I have a simple LINQ EF query below using the method syntax. I'm using my Include statement to join four tables: Event and Doc are the two main tables, EventDoc is a many-to-many link table, and DocUsage is a lookup table.
My challenge is that I'd like to shape my results by only selecting specific columns from each of the four tables. But, the compiler is giving a compiler is giving me the following error:
'System.Data.Objects.DataClasses.EntityCollection does not contain a definition for "Doc' and no extension method 'Doc' accepting a first argument of type 'System.Data.Objects.DataClasses.EntityCollection' could be found.
I'm sure this is something easy but I'm not figuring it out. I haven't been able to find an example of someone using the multi-table include but also shaping the projection.
Thx,Mark
var qry= context.Event
.Include("EventDoc.Doc.DocUsage")
.Select(n => new
{
n.EventDate,
n.EventDoc.Doc.Filename, //<=COMPILER ERROR HERE
n.EventDoc.Doc.DocUsage.Usage
})
.ToList();
EventDoc ed;
Doc d = ed.Doc; //<=NO COMPILER ERROR SO I KNOW MY MODEL'S CORRECT
DocUsage du = d.DocUsage;
Very difficult to know what is going on without a screencap of your model, including the navigational properties on each entity.
But if your saying it's a many-to-many between Event and Doc (with EventDoc being the join table), and assuming your join table has nothing but the FK's and therefore doesn't need to be mapped, then shouldn't a single Event have many Doc's?
This query:
var query = ctx.Event.Include("EventDoc.Doc");
Would imply (based on the lack of pluralization): a single Event has a single EventDoc which has a single Doc.
But shouldn't that be: a single Event has a single EventDoc which has many Doc's.
Therefore your projection doesn't really make sense. Your trying to project to an anonymous type, with EventDate and Filename for a single Doc, but an Event has many Docs.
Maybe a projection like this would be more suitable:
var query = ctx.Event.Include("EventDoc.Docs.DocUsage")
.Select(x => new
{
EventDate = x.EventDate,
DocsForEvent = x.EventDocs.Docs
}).ToList();
And for that you work you need to fix up your model. Im surprised it even validates/compiles.
Either your model is wrong or your description of the database cardinalities in your question is. :)
Of course, i could be completely misunderstanding your database and/or model - so if i am let me know and i'll remove this answer.
Related
I have a query that fetches books, I'm new to LINQ so I don't know the syntax:
var books = (from book in db.Books
join borrow in db.Borrows on book equals borrow.Book
select new BookDTO { Title = book.Title,
Borrows = book.Borrows.ToList() }).ToList(); // book.Borrows.ToList() <- use dto's instead
How can I select Book.Borrows as a list of objects (BorrowDTO's)? Is there something like Borrows = new List<BorrowDTO>(book.Borrows)
You can use .Select() to project the list into a different type. So instead of this:
Borrows = book.Borrows.ToList()
you would have something like this:
Borrows = book.Borrows.Select(b => new BorrowDTO { /* properties here */ }).ToList()
Note that, depending on your data source, there may be more efficient ways to approach selecting your data. If you're pulling directly from LINQ To Entities then you may run into problems trying to materialize a type within the query that isn't known to the DB, or any other operation that can't be translated into SQL. It's also not necessarily wise to toss in a bunch of .ToList() operations without a specific purpose.
But that's all theoretical at this point in the question. Based on the code shown and on LINQ syntax itself, you can select from a list just fine. (I'd even recomment using the extension method syntax more than the query syntax that you currently use. Personal preference of course, but I find it easier and more intuitive to build nested operations like this. Though you can just as well use the from ... select ... syntax after Borrows =, I would imagine.)
Just select book.Borrows instead of creating a new temporal object.
That query is going to return a IEnumerable of the Borrows type; and you'll be able to iterate through it and convert it into a List if you please
Is there a way to get the IQueryable object that the LinqDataSource has used to retrieve data? I thought that it might be possible from the selected event, but it doesn't appear to be.
Each row in my table has a category field, and I want to determine how many rows there are per category in the results.
I should also note that I'm using a DataPager, so not all of the rows are being returned. That's why I want to get the IQueryable, so that I can do something like
int count = query.Where(i => i.Category == "Category1").Count();
Use the QueryCreated event. QueryCreatedEventArgs has a Query property that contains the IQueryable.
The event is raised after the original LINQ query is created, and contains the query expression before to it is sent to the database, without the ordering and paging parameters.
There's no "Selected" event in IQueryable. Furthermore, if you're filtering your data on the server, there'd be no way you can access it, even if the API exposed it, but to answer a part of the question, let's say you have category -> product where each category has many products and you want the count of the products in each category. It'd be a simple LINQ query:
var query = GetListOfCategories();
var categoryCount = query.Select(c => c.Products).Count();
Again, depending on the type of object GetListOfCategories return, you might end up having correct value for all the entries, or just the ones that are loaded and are in memory, but that's the different between Linq-to-Objects (in memory) and Linq-to-other data sources (lazy loaded).
suppose my LINQ query is like
var qry = from c in nwEntitiesContext.CategorySet.AsEnumerable()
let products = this.GetProducts().WithCategoryID(c.CategoryID)
select new Model.Category
{
ID = c.CategoryID,
Name = c.CategoryName,
Products = new Model.LazyList<Core.Model.Product>(products)
};
return qry.AsQueryable();
i just want to know what query it will generate at runtime....how to see what query it is generating from VS2010 IDE when we run the code in debug mode....guide me step by step.
There is not much to see here - it will just select all fields from the Category table since you call AsEnumerable thus fetching all the data from the Category table into memory. After that you are in object space. Well, depending on what this.GetProducts() does - and my guess it makes another EF query fetching the results into memory. If that's the case, I would strongly recommend you to post another question with this code and the code of your GetProducts method so that we can take a look and rewrite this in a more optimal way. (Apart from this, you are projecting onto a mapped entity Model.Category which again won't (and should not) work with Linq-to-Entities.)
Before reading into your query I was going to recommend doing something like this:
string sqlQueryString = ((ObjectQuery)qry).ToTraceString();
But that won't work since you are mixing Linq-to-Entities with Linq-to-objects and you will actually have several queries executed in case GetProducts queries EF. You can separate the part with your EF query and see the SQL like this though:
string sqlString = nwEntitiesContext.CategorySet.ToTraceString();
but as I mentioned earlier - that would just select everything from the Categories table.
In your case (unless you rewrite your code in a drastic way), you actually want to see what queries are run against the DB when you execute the code and enumerate the results of the queries. See this question:
exact sql query executed by Entity Framework
Your choices are SQL Server Profiler and Entity Framework Profiler. You can also try out LinqPad, but in general I still recommend you to describe what your queries are doing in more detail (and most probably rewrite them in a more optimal way before proceeding).
Try Linqpad
This will produce SELECT * FROM Categories. Nothing more. Once you call AsEnumerable you are in Linq-to-objects and there is no way to get back to Linq-to-entities (AsQueryable doesn't do that).
If you want to see what query is generated use SQL Profiler or any method described in this article.
I have a LINQ query mapped with the Entity Framework that looks something like this:
image = this.Context.ImageSet
.Where(n => n.ImageId == imageId)
.Where(n => n.Albums.IsPublic == true)
.Single();
This returns a single image object and works as intended.
However, this query returns all the properties of my Image table in the DB.
Under normal circumstances, this would be fine but these images contain a lot of binary data that takes a very long time to return.
Basically, in it current state my linq query is doing:
Select ImageId, Name, Data
From Images
...
But I need a query that does this instread:
Select ImageId, Name
From Images
...
Notice i want to load everything except the Data. (I can get this data on a second async pass)
Unfortunately, if using LINQ to SQL, there is no optimal solution.
You have 3 options:
You return the Entity, with Context tracking and all, in this case Image, with all fields
You choose your fields and return an anonymous type
You choose your fields and return a strongly typed custom class, but you lose tracking, if thats what you want.
I love LINQ to SQL, but thats the way it is.
My only solution for you would be to restructure your DataBase, and move all the large Data into a separate table, and link to it from the Image table.
This way when returning Image you'd only return a key in the new DataID field, and then you could access that heavier Data when and if you needed it.
cheers
This will create a new image with only those fields set. When you go back to get the Data for the images you select, I'd suggest going ahead and getting the full dataset instead of trying to merge it with the existing id/name data. The id/name fields are presumably small relative to the data and the code will be much simpler than trying to do the merge. Also, it may not be necessary to actually construct an Image object, using an anonymous type might suit your purposes just as well.
image = this.Context.ImageSet
.Where(n => n.ImageId == imageId)
.Where(n => n.Albums.IsPublic == true)
.Select( n => new Image { ImageId = n.ImageId, Name = n.Name }
.Single();
[If using Linq 2 SQL] Within the DBML designer, there is an option to make individual table columns delay-loaded. Set this to true for your large binary field. Then, that data is not loaded until it is actually used.
[Question for you all: Does anyone know if the entity frameworks support delayed loaded varbinary/varchar's in MSVS 2010? ]
Solution #2 (for entity framework or linq 2 sql):
Create a view of the table that includes only the primary key and the varchar(max)/varbinary(max). Map that into EF.
Within your Entity Framework designer, delete the varbinary(max)/varchar(max) property from the table definition (leaving it defined only in the view). This should exclude the field from read/write operations to that table, though you might verify that with the logger.
Generally you'll access the data through the table that excludes the data blob. When you need the blob, you load a row from the view. I'm not sure if you'll be able to write to the view, I'm not sure how you would do writes. You may be able to write to the view, or you may need to write a stored procedure, or you can bust out a DBML file for the one table.
You cannot do it with LINQ at least for now...
The best approach I know is create View for the table you need without large fields and use LINQ with that View.
Alternatively you could use the select new in the query expression...
var image =
(
from i in db.ImageSet
where i.ImageId == imageId && i.Albums.IsPublic
select new
{
ImageId = i.ImageId,
Name = i.Name
}
).Single()
The LINQ query expressions actually get converted to the Lambda expression at compile time, but I prefair using the query expression generally because i find it more readable and understandable.
Thanks :)
So I'm extremely new to Linq in .Net 3.5 and have a question. I use to use a custom class that would handle the following results from a store procedure:
Set 1: ID Name Age
Set 2: ID Address City
Set 3: ID Product Price
With my custom class, I would have received back from the database a single DataSet with 3 DataTables inside of it with columns based on what was returned from the DB.
My question is how to I achive this with LINQ? I'm going to need to hit the database 1 time and return multiple sets with different types of data in it.
Also, how would I use LINQ to return a dynamic amount of sets depending on the parameters (could get 1 set back, could get N amount back)?
I've looked at this article, but didn't find anything explaining multiple sets (just a single set that could be dynamic or a single scalar value and a single set).
Any articles/comments will help.
Thanks
I believe this is what you're looking for
Linq to SQL Stored Procedures with Multiple Results - IMultipleResults
I'm not very familiar with LINQ myself but here is MSDN's site on LINQ Samples that might be able to help you out.
EDIT: I apologize, I somehow missed the title where you mentioned you wanted help using LINQ with Stored Procedures, my below answer does not address that at all and unfortunately I haven't had the need to use sprocs with LINQ so I'm unsure if my below answer will help.
LINQ to SQL is able hydrate multiple sets of data into a object graph while hitting the database once. However, I don't think LINQ is going to achieve what you ultimately want -- which as far as I can tell is a completely dynamic set of data that is defined outside of the query itself. Perhaps I am misunderstanding the question, maybe it would help if you provide some sample code that your existing application is using?
Here is a quick example of how I could hydrate a anonymous type with a single database call, maybe it will help:
var query = from p in db.Products
select new
{
Product = p,
NumberOfOrders = p.Orders.Count(),
LastOrderDate = p.Orders.OrderByDescending().Take(1).Select(o => o.OrderDate),
Orders = p.Orders
};