Let say I have the following classes:
Product { ID, Name }
Meta { ID, Object, Key, Value }
Category { ID, Name }
Relation {ID, ChildID, ParentID } (Child = Product, Parent = Category)
and some sample data:
Product:
ID Name
1 Chair
2 Table
Meta
ID Object Key Value
1 1 Color "Red"
2 1 Size "Large"
3 2 Color "Blue"
4 2 Size "Small"
Category
ID Name
1 Indoor
2 Outdoor
Relation
ID ChildID ParentID
1 1 1
2 1 2
3 2 1
Can we use Distinct and Group by to produce the following format (ProductDetail)
ID=1,
Name=Chair,
Parent=
{
{ ID=1, Name="Indoor" },
{ ID=2, Name="Outdoor" }
},
Properties { Color="Red", Size="Large" }
ID=2,
Name=Table,
Parent=
{
{ ID=1, Name="Indoor"}
},
Properties { Color = "Blue", Size = "Small" }
which we can get the "Color" value of the first item by using
ProductDetails[0].Properties.Color
Any helps would be appreciated!
No, you can't do this based on what you've said - because "Color" and "Size" are part of the data, rather than part of the model. They're only known at execution time, so unless you use dynamic typing, you're not going to be able to access it by Properties.Color.
You could, however, use Properties["Color"] potentially:
var query = from product in db.Products
join meta in db.Meta
on product.ID equals meta.Object
into properties
select new { Product = product,
Properties = properties.ToDictionary(m => m.Key,
m => m.Value) };
So for each product, you'll have a dictionary of properties. That works logically, but you may need to tweak it to get it to work in the entity framework - I don't know how well that supports ToDictionary.
EDIT: Okay, I'll leave the above up as the "ideal" solution, but if EF doesn't support ToDictionary, you'd have to do that part in-process:
var query = (from product in db.Products
join meta in db.Meta
on product.ID equals meta.Object
into properties
select new { product, properties })
.AsEnumerable()
.Select(p => new {
Product = p.product,
Properties = p.properties.ToDictionary(m => m.Key,
m => m.Value) });
I just came across this question while learning LINQ, but I wasn't satisfied that Jon's output matched the question (sorry Jon). The following code returns a List of anonymously-typed objects that better match the structure of your question:
var ProductDetails = (from p in Product
let Parents = from r in Relation
where r.ChildID == p.ID
join c in Category on r.ParentID equals c.ID
into RelationCategory
from rc in RelationCategory
select new
{
rc.ID,
rc.Name
}
join m in Meta on p.ID equals m.Object into ProductMeta
select new
{
p.ID,
p.Name,
Parent = Parents.ToList(),
ProductMeta
})
.AsEnumerable()
.Select(p => new
{
p.ID,
p.Name,
p.Parent,
Properties = p.ProductMeta
.ToDictionary(e => e.Key, e => e.Value)
}).ToList();
Credit goes mostly to Jon Skeet and the Visual Studio debugger ;)
I realise that you've probably moved on by now but hopefully this might help someone else looking to learn LINQ, as I was.
Related
A newbie asks...
Part 1
Suppose I have 3 classes (and their equivalent SQL tables) :
Product
{
int Id;
List<Keyword> Keywords;
List<Serial> Serials;
}
Keyword
{
int Id;
int ProductId; // FK to Product
string Name;
}
Serial
{
int Id;
int ProductId; // FK to Product
string SerialNumber;
}
When loading PRODUCT == 123, the we could do this:
item = db.Products.FirstOrDefault(p => p.Id == 123);
item.Keywords = db.Keywords.Where(p => p.ProductId == 123).ToList();
item.Serials = db.Serials.Where(p => p.ProductId == 123).ToList();
which is 3 SQL statements.
Or we could do this:
from product in db.Products.AsNoTracking()
join link1 in Db.Keywords.AsNoTracking()
on product.Id equals link1.ProductId into kwJoin
from keyword in kwJoin.DefaultIfEmpty()
join link2 in Db.Serials.AsNoTracking()
on product.Id equals link2.ProductId into serJoin
from serial in serJoin.DefaultIfEmpty()
where product.Id == 123
select new { product, keyword, serial };
which gives 1 SQL statement but produces far too many rows (number of keywords x number of serials) that need to be coalesced together
Both seem less than efficient. Is there a better way?
Part 2
As another question, but using the same example, when we have a join like so:
from product in db.Products.AsNoTracking()
join link1 in Db.Keywords.AsNoTracking()
on product.Id equals link1.ProductId into kwJoin
from keyword in kwJoin.DefaultIfEmpty()
select new { product, keyword };
Is there a way to assign the keywords directly in the product, in select statement?
select new { product, product.Keywords = keyword };
Thanks for any help!
If the FKs exist, depending on how you have setup your DB context, the properties will automatically be fetched. No joins required. Part 1 query is simple as it has a filter. Part 2 might have issues depending on how many records needs to be fetched from the database. You can map the fields to anonymous objects(or DTOs) after the fact that you have keyword objects for each product in the list.
Part 1
item = db.Products
.Include(p=>p.Keywords)
.Include(s=>s.Serials)
.Where(p => p.Id == 123)
.FirstOrDefault();
Part 2
products = db.Products.Include(p=>p.Keywords).ToList();
Given the following code:
var persons = context.PERSONs.Select(
x =>
new
{
personId = x.PERSON_ID,
personName = x.PERSON_NAME,
items = x.ITEMs.Select(
y =>
new
{
itemID = y.ITEM_ID,
itemName = y.ITEM_NAME,
properties = y.PROPERTies.Select(
z =>
new
{
z.PROPERTY_ID,
z.PROPERTY_NAME
}
)
}
)
}
).ToList();
How can I avoid select n + 1 problems with it? Tried .Include("ITEMs").Include("ITEMs.PROPERTies") but it didn't help. Would expect a single query with 2 left outer joins.
Note - would like a generic answer because I'm working on an OData background where it's hard to craft queries for each entity by hand
-edit-
Database: MS SQL Server
Entity Framework version: 6
Can confirm that all properties are simple mapped properties (ints and strings actullay, no functions nor computed values)
Given a table of players (users) with several fields. One of this is their rating with respect other players.
I'd like to implement via LINQ following SQL query:
SELECT p.*,
(select COUNT(*) from Users where (Rating > p.Rating)) as Rank
FROM Users as p
ORDER BY p.Rating DESC
In other words, last field (RANK) should give the rank of each user with respect the others:
Id Username ... Rating Rank
43 player41 ... 1002,333 0
99 player97 ... 1002 1
202 player200 ... 1002 1
53 player51 ... 1000,667 2
168 player166 ... 1000,667 2
56 player54 ... 1000 3
32 player30 ... 999,342 4
This attempt does not work:
var q = from u in Users
orderby u.Rating descending
group u by u.Id into g
select new
{
MyKey = g.Key,
User = g.First(),
cnt = Users.Count(uu => uu.Rating > g.First().Rating) + 1
};
Just for your knowledge, note that the table Users is mapped to a EF entity named User with a 'NotMapped' int? field named Rank where I'd like to manually copy the rank:
class User {
...
[NotMapped]
public virtual int? Rank { get; internal set; }
}
You'll want something like:
var rankedUsers = db.Users
.Select(user => new
{
User = user,
Rank = db.Users.Count(innerQueryUser => innerQueryUser.Rating > user.Rating)
})
.ToList();
Which will give you a list of users and their Rank as an anonymous type.
You'll then be able to do something like:
List<User> users = rankedUsers.Select(rankedUser=>
{
rankedUser.User.Rank = rankedUser.Rank;
return rankedUser.User;
})
.ToList();
Try this:
var q = (from u in Users
select new
{
UserId = u.Id,
UserObj = u,
Rank = (from u1 in Users where u1.Rating>u.Rating select u1).Count()
}).ToList();
I've got an EDM with two tables Product and ProductCategory with a many-to-many relationship between both.
What I'm currently trying to do is to build a dynamic query to select the products related to the categories the user has selected through the UI.
In short I should build a query like the following but based one or more categories that I don't know at compile time.
var productQuery = context.Product.Where
(p => p.ProductCategories.Any(c => c.CategoryId == id1 ||
c.CategoryId == id2 || ...));
I've read a lot of things and I'm actually very new to linq so I really don't know where to start.
What is the best approach to do such queries ?
Thank you for your time.
var ids = new [] { id1, id2, // ...
var productQuery = context.Product.Where(
p => p.ProductCategories.Any(c => ids.Contains(c.CategoryId)));
I have a People table, I want a method that return one Person by id, with numbers of Cars, number of houses, etc.
I need to Load this tables together? I see SQL generated by EF, is a monster.
Ex:
public Person Get()
{
return context.People.Include("Cars").Include("Houses").Where(x=> x.Id = 1).First();
}
My view use this:
Name: <%= people.Name%>
Cars: <%= people.Cars.Count%>
Houses: <%= people.Houses.Count%>
You can do this
var result = (
from p in ctx.People
where p.Id == 1
select new {
Person = p,
Cars = p.Cars.Count(),
Houses = p.Houses.Count()
}).FirstOrDefault();
Which will just bring back the counts.