I have two table Company and Employee. And there relation is Company(one) - Employee(many).
And I want to concatenate all the Employees' name to a string and output.
I know I can write such a query :
String names = "";
foreach(var emp in Company.Employee)
{
names += emp.name;
}
But If I use this mean, I would load all the employee records into memory, and then make comparison, which is a waste of time and memory, and would low the performance.
So in linq, is it possible to craft such a query that can return all concatenated names within a single SQL ?
Thanks in advance ! Any recommendations will be greatly appreciated !
var employeeNames = context
.Company
.Where(c => c.Id = 0xF00)
.SelectMany(c => c.Employee)
.Select(e => e.Name)
.ToArray();
var result = String.Join(" ", employeeNames);
You can vary the part of the query selecting the company a bit depending on the exact semantics and Entity Framework version. In .NET 4.0 the Entity Framework supports Single(). If you don't care about minor semantic differences you can use First() instead of SelectMany() prior to .NET 4.0.
This is a simplified variation of Daniel's but I think it should work (making assumptions about the schema)
var employeeNames = (from e in context.Employee
where e.CompanyId = 0xF00
select e.Name)
.ToArray();
var result = String.Join(" ", employeeNames);
Related
I want to return a list of objects stored in a database using LINQ queries.
I tried the following
public BO.Hotel getHotels()
{
TripBagEntities db = new TripBagEntities();
var hotels = (from m in db.HotelEntities
where m.id < 10
select m).ToList().First();
return Mapper.ToHotelObject(hotels);
}
This returns only the first item in the list. How can I return the entire list?
Thanks in advance
Firstly, as per the comment, you should understand exactly what each line of your existing code does first. (You should also try to follow .NET naming conventions.) If you're guessing around which bit of your code does what, it would be a good idea to read a good tutorial geared towards the LINQ provider you're using (Entity Framework?).
We don't really know what Mapper.ToHotelObject does, or whether there's already a method for converting a whole sequence. This should work though:
public List<BO.Hotel> GetHotels()
{
// Note: you may want a using statement here...
TripBagEntities db = new TripBagEntities();
var hotels = db.HotelEntities
.Where(m => m.id < 10)
.AsEnumerable()
.Select(Mapper.ToHotelObject)
.ToList();
}
Or if the method group conversion doesn't work:
public List<BO.Hotel> GetHotels()
{
// Note: you may want a using statement here...
TripBagEntities db = new TripBagEntities();
var hotels = db.HotelEntities
.Where(m => m.id < 10)
.AsEnumerable()
.Select(m => Mapper.ToHotelObject(m))
.ToList();
}
Note that I've used "dot notation" for the whole query, as it makes life simpler when you're using things like AsEnumerable and ToList, and your query expression wasn't complicated anyway.
The AsEnumerable call "shifts" the query into LINQ to Objects, so that the query-to-SQL translation part doesn't need to try to convert Mapper.ToHotelObject into SQL, which I assume would fail.
You need to make your function return List<BO.Hotel> instead of a Hotel, and adjust your query and Mapper function accordingly.
public List<BO.Hotel> getHotels()
{
TripBagEntities db = new TripBagEntities();
return (from m in db.HotelEntities
where m.id < 10
select Mapper.ToHotelObject(m)).ToList();
}
I have the following linq-to-entities query with 2 joined tables that I would like to add pagination to:
IQueryable<ProductInventory> data = from inventory in objContext.ProductInventory
join variant in objContext.Variants
on inventory.VariantId equals variant.id
where inventory.ProductId == productId
where inventory.StoreId == storeId
orderby variant.SortOrder
select inventory;
I realize I need to use the .Join() extension method and then call .OrderBy().Skip().Take() to do this, I am just gettting tripped up on the syntax of Join() and can't seem to find any examples (either online or in books).
NOTE: The reason I am joining the tables is to do the sorting. If there is a better way to sort based on a value in a related table than join, please include it in your answer.
2 Possible Solutions
I guess this one is just a matter of readability, but both of these will work and are semantically identical.
1
IQueryable<ProductInventory> data = objContext.ProductInventory
.Where(y => y.ProductId == productId)
.Where(y => y.StoreId == storeId)
.Join(objContext.Variants,
pi => pi.VariantId,
v => v.id,
(pi, v) => new { Inventory = pi, Variant = v })
.OrderBy(y => y.Variant.SortOrder)
.Skip(skip)
.Take(take)
.Select(x => x.Inventory);
2
var query = from inventory in objContext.ProductInventory
where inventory.ProductId == productId
where inventory.StoreId == storeId
join variant in objContext.Variants
on inventory.VariantId equals variant.id
orderby variant.SortOrder
select inventory;
var paged = query.Skip(skip).Take(take);
Kudos to Khumesh and Pravin for helping with this. Thanks to the rest for contributing.
Define the join in your mapping, and then use it. You really don't get anything by using the Join method - instead, use the Include method. It's much nicer.
var data = objContext.ProductInventory.Include("Variant")
.Where(i => i.ProductId == productId && i.StoreId == storeId)
.OrderBy(j => j.Variant.SortOrder)
.Skip(x)
.Take(y);
Add following line to your query
var pagedQuery = data.Skip(PageIndex * PageSize).Take(PageSize);
The data variable is IQueryable, so you can put add skip & take method on it. And if you have relationship between Product & Variant, you donot really require to have join explicitly, you can refer the variant something like this
IQueryable<ProductInventory> data =
from inventory in objContext.ProductInventory
where inventory.ProductId == productId && inventory.StoreId == storeId
orderby inventory.variant.SortOrder
select new()
{
property1 = inventory.Variant.VariantId,
//rest of the properties go here
}
pagedQuery = data.Skip(PageIndex * PageSize).Take(PageSize);
My answer here based on the answer that is marked as true
but here I add a new best practice of the code above
var data= (from c in db.Categorie.AsQueryable().Join(db.CategoryMap,
cat=> cat.CategoryId, catmap => catmap.ChildCategoryId,
cat, catmap) => new { Category = cat, CategoryMap = catmap })
select (c => c.Category)
this is the best practice to use the Linq to entity because when you add AsQueryable() to your code; system will converts a generic System.Collections.Generic.IEnumerable to a generic System.Linq.IQueryable which is better for .Net engine to build this query at run time
thank you Mr. Khumesh Kumawat
You would simply use your Skip(itemsInPage * pageNo).Take(itemsInPage) to do paging.
I'm trying to add a column to the following LINQ expression. I want the column to contain a string concatenation of a text value in a many table called WasteItems. The join would be on "Waste.WasteId = WasteItem.WasteId". My problem is I need to display in a single dynamic column a string such as "EW (5); EX (3)" if there was 8 records in WasteItem and the column containing the 2 character string was called WasteItem.EWC. Hope that makes sense, there must be an efficient way since I realise LINQ is very powerfull. I'm new to it and not sure how to start or go about this:
return from waste in this._db.Wastes
where (from u in _db.UsersToSites.Where(p => p.UserId == userId && p.SystemTypeId == SystemType.W)
select u.SiteId)
.Contains(waste.SiteId)
orderby waste.Entered descending select waste;
THANKS IN ADVANCE
Something like this should do:
wastes.GroupJoin(db.WasteItems, w => w.WastId, wi => wi.WasteId, (w,wi) => new { w, wi })
.AsEnumerable()
.Select(x => new
{
x.w.Name,
Items = string.Join(", ", x.wi.GroupBy(wi => wi.EWC).Select(g => string.Format("{0} ({1})", g.Key, g.Count())))
})
Where wastes is the result from your query. The AsEnumerable() is necessary because Entity Framework can not handle string.Join, so that part must be dealt with in memory.
I could not check the syntax, obviously, but at least it may show you the way to go.
I'm trying to get Entity Framework to select an object and filter its collection at the same time. I have a JobSeries object which has a collection of jobs, what I need to do is select a jobseries by ID and filter all the jobs by SendDate but I can't believe how difficult this simple query is!
This is the basic query which works:
var q = from c in KnowledgeStoreEntities.JobSeries
.Include("Jobs.Company")
.Include("Jobs.Status")
.Include("Category")
.Include("Category1")
where c.Id == jobSeriesId
select c;
Any help would be appreciated, I've been trying to find something in google and what I want to do is here:http://blogs.msdn.com/bethmassi/archive/2009/07/16/filtering-entity-framework-collections-in-master-detail-forms.aspx
It's in VB.NET though and I couldn't convert it to C#.
EDIT: I've tried this now and it doesn't work!:
var q = from c in KnowledgeStoreEntities.JobSeries
.Include("Jobs")
.Include("Jobs.Company")
.Include("Jobs.Status")
.Include("Category")
.Include("Category1")
where (c.Id == jobSeriesId & c.Jobs.Any(J => J.ArtworkId == "13"))
select c;
Thanks
Dan
Include can introduce performance problems. Lazy loading is guaranteed to introduce performance problems. Projection is cheap and easy:
var q = from c in KnowledgeStoreEntities.JobSeries
where c.Id == jobSeriesId
select new
{
SeriesName = c.Name,
Jobs = from j in c.Jobs
where j.SendDate == sendDate
select new
{
Name = j.Name
}
CategoryName = c.Category.Name
};
Obviously, I'm guessing at the names. But note:
Filtering works.
SQL is much simpler.
No untyped strings anywhere.
You always get the data you need, without having to specify it in two places (Include and elsewhere).
No bandwith penalties for retrieving columns you don't need.
Free performance boost in EF 4.
The key is to think in LINQ, rather than in SQL or in materializing entire entities for no good reason as you would with older ORMs.
I've long given up on .Include() and implemented Lazy loading for Entity Framework
In another posting: Does Linq-To-Sql support composable queries there was discussion on how to compose/concat where clauses dynamically. This appears to be done with an "AND" (i.e. the first where clause and the second where clause are joined by an AND). What I am wondering is if there is a way to compose Linq queries with an OR.
Example:
var people = from p in Person
where p.age < 18
select p
var otherPeople = from p in people
where p.firstName equals "Daniel"
select p
This gives people with a first name of "Daniel" and that are under 18. I'm looking for the syntax to join these to find people who have a first name of "Daniel" or are under 18.
Note: I am using ADO.net Data Services so I do not have .Contains() available to me.
EDIT: The Union Suggestion (by Garry Shutler) is exactly what I am looking for functionality-wise. I did run into two possible issues with it:
It looks like it would make multiple database hits if I was to do a third condition (union seems to take an IEnumerable as its parameter) - I was hoping to build up multiple AND and OR statements in code and then execute one request.
Union is not supported by ADO.Net Data Services (very disappointing)
Is what you want as simple as:
var people = from p in Person
where p.age < 18 || p.firstName == "Daniel"
select p;
or have you just given a simple example?
In which case you can use:
var under18 = from p in Person
where p.age < 18
select p;
var daniels = from p in Person
where p.firstName == "Daniel"
select p;
var combined = under18.Union(daniels);
LinqToSql may be intelligent enough to convert that to an OR but I'm not so sure.
What about using PredicateBuilder by Joe Albahari?
var predicate = PredicateBuilder.False<Person>();
predicate = predicate.Or(p => p.age < 18);
predicate = predicate.Or(p => p.firstName == "Daniel");
var query = Person.Where(predicate);
The predicate option is the way to go. The Union option DOES NOT build good sql. Reference http://social.msdn.microsoft.com/forums/en-US/linqprojectgeneral/thread/925b245d-5529-4a64-8cd4-4bc83ee6fe7a/
I wrote about how to achieve queries which search for a key value within a set on my blog .
Here are the relevant links.
Contains Operations in ADO.NET Data Services Part I
Contains Operations in ADO.NET Data Services Part II
Using this , you can write queries which look like this
//The set in which we have to search for a match
List<string> citiesIWillVisit = new List<string>() {"London","Berlin","Prague"};
var customersAround = nwContext.Customers
.IsIn<Customers>(citiesIWillVisit, c=> c.City);
foreach (Customers localCustomer in customersAround) {
System.Console.WriteLine(localCustomer.ContactName);
}