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
Related
How can i dynamically transform an SQL-Query?
I know there is a Select.getSelect(), but how can i add fields in the select-query?
Use-case: for a Rest-Query i have a lot of paginated resources and i have an abstraction to create the paginated-query. It takes the SelectConditionStep and adds the rest, depending on additional parameters. It works really well for simple queries, but for queries containing joins a little bit of transformation of the query would required. (Mainly because i can't naively limit the number results, since the join can be a one to many relationship)
The easiest way is to keep a List<Field<?>> where you add the fields for your select() clause, and then create the Select statement only when you actually execute it, instead of passing a Select object around. Example:
List<Field<?>> fields = new ArrayList<>();
// Just some examples:
fields.addAll(getDefaultFields());
fields.addAll(getFieldsFromUI());
fields.addAll(getCalculatedFields());
// Much later on, you finally create the statement:
DSL.using(configuration)
.select(fields)
.from(...)
.fetch();
I'm looking for a clean way to write this Linq query.
Basically I have a collection of objects with id's, then using nhibernate and Linq, I need to check if the nhibernate entity has a subclass collection where all id's in object collection exist in the nhibernate subclass collection.
If there was just one item this would work:
var objectImCheckingAgainst = ... //irrelevant
where Obj.SubObj.Any(a => a.id == objectImCheckingAgainst.Id)
Now I want to instead somehow pass a list of objectImCheckingAgainst and return true only if the Obj.SubObj collection contains all items in list of objectImCheckingAgainst based on Id.
I like to use GroupJoin for this.
return objectImCheckingAgainst.GroupJoin(Obj.SubObj,
a => a.Id,
b => b.id,
(a, b) => b.Any())
.All(c => c);
I believe this query should be more or less self-explanatory, but essentially, this joins the two collections using their respective ids as keys, then groups those results. Then for each of those groupings, it determines whether any matches exist. Finally, it ensures that all groupings had matches.
A useful alternative that I sometimes use is .Count() == 1 instead of the .Any(). Obviously, the difference there is whether you want to support multiple elements with the same id matching. From your description, it sounded like that either doesn't matter or is enforced by another means. But that's an easy swap, either way.
An important concept in GroupJoin that I know is relevant, but may or may not be obvious, is that the first enumerable (which is to say, the first argument to the extension method, or objectImCheckingAgainst in this example) will have all its elements included in the result, but the second one may or may not. It's not like Join, where the ordering is irrelevant. If you're used to SQL, these are the elementary beginnings of a LEFT OUTER JOIN.
Another way you could accomplish this, somewhat more simply but not as efficiently, would be to simply nest the queries:
return objectImCheckingAgainst.All(c => Obj.SubObj.Any(x => x.id == c.Id));
I say this because it's pretty similar to the example you provided.
I don't have any experience with NHibernate, but I know many ORMs (I believe EF included) will map this to SQL, so efficiency may or may not be a concern. But in general, I like to write LINQ as close to par as I can so it works as well in memory as against a database, so I'd go with the first one I mentioned.
I'm not well versed in LINQ-to-NHibernate but when using LINQ against any SQL backen it's always important to keep an eye on the generated SQL. I think this where clause...
where Obj.SubObj.All(a => idList.Contains(a.id))
...will produce the best SQL (having an IN statement).
idList is a list of Ids extracted from the list of objectImCheckingAgainst objects.
I'm having a problem joining two LINQ queries.
Currently, my (original) code looks like this
s.AnimalTypes.Sort((x, y) => string.Compare(x.Type, y.Type));
What I'm needing to do is change this based on a date, then select all data past that date, so I have
s.AnimalTypes.Select(t=>t.DateChanged > dateIn).ToList()
s.AnimalTypes.Sort((…
This doesn't look right as it's not sorting the data selected, rather sorting everything in s.AnimalTypes.
Is there a way to concatenate the two LINQ lines? I've tried
s.AnimalTypes.Select(t=>t.DateChanged > dateIn).ToList().Sort((…
but this gives me an error on the Sort section.
Is there a simple way to do this? I've looked around and Grouo and OrderBy keep cropping up, but I'm not sure these are what I need here.
Thanks
From your description, I believe you want something like:
var results = s.AnimalTypes.Where(t => t.DateChanged > dateIn).OrderBy(t => t.Type);
You can call ToList() to convert to a List<T> at the end if required.
There are a couple of fundamental concepts I believe you are missing here -
First, unlike List<T>.Sort, the LINQ extension methods don't change the original collections, but rather return a new IEnumerable<T> with the filtered or sorted results. This means you always need to assign something to the return value (hence my var results = above).
Second, Select performs a mapping operation - transforming the data from one form to another. For example, you could use it to extract out the DateChanged (Select(t => t.DateChanged)), but this would give you an enumeration of dates, not the original animal types. In order to filter or restrict the list with a predicate (criteria), you'd use Where instead.
Finally, you can use OrderBy to reorder the resulting enumerable.
You are using Select when you actually want to use Where.
Select is a projection from one a collection of one type into another type - you won't increase or reduce the number of items in a collection using Select, but you can instead select each object's name or some other property.
Where is what you would use to filter a collection based on a boolean predicate.
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 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.