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How do I group one to many in a nested fashion with linq to entities?

I have three tables, Chart, ChartQuestion, ChartAnswer
A Chart has many ChartQuestions, a ChartQuestion has many ChartAnswers.
I'd like to do a linq query that gives me an object containing all of this data, so it'd be a ChartObject containing List<ChartQuestion> and each ChartQuestion contains a List<ChartAnswer>
I started with this:
(from chart in db.Chart
join chartQuestion in db.chartQuestion on chart.ChartId equals chartQuestion.ChartId into chartQuestions)
This seems to be the first step. However I want to include the ChartAnswers now, so I have to do another join to pull back the ChartAnswers but I don't know how to do this.
I can't do a join, and while I can do a from, I am not sure of the exact syntax.
(from chart in db.Chart
join chartQuestion in db.chartQuestion on chart.ChartId equals chartQuestion.ChartId into chartQuestions
from chartQuestionsSelection in chartQuestions
join chartAnswer in context.ChartAnswers on chartQuestions.ChartAnswerId equals chartAnswer.ChartAnswerId into chartAnswers // This is wrong
)
With that code above you end up as chartAnswers being separate to the chartQuestions rather than belonging to them, so I don't think it is correct.
Any idea?

joining a table destructors it i.e. you now have the row as a variable if you need it in a nested fashion then select in like this
(from chart in db.Chart
select new { //can also be your own class/record, perhaps a DTO
chart.Id,
chart.Name,
questions = (from chartQuestion in db.chartQuestion
where chart.ChartId == chartQuestion.ChartId
select new { //perhaps a dto
chartQuestion.Id,
chartQuestion.Question,
Answers =
(from chartAnswer in context.ChartAnswers
where chartAnswer.ChartAnswerId == chartQuestion.ChartAnswerId
select chartAnswer).ToList() //this is translated and not evaluated
}).ToList() //this is translated not evaluated
}).ToListAsync(cancellationToken) //this will evaluate the expression
If you need it by joins then you can group join it like this:
from m in _context.Chart
join d in _context.ChartQuestions
on m.ID equals d.ID into mdJoin
select new
{
chartId = m.ID,
chartName = "m.name",
quess = from d in mdJoin
join dd in _context.ChartAnswer
on d.Iddomain equals dd.DomainId into anJoin
select new
{
quesId = d.ID,
quesName = d.Question,
anss = anJoin
}
}
A better way: If you edit your DbConfigurations to include navigation properties of each then the code will become way simpler.
example:
db.Chart
.Include(x => x.chartQuestions)
.ThenInclude(x => x.chartAnswers)
You can search more about how to do navigation properties in EFCore, but ofcourse if the code base is large then this might not be feasible for you.

Try following :
(from chart in db.Chart
join chartQuestion in db.chartQuestion on chart.ChartId equals chartQuestion.ChartId
join chartAnswer in context.ChartAnswers on chartAnswer.ChartAnswerId equals chartQuestion.ChartAnswerId
)

Linq - how to filter dataset based on data in another table?

I'm looking to filter the data in a dataset much like you'd do a where <value> in (select <value> from other_table where year=2016)
So I have a list of the "values":
var BUs = (from b in dc.BusinessUnits
where b.Year == int.Parse(ddlYears.SelectedValue)
orderby b.BuName
select new { b.BUID }).ToList();
So what I need to do is filter this dataset based on the BUID list returned in the BUs var.
IQueryable<Market> markets = (from p in dc.Markets
orderby p.MarketName
select p);
Help? I'm 100% new to linq so I need a concise solution.

Well if you Market entity has a BUID property and this property it's a primitive type (int, string..) or an enum, you can use Contains method:
var BUs = (from b in dc.BusinessUnits
where b.Year == int.Parse(ddlYears.SelectedValue)
orderby b.BuName
select b.BUID );
IQueryable<Market> markets = (from p in dc.Markets
where BUs.Contains(p.BUID)
orderby p.MarketName
select p);

The standard way of filtering by in memory id list is to use Enumerable.Contains method. But you need first to make sure your list contains ids - they way you wrote it it will contain anonymous type with a property called BUID, by changing the first query like this
int year = int.Parse(ddlYears.SelectedValue);
var BUIDs = (from b in dc.BusinessUnits
where b.Year == year
orderby b.BuName
select b.BUID).ToList();
and then use
var markets = (from p in dc.Markets
where BUIDs.Contains(p.BUID)
orderby p.MarketName
select p);
But note that this will be inefficient. Much better option would be to not use the list of BUIDs for filtering, but combining the 2 queries so the whole thing becomes a single query executed in the database, like this
var markets = (from p in dc.Markets
where dc.BusinessUnits.Any(bu => b.Year == year && b.BUID == p.BUID)
orderby p.MarketName
select p);
This is the exact equivalent of, if using your words, much like you'd do a "where in (select from other_table where year=2016)".

How to assign to a List<string> in LinqToEntities C#

Hoping that I'm just missing something obvious but here's my query
var data = (from project in _db.Projects
where project.Id == id
let primaryCategory = (from c in _db.Categories
where c.CategoryID == project.PrimaryCategory
select c.Name)
let categories = (from c in _db.ProjectCategories
join pc in _db.Projects_ProjectCategories on c.ProjectCategoryID equals pc.ProjectCategoryID
where pc.ProjectID == project.ProjectID
select c.Name)
let owner = (from o in _db.Owners
join po in _db.App_Projects_Owners on o.OwnerID equals po.OwnerID
where po.ProjectID == project.ProjectID
select new OwnerModel
{
Owner = o,
Project = project,
PrimaryCategory = primaryCategory.FirstOrDefault(),
Categories = categories.ToList()
})
select new
{
owner,
project
}).FirstOrDefault();
In there OwnerModel.Categories is a List of strings. I can't use ToList() in the query because it gives a materialization error. I've added a custom setter that takes the IQueryable, but that still makes another round trip to the database for every owner that the query returns.
So how are you supposed to assign basic lists in a subquery?
EDIT AND ANSWER (since Robert McKee lead me to the answer in his comment).
The answer is to use the group by clause like so
var data = (from project in _db.Projects
where project.Id == id
let primaryCategory = (from c in _db.Categories
where c.CategoryID == project.PrimaryCategory
select c.Name)
let categories = (from c in _db.ProjectCategories
join pc in _db.Projects_ProjectCategories on c.ProjectCategoryID equals pc.ProjectCategoryID
where pc.ProjectID == project.ProjectID
group c.Name by pc.ProjectCategoryID into x
select x.ToList())
let owner = (from o in _db.Owners
join po in _db.App_Projects_Owners on o.OwnerID equals po.OwnerID
where po.ProjectID == project.ProjectID
select new OwnerModel
{
Owner = o,
Project = project,
PrimaryCategory = primaryCategory.FirstOrDefault(),
Categories = categories
})
select new
{
owner,
project
}).FirstOrDefault();
Specifically note the bits involving
group c.Name by pc.ProjectCategoryID into x
select x.ToList()
It may seem counter-intuitive that it works like this until you dig into exactly what's going on. The method that I was calling above with categories.ToList() was trying to use the System.Collection.Generics ToList function, and that list didn't have any way to convert the expression to sql. However by using the group by clause I was creating a specialized Enumerable IGrouping and calling the ToList function on that. This function is able to be translated into a sql statement and thus doesn't throw the exception.
Learn something new every day.

Set up your navigation properties, and then your query becomes something like:
var data=db.Projects
.Include(p=>p.PrimaryCategory)
.Include(p=>p.Categories)
.Include(p=>p.Owner) // or .Include(p=>p.Owners) if projects can have multiple owners
.First(p=>p.Id == id);
As for child objects, you are looking for the group by clause or .GroupBy method.

Linq how to write a JOIN

Linq to EF, I'm using asp.net 4, EF 4 and C#.
Here are two ways I came up with to query my data. Ways A and C are working fine. B however needs to implement and additional WHERE statement (as "where c.ModeContent == "NA").
My question is:
Regarding this kind of join (outer join, I suppose) what is the best approach in term of performance?
Could you show me some code to implement additional WHERE statement in B?
Any way to improve this code?
Thanks for your time! :-)
// A
var queryContents = from c in context.CmsContents
where c.ModeContent == "NA" &&
!(from o in context.CmsContentsAssignedToes select o.ContentId)
.Contains(c.ContentId)
select c;
// B - I need to implent where c.ModeContent == "NA"
var result01 = from c in context.CmsContents
join d in context.CmsContentsAssignedToes on c.ContentId equals d.ContentId into g
where !g.Any()
select c;
// C
var result02 = context.CmsContents.Where(x => x.ModeContent == "NA").Where(item1 => context.CmsContentsAssignedToes.All(item2 => item1.ContentId != item2.ContentId));

Regarding query B you can apply the condition like this:
var result01 = from c in context.CmsContents where c.ModeContent == "NA"
join d in context.CmsContentsAssignedToes on c.ContentId equals d.ContentId into g
where !g.Any()
select c;

Your query will be far more readable and maintainable (and perform at least as well) if you use your association properties instead of join:
var result = from c in context.CmsContents
where c.ModeContent == "NA"
&& !c.AssignedToes.Any()
select c;
I'm guessing that the navigation on CmsContent to CmsContentsAssignedToes is called AssignedToes. Change the name in my query if it's actually called something else.
This query can be read out loud and you know exactly what it means. The join versions you have to think about.

How to Join 2 Generic IEnumerators

I'm wondering if its possible to join together IEnumerable's.
Basically I have a bunch of users and need to get their content from the database so I can search and page through it.
I'm using LINQ to SQL, my code at the moment it:
public IEnumerable<content> allcontent;
//Get users friends
IEnumerable<relationship> friends = from f in db.relationships
where f.userId == int.Parse(userId)
select f;
IEnumerable<relationship> freindData = friends.ToList();
foreach (relationship r in freindData)
{
IEnumerable<content> content = from c in db.contents
where c.userId == r.userId
orderby c.contentDate descending
select c;
// This is where I need to merge everything together
}
I hope that make some sense!
Matt

If I understand correctly what you are trying to do, why don't you try doing:
var result = from r in db.relationships
from c in db.contents
where r.userId == int.Parse(userId)
where c.userId == r.UserId
orderby c.contentDate descending
select new {
Relationship = r,
Content = c
}
This will give you an IEnumerable<T> where T is an anonymous type that has fields Relationship and Content.

If you know your users will have less than 2100 friends, you could send the keys from the data you already loaded back into the database easily:
List<int> friendIds = friendData
.Select(r => r.UserId)
.Distinct()
.ToList();
List<content> result = db.contents
.Where(c => friendIds.Contains(c.userId))
.ToList();
What happens here is that Linq translates each Id into a parameter and then builds an IN clause to do the filtering. 2100 is the maximum number of parameters that SQL server will accept... if you have more than 2100 friends, you'll have to break the ID list up and combine (Concat) the result lists.
Or, if you want a more literal answer to your question - Concat is a method that combines 2 IEnumerables together by creating a new IEnumerable which returns the items from the first and then the items from the second.
IEnumerable<content> results = Enumerable.Empty<content>();
foreach (relationship r in friendData)
{
IEnumerable<content> content = GetData(r);
results = results.Concat(content);
}

If you're doing an INNER join, look at the .Intersect() extension method.

Which things are you merging?
There are two main options you could use: .SelectMany(...) or .Concat(...)