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C# LINQ Join multiple tables into one object

Main problem to solve:
How do I get "2 levels down" to get Owner.Pet[n].Toys and put it all in one object right away.
There are three entities in the system: Owner, Pet and Toy.
I am trying to write a query (using LINQ's SQL-like notation) that will bring me single object of class Owner with its inner properties filled = all pets assigned to that owner + each pet having all its toys.
This is what I have so far, it's not working as expected. SQL feelings tell me that I am missing GROUP JOIN somewhere...
var singleOwnerQuery =
from o in owners
join p in pets on o.FirstName = p.OwnerName
join t in toys on p.PetId = t.ToyUniqueId // each toy is unique - there are not "re-usable" toys
where o.Name == "..." && o.LastName == "..."
select new Owner
{
Pets = pets // this should already assign all the toys for each of the pets
};
Any help will be appreciated.
I skipped lot of other properties inside each of the classes to make it simpler

Try the following query. Without model, this is just direction how to do Eager Loading query as in your case.
var singleOwnerQuery =
from o in owners
where o.Name == "..." && o.LastName == "..."
select new Owner
{
Pets = pets.Where(p => o.FirstName == p.OwnerName)
.Select(p => new
{
p.PetId,
Toys = toys.Where(t => p.PetId == t.ToyUniqueId).ToList()
})
.tolist()
};

How to understand this Linq query? [duplicate]

When writing LINQ queries in C#, I know I can perform a join using the join keyword. But what does the following do?
from c in Companies
from e in c.Employees
select e;
A LINQ book I have say it's a type of join, but not a proper join (which uses the join keyword). So exactly what type of join is it then?

Multiple "from" statements are considered compound linq statments. They are like nested foreach statements. The msdn page does list a great example here
var scoreQuery = from student in students
from score in student.Scores
where score > 90
select new { Last = student.LastName, score };
this statement could be rewritten as:
SomeDupCollection<string, decimal> nameScore = new SomeDupCollection<string, float>();
foreach(Student curStudent in students)
{
foreach(Score curScore in curStudent.scores)
{
if (curScore > 90)
{
nameScore.Add(curStudent.LastName, curScore);
}
}
}

This will get translated into a SelectMany() call. It is essentially a cross-join.
Jon Skeet talks about it on his blog, as part of the Edulinq series. (Scroll down to Secondary "from" clauses.)

The code that you listed:
from c in company
from e in c.Employees
select e;
... will produce a list of every employee for every company in the company variable. If an employee works for two companies, they will be included in the list twice.
The only "join" that might occur here is when you say c.Employees. In an SQL-backed provider, this would translate to an inner join from the Company table to the Employee table.
However, the double-from construct is often used to perform "joins" manually, like so:
from c in companies
from e in employees
where c.CompanyId == e.CompanyId
select e;
This would have a similar effect as the code you posted, with potential subtle differences depending on what the employees variable contains. This would also be equivalent to the following join:
from c in companies
join e in employees
on c.CompanyId equals e.CompanyId
select e;
If you wanted a Cartesian product, however, you could just remove the where clause. (To make it worth anything, you'd probably want to change the select slightly, too, though.)
from c in companies
from e in employees
select new {c, e};
This last query would give you every possible combination of company and employee.

All the first set of objects will be joined with all the second set of objects. For example, the following test will pass...
[TestMethod()]
public void TestJoin()
{
var list1 = new List<Object1>();
var list2 = new List<Object2>();
list1.Add(new Object1 { Prop1 = 1, Prop2 = "2" });
list1.Add(new Object1 { Prop1 = 4, Prop2 = "2av" });
list1.Add(new Object1 { Prop1 = 5, Prop2 = "2gks" });
list2.Add(new Object2 { Prop1 = 3, Prop2 = "wq" });
list2.Add(new Object2 { Prop1 = 9, Prop2 = "sdf" });
var list = (from l1 in list1
from l2 in list2
select l1).ToList();
Assert.AreEqual(6, list.Count);
}

Linq grouping - <k,v> where does my 'V' go?

In the following code:
var finalArticles =
from domainArticle in articlesFoundInDomain
join articleCategoryVersion in dbc.ArticlesCategoriesVersions
on domainArticle.ArticleID equals articleCategoryVersion.ArticleID
join articleCategory in dbc.ArticleCategories
on articleCategoryVersion.CategoryID equals articleCategory.CategoryID
where articleCategory.ParentID == 52
group articleCategory by articleCategory.CategoryID
into newArticleCategoryGroup
I understand that the group clause should be returning an IEnumerable where k is the Key, in this case CategoryID.
I think I'm misunderstanding Linq at this point because I assume that for each 'k' there should be a list of articles in 'v', but I don't understand the mechanisms or terminology or something. When I try to project this statement into a new anonymous object I don't seem to get any articles... where are they?
Edit:
Okay so I've got a piece of code that is working, but unfortunately it's hitting the SQL server multiple times:
var articlesAssociatedWithKnowledgeTypes =
from categories in dbc.ArticleCategories
join categoryVersions in dbc.ArticlesCategoriesVersions
on categories.CategoryID equals categoryVersions.CategoryID
join articles in articlesFoundInGivenDomain
on categoryVersions.ArticleID equals articles.ArticleID
where categories.ParentID == 52 && articles.Version == categoryVersions.Version
select new
{
ArticleID = articles.ArticleID,
ArticleTitle = articles.Title,
ArticleVersion = articles.Version,
CategoryID = categories.CategoryID,
CategoryName = categories.Name
} into knowledgeTypesFlat
group knowledgeTypesFlat by new { knowledgeTypesFlat.CategoryID, knowledgeTypesFlat.CategoryName } into knowledgeTypesNested
select new
{
CategoryID = knowledgeTypesNested.Key.CategoryID,
CategoryName = knowledgeTypesNested.Key.CategoryName,
Articles = knowledgeTypesNested.ToList()
};
I thought the ToList() on Articles would sort that out but it doesn't. But, the code works although I'm not sure if this is optimal?

The grouping returns an enumeration of IGroupings. IGrouping<K, V> itself implements IEnumerable<V>. Think of each group as an enumerable of all the members of that group plus an extra property Key

In your first query you are showing a group by and the second one is a group join, both return different results. The group by returns an IEnumerable<IGrouping<TKey, TElement>>. To get the result you're expecting you could group by CategoryId and CategoryName and project as I show below:
var finalArticles =
from domainArticle in articlesFoundInDomain
join articleCategoryVersion in dbc.ArticlesCategoriesVersions
on domainArticle.ArticleID equals articleCategoryVersion.ArticleID
join articleCategory in dbc.ArticleCategories
on articleCategoryVersion.CategoryID equals articleCategory.CategoryID
where articleCategory.ParentID == 52
group articleCategory by new{ articleCategory.CategoryID,articleCategory.CategoryName}
into g
select new {CatId=g.Key.CategoryID, CatName=g.Key.CategoryName,Articles =g.ToList() };
When you need the grouped elements you can call ToList or ToArray as I did above

Your finalArticles query results in a IEnumerable<IGrouping<int, Article>> (assuming CategoryID is int and your articles are of type Article).
These IGrouping<int, Article> provides a Key property of type int (your CategoryID and also the IEnumerable<Article> representing the sequence of articles for that CategoryID.
You can turn this for example into a Dictionary<int, List<Article>> mapping CategoryIDs to the lists of articles:
var dictionary = finalArticles.ToDictionary(group => group.Key, group => group.ToList());
or to a list of categories containing articles:
var categories = finalArticles.Select(group => new {
CategoryID = group.Key,
Articles = group.ToList()}).ToList();
Update after your comment:
var finalArticles =
from domainArticle in articlesFoundInDomain
join articleCategoryVersion in dbc.ArticlesCategoriesVersions
on domainArticle.ArticleID equals articleCategoryVersion.ArticleID
join articleCategory in dbc.ArticleCategories
on articleCategoryVersion.CategoryID equals articleCategory.CategoryID
where articleCategory.ParentID == 52
group articleCategory by new {articleCategory.CategoryID, articleCategory.Name}
into newArticleCategoryGroup
select new
{
CategoryID = newArticleCategoryGroup.Key.CategoryID,
CategoryName = newArticleCategoryGroup.Key.Name,
Articles = newArticleCateGroup.ToList()
}

LINQ using dictionary in where clause

er have the following query in linq...
Whenever I try to run it I get a No comparison operator for type System.Int[] exception.
It's got something to do with the dictionary I am sure, but I don't understand why this isn't valid and was wondering if someone could explain?
// As requested... not sure it will help though.
var per = (
from p in OtherContext.tblPeriod
where activeContractList.Select(c => c.DomainSetExtensionCode).Contains(p.DomainSetExtensionCode)
select p).ToArray();
var com = (
from c in MyContext.tblService
join sce in MyContext.tblServiceExtension
on c.ServiceExtensionCode equals sce.ServiceExtensionCode
join sc in MyContext.tblServiceContract
on sce.ServiceContractCode equals sc.ContractCode
group sc by c.Period into comG
select new
{
PeriodNumber = comG.Key,
Group = comG,
}).ToArray();
var code =
(from c in com
join p in per on c.PeriodNumber equals p.PeriodNumber
select new
{
p.Code,
c.Group
}).ToArray();
var payDictionary = new Dictionary<int, int[]>();
// This is another linq query that returns an anonymous type with
// two properties, and int and an array.
code.ForEach(c => payDictionary.Add(c.Code, c.Group.Select(g => g.Code).ToArray()));
// MyContext is a LINQ to SQL DataContext
var stuff = (
from
p in MyContext.tblPaySomething
join cae in MyContext.tblSomethingElse
on p.PaymentCode equals cae.PaymentCode
join ca in MyContext.tblAnotherThing
on cae.SomeCode equals ca.SomeCode
where
// ca.ContractCode.Value in an int?, that should always have a value.
payDictionary[p.Code].Contains(ca.ContractCode.Value)
select new
{
p.Code,
p.ExtensionCode,
p.IsFlagged,
p.Narrative,
p.PayCode,
ca.BookCode,
cae.Status
}).ToList();

You won't be able to do this with a dictionary. The alternative is to join the three linq queries into one. You can do this with minimal impact to your code by not materializing the queries with ToArray. This will leave com and code as IQueryable<T> and allow for you compose other queries with them.
You will also need to use a group rather than constructing a dictionary. Something like this should work:
var per = (
from p in OtherContext.tblPeriod
where activeContractList.Select(c => c.DomainSetExtensionCode).Contains(p.DomainSetExtensionCode)
select p.PeriodNumber).ToArray(); // Leave this ToArray because it's materialized from OtherContext
var com =
from c in MyContext.tblService
join sce in MyContext.tblServiceExtension on c.ServiceExtensionCode equals sce.ServiceExtensionCode
join sc in MyContext.tblServiceContract on sce.ServiceContractCode equals sc.ContractCode
group sc by c.Period into comG
select new
{
PeriodNumber = comG.Key,
Group = comG,
}; // no ToArray
var code =
from c in com
where per.Contains(c.PeriodNumber) // have to change this line because per comes from OtherContext
select new
{
Code = c.PeriodNumber,
c.Group
}; // no ToArray
var results =
(from p in MyContext.tblPaySomething
join cae in MyContext.tblSomethingElse on p.PaymentCode equals cae.PaymentCode
join ca in MyContext.tblAnothThing on cae.SomeCode equals ca.SomeCode
join cg in MyContext.Codes.GroupBy(c => c.Code, c => c.Code) on cg.Key equals p.Code
where cg.Contains(ca.ContractCode.Value)
select new
{
p.ContractPeriodCode,
p.DomainSetExtensionCode,
p.IsFlagged,
p.Narrative,
p.PaymentCode,
ca.BookingCode,
cae.Status
})
.ToList();
Side Note: I also suggest using navigation properties where possible instead of joins. It makes it much easier to read and understand how objects are related and create complex queries.

Linq to SQL joining two tables and populate GridView

I have two tables in my database, Building and Town. They look like this:
Building:
buildingid
buildingname
Town:
id
userid
buildingid
In Town there is one entry for each building a user has.
What i want is to populate a GridView for a user with a given userid. This GridView should include the buildingname and the number of buildings.
Building. I have tried this:
var buildings = (from Town in dc.Towns
join Building in dc.Buildings
on Town.buildingid equals Building.buildingid
select Building.buildingname);
gvBuildings.DataSource = buildings;
gvBuildings.DataBind();
But I don't know how to get the numbers for each building.
I have now been working on this for a while and a couple of your answers work. I have used this code:
var buildings = dc.Towns
.Where(t => t.userid == userid)
.GroupJoin(dc.Buildings,
t => t.buildingid,
b => b.buildingid,
(Town, Buildings) => new
{
BuildningName = Buildings.First().buildingname,
Count = Buildings.Count()
});
gvBuildings.DataSource = buildings.ToList();
gvBuildings.DataBind();
When i run this code my GridView ends up looking like this:
I need the buildings to be shown in groups, grouped by the buildingname. I have tried all of the suggestions but i cant get it to work.

Try grouping:
var buildings = dc.Towns
.Where(t => t.UserId == userId)
.GroupJoin(dc.Buildings,
t => t.BuildingId,
b => b.BuildingId,
(town, buildings) => new
{
BuildingName = buildings.First().BuildingName,
Count = buildings.Count
});
Keep in mind that when binding to a control you must supply a collection of type (or implementing) IList. This can be accomplished by calling ToList() on the buildings collection:
gvBuildings.DataSource = buildings.ToList();
gvBuildings.DataBind();

check linq differed execution
and than try the blow code might work for you
var buildings =
(from j in dc.Town
join i in dc.Buildings
on j.buildingId equals i.buildingId
where j.Userid = varUSerid
group new {i, j}
by new
{ i.BuildingID }
into
g
select new {
BuildingName = g.First<k=>k.BuildingName)
, count = g.Count() } ).ToList();
gvBuildings.DataSource = buildings;
gvBuildings.DataBind();

var buildings = (from Town in dc.Towns
join Building in dc.Buildings
on Town.buildingid equals Building.buildingid
into results
from r in results.DefaultIfEmpty()
group Town by new
{
r.BuildingId
} into groupedResults
where Town.UserID == parameteruserId
select new
{
BuildingName = Building.buildingname,
BuildingCount = groupedResults.Count()
});
Try this.. it should work.. i have a similar requirement..

manDbDataContext db = new DbDataContext();
var estimatedTotal = ( from est in db.AssignmentEstimatedMaterials
where est.assignment_id == Convert.ToInt32(Label_assignmentId.Text)
join materialdetail in db.Materials on est.material_id equals materialdetail.material_id
select new { est.qty,est.total_amount, materialdetail.material_name}).ToList();
GridView_estiamte_material.DataSource = estimatedTotal;
GridView_estiamte_material.DataBind();
Note, you should select individual data and it works.