();
How do I do a eager query of a parent child relationship that:
filters a on child fields
sorts on both parent and child
return a List or Parents with the children pre-populated
If I try
from p in _context.Parents.Include("children")
join c in _context.childrenon p.Id equals c.ParentId
where d.DeletedDate == null
orderby p.Name ascending, c.Name
select p
Then I get the Parent object back but each Parent has NULL for children
if I try
from p in _context.Parents.Include("children")
orderby p.Name ascending
select p
The query returns all Parents and children but they are not filtered or sorted.
The result I want back is a IEnumerable<Parent>
i.e.
Parent[0].name = "foo"
Parent[0].children = IEnumerable<Child>
Parent[1].name = "bar"
Parent[1].children = IEnumerable<Child>
There is no direct way of doing this, but you can use somewhat of a workaround - project the parent and children onto an annonymous object and then select and return the parent from the object.
See similar question: Linq To Entities - how to filter on child entities
In your case you will have something along the lines of:
var resultObjectList = _context.
Parents.
Where(p => p.DeletedDate == null).
OrderBy(p => p.Name).
Select(p => new
{
ParentItem = p,
ChildItems = p.Children.OrderBy(c => c.Name)
}).ToList();
List<Parent> resultingCollection = resultObjectList.Select(o => o.ParentItem).ToList();
The solution depends on what exactly you are trying to do.
The first query gives the impression that you want to "flatten out" the results in objects, like this (pseudocode, I hope it's clear what I mean):
{ Parent1, Child1 }
{ Parent1, Child2 }
{ Parent1, Child3 }
{ Parent2, Child1 }
In this case each result "row" would be an object having a Parent and a Child property, and you could sort by parent name and then by child name.
The second query just returns the Parent objects and (you don't show it but I assume EF has been instructed to do that) each one has a Children collection. In this case you can only sort by parent name; if you want to sort each Parent's children, sort the Children collection on that object by itself.
Which of the two do you want to do?
Update
OK, it seems you want the second one. I don't believe it can be done directly. You can just do it when you enumerate the results - since the Parents are already sorted, simply sort each one's children:
var sortedChildren = parent.Children.OrderBy(c => c.Name);
prefetching child fields:
using (BlogDataContext context = new BlogDataContext())
{
DataLoadOptions options = new DataLoadOptions();
options.LoadWith<Blog>(c => c.Categories);
options.LoadWith<Blog>(c => c.Title);
context.LoadOptions = options;
Blog blog = context.Blogs.Single<Blog>(c => c.BlogId == 1);
}