I want to create an Entity Object from a LinQ statement, but I don't want to load all its columns.
My ORDERS object has a lot of columns, but I just want to retrieve the REFERENCE and OPERATION columns so the SQL statement and result will be smaller.
This LinQ statement works properly and loads all my object attributes:
var orders = (from order in context.ORDERS
select order);
However the following statement fails to load only two properties of my object
var orders = (from order in context.ORDERS
select new ORDERS
{
REFERENCE = order.REFERENCE,
OPERATION = order.OPERATION
});
The error thrown is:
The entity or complex type
'ModelContextName.ORDERS' cannot be
constructed in a LINQ to Entities
query.
What is the problem? Isn't it possible to partially load an object this way?
Thank you in advance for your answers.
ANSWER
Ok I should thank you both Yakimych and Dean because I use both of your answers, and now I have:
var orders = (from order in context.ORDERS
select new
{
REFERENCE = order.REFERENCE,
OPERATION = order.OPERATION,
})
.AsEnumerable()
.Select(o =>
(ORDERS)new ORDERS
{
REFERENCE = o.REFERENCE,
OPERATION = o.OPERATION
}
).ToList().AsQueryable();
And I get exactly what I want, the SQL Statement is not perfect but it returns only the 2 columns I need (and another column which contains for every row "1" but I don't know why for the moment) –
I also tried to construct sub objects with this method and it works well.
No, you can't project onto a mapped object. You can use an anonymous type instead:
var orders = (from order in context.ORDERS
select new
{
REFERENCE = order.REFERENCE,
OPERATION = order.OPERATION
});
The problem with the above solution is that from the moment you call AsEnumerable(), the query will get executed on the database. In most of the cases, it will be fine. But if you work with some large database, fetching the whole table(or view) is probably not what you want. So, if we remove the AsEnumerable, we are back to square 1 with the following error:
The entity or complex type 'ModelContextName.ORDERS' cannot be constructed in a LINQ to Entities query.
I have been struggling with this problem for a whole day and here is what I found. I created an empty class inheriting from my entity class and performed the projection using this class.
public sealed class ProjectedORDERS : ORDERS {}
The projected query (using covariance feature):
IQueryable<ORDERS> orders = (from order in context.ORDERS
select new ProjectedORDERS
{
REFERENCE = order.REFERENCE,
OPERATION = order.OPERATION,
});
Voilà! You now have a projected query that will map to an entity and that will get executed only when you want to.
I think the issue is creating new entities within the query itself, so how about trying this:
context.ORDERS.ToList().Select(o => new ORDERS
{
REFERENCE = o.REFERENCE,
OPERATION = o.OPERATION
});
Related
I am using LinqPad to test my query. This query works when the LInqPad connection is to my database (LInq to SQL) but it does not work when I change the connection to use my Entity Framework 5 Model.dll. (Linq to Entity). This is in C#.
I have two tables called Plan and PlanDetails. Relationship is one Plan to many PlanDetails.
var q = from pd in PlanDetails
select new {
pd.PlanDetailID,
ThePlanName = (from p in this.Plans
where p.PlanID == pd.PlanID
select p.PlanName)
};
var results = q.ToList();
q.Dump(); //This is a linqpad method to output the result.
I get this error "NotSupportedException: Unable to create a constant value of type 'Domain.Data.Plan'. Only primitive types or enumeration types are supported in this context." Any ideas why this only works with Linq to SQL?
basically it means you are using some complex datatype inside the query for comparison.
in your case i suspect from p in this.Plans where p.PlanID == pd.PlanID is the culprit.
And it depends on DataProvider. It might work for Sql Data Provider, but not for SqlCE data Provider and so on.
what you should do is to convert your this.Plans collection into a primitive type collection containing only the Ids i.e.
var integers = PlanDetails.Plans.Select(s=>s.Id).ToList();
and then use this list inside.
var q = from pd in PlanDetails
select new {
pd.PlanDetailID,
ThePlanName = (from p in integers
where p == pd.PlanID
select pd.PlanName)
};
I got this error when i was trying to null check for a navigational property in the entity framework expression
I resolved it by not using the not null check in the expression and just using Any() function only.
protected Expression<Func<Entities.Employee, bool>> BriefShouldAppearInSearchResults(
IQueryable<Entities.Employee> briefs, string username)
{
var trimmedUsername = NameHelper.GetFormattedName(username);
Expression<Func<Entities.Employee, bool>> filterExpression = cse =>
cse.Employee.Cars.All(c =>
c.Employee.Cars!=null && <--Removing this line resolved my issue
c.Employee.Cars.Any(cur => cur.CarMake =="Benz")));
return filterExpression;
}
Hope this helps someone!
This is a Linqpad bug if you like (or a peculiarity). I found similar behaviour myself. Like me, you may find that your query works with an ObjectContext, but not a DbContext. (And it works in Visual Studio).
I think it has to do with Linqpad's inner structure. It adds MergeAs (AppendOnly) to collections and the context is a UserQuery, which probably contains some code that causes this bug.
This is confirmed by the fact that the code does work when you create a new context instance in the Linqpad code and run the query against this instance.
If the relationship already exists.
Why not simply say.
var q = from pd in PlanDetails
select new {
pd.PlanDetailID,
ThePlanName = pd.Plan.PlanName
};
Of course i'm assuming that every PlanDetail will belong to a Plan.
Update
To get better results from LinqPad you could tell it to use your own assembly (which contains your DbContext) instead of the default Datacontext it uses.
I have been trying for hours to get a Distinct to work for my code.
I am using EF 4.3, MVC3, Razor and trying to get a list downto product id and name.
When I run the Sql query against the DB, it's fine.
Sql Query is
SELECT DISTINCT [ProductId]
,[Product_Name]
FROM [dbo].[PRODUCT]
The only other column in that table is a country code so that's why a standard distinct() isn't working.
I have gone as far as creating an IEqualityComparer
Here is code:
public class DistinctProduct : IEqualityComparer<PRODUCT>
{
public bool Equals(PRODUCT x, PRODUCT y)
{
return x.ProductId.Equals(y.ProductId);
}
public int GetHashCode(PRODUCT obj)
{
return obj.ProductId.GetHashCode();
}
}
here is where I called it.
IEqualityComparer<PRODUCT> customComparer = new DistinctProduct();
IEnumerable<PRODUCT> y = db.PRODUCTs.Distinct(customComparer);
But when it hit's that Last line I get an error out of it stating...
LINQ to Entities does not recognize the method 'System.Linq.IQueryable`1[MyService.Models.PRODUCT] Distinct[PRODUCT](System.Linq.IQueryable`1[MyService.Models.PRODUCT], System.Collections.Generic.IEqualityComparer`1[MyService.Models.PRODUCT])' method, and this method cannot be translated into a store expression.
Can anyone tell me what I'm doing wrong?
Thanks,
David
Is there any reason you could just not use a distinct like the following?
var distinctProdcts = (from p in db.PRODUCTs
select new {
ProductId = p.ProductId,
Product_Name = p.ProductName
}).Distinct();
This would remove the country code from the query before you do the distinct.
Entity Framework is trying to translate your query to a SQL query. Obviously it does not know how to translate the IEqualityComparerer. I think the question is whether you want to do the Distinct in the datbase (in which case your client gets only filtered results) or you are OK with bringing all the data to the client and select distinct on the client. If you want the filtering to happen on the database side (which will make your app perform much better) and you want to be able to use different strategies for comparing you can come up with a code that builds distinct criteria on top of your query. If you are fine with bringing your data to the client (note that it can be a lot of data) you should be able just to do (.ToList() will trigger querying the database and materializing results):
IEnumerable<PRODUCT> y = db.PRODUCTs.ToList().Distinct(customComparer);
I have an Abstract type called Product, and five "Types" that inherit from Product in a table per type hierarchy fashion as below:
I want to get all of the information for all of the Products, including a smattering of properties from the different objects that inherit from products to project them into a new class for use in an MVC web page. My linq query is below:
//Return the required products
var model = from p in Product.Products
where p.archive == false && ((Prod_ID == 0) || (p.ID == Prod_ID))
select new SearchViewModel
{
ID = p.ID,
lend_name = p.Lender.lend_name,
pDes_rate = p.pDes_rate,
pDes_details = p.pDes_details,
pDes_totTerm = p.pDes_totTerm,
pDes_APR = p.pDes_APR,
pDes_revDesc = p.pDes_revDesc,
pMax_desc = p.pMax_desc,
dDipNeeded = p.dDipNeeded,
dAppNeeded = p.dAppNeeded,
CalcFields = new DAL.SearchCalcFields
{
pDes_type = p.pDes_type,
pDes_rate = p.pDes_rate,
pTFi_fixedRate = p.pTFi_fixedRate
}
}
The problem I have is accessing the p.pTFi_fixedRate, this is not returned with the Products collection of entities as it is in the super type of Fixed. How do I return the "super" type of Products (Fixed) properties using Linq and the Entity Framework. I actually need to return some fields from all the different supertypes (Disc, Track, etc) for use in calculations. Should I return these as separate Linq queries checking the type of "Product" that is returned?
This is a really good question. I've had a look in the Julie Lerman book and scouted around the internet and I can't see an elegant answer.
If it were me I would create a data transfer object will all the properties of the types and then have a separate query for each type and then union them all up. I would insert blanks into the DTO properies where the properties aren't relevant to that type. Then I would hope that the EF engine makes a reasonable stab at creating decent SQL.
Example
var results = (from p in context.Products.OfType<Disc>
select new ProductDTO {basefield1 = p.val1, discField=p.val2, fixedField=""})
.Union(
from p in context.Products.OfType<Fixed>
select new ProductDTO {basefield1 = p.val1, discField="", fixedField=p.val2});
But that can't be the best answer can it. Is there any others?
So Fixed is inherited from Product? If so, you should probably be querying for Fixed instead, and the Product properties will be pulled into it.
If you are just doing calculations and getting some totals or something, you might want to look at using a stored procedure. It will amount to fewer database calls and allow for much faster execution.
Well it depends on your model, but usually you need to do something like:
var model = from p in Product.Products.Include("SomeNavProperty")
.... (rest of query)
Where SomeNavProperty is the entity type that loads pTFi_fixedRate.
I am trying to bind distinct records to a dropdownlist. After I added distinct function of the linq query, it said "DataBinding: 'System.String' does not contain a property with the name 'Source'. " I can guarantee that that column name is 'Source'. Is that name lost when doing distinct search?
My backend code:
public IQueryable<string> GetAllSource()
{
PromotionDataContext dc = new PromotionDataContext(_connString);
var query = (from p in dc.Promotions
select p.Source).Distinct();
return query;
}
Frontend code:
PromotionDAL dal = new PromotionDAL();
ddl_Source.DataSource = dal.GetAllSource();
ddl_Source.DataTextField = "Source";
ddl_Source.DataValueField = "Source";
ddl_Source.DataBind();
Any one has a solution? Thank you in advance.
You're already selecting Source in the LINQ query, which is how the result is an IQueryable<string>. You're then also specifying Source as the property to find in each string in the databinding. Just take out the statements changing the DataTextField and DataValueField properties in databinding.
Alterantively you could remove the projection to p.Source from your query and return an IQueryable<Promotion> - but then you would get distinct promotions rather than distinct sources.
One other quick note - using query syntax isn't really helping you in your GetAllSources query. I'd just write this as:
public IQueryable<string> GetAllSource()
{
PromotionDataContext dc = new PromotionDataContext(_connString);
return dc.Promotions
.Select(p => p.Source)
.Distinct();
}
Query expressions are great for complicated queries, but when you've just got a single select or a where clause and a trivial projection, using the dot notation is simpler IMO.
You're trying to bind strings, not Promotion objects... and strings do not have Source property/field
Your method returns a set of strings, not a set of objects with properties.
If you really want to bind to a property name, you need a set of objects with properties (eg, by writing select new { Source = Source })
I am using Entity Framework for the first time and noticed that the entities object returns entity collections.
DBEntities db = new DBEntities();
db.Users; //Users is an ObjectSet<User>
User user = db.Users.Where(x => x.Username == "test").First(); //Is this getting executed in the SQL or in memory?
user.Posts; //Posts is an EntityCollection<Post>
Post post = user.Posts.Where(x => x.PostID == "123").First(); //Is this getting executed in the SQL or in memory?
Do both ObjectSet and EntityCollection implement IQueryable? I am hoping they do so that I know the queries are getting executed at the data source and not in memory.
EDIT: So apparently EntityCollection does not while ObjectSet does. Does that mean I would be better off using this code?
DBEntities db = new DBEntities();
User user = db.Users.Where(x => x.Username == "test").First(); //Is this getting executed in the SQL or in memory?
Post post = db.Posts.Where(x => (x.PostID == "123")&&(x.Username == user.Username)).First(); // Querying the object set instead of the entity collection.
Also, what is the difference between ObjectSet and EntityCollection? Shouldn't they be the same?
Thanks in advance!
EDIT: Sorry, I'm new to this. I'm trying to understand. Attached EntityCollections are lazy loaded, so if I access them then memory is populated with them. Rather than doing two querys to the object sets like in my last edit, I am curious if this query would be more what I was after:
DBEntities db = new DBEntities();
User user = (from x in db.Users
from y in x.Posts
where x.Username == "test"
where y.PostID == 123
select x).First();
ObjectSet<T> does implement IQueryable<T>, but EntityCollection<T> does not.
The difference is that ObjectSet<T> is meant to be used for querying directly (which is why it does implement the interface). EntityCollection<T>, on the other hand, is used for the "many" end of a result set, typically returned in a query done on an ObjectSet<T>. As such, it impelments IEnumerable<T>, but not IQueryable<T> (as it's already the populated results of a query).
I was almost ready to say yes, they both do. Luckily I check the documentation first.
EntityCollection does not implement IQueryable.
As for the difference, ObjectSet<TEntity> represents the the objects generated from a table in a database. EntityCollection<TEntity> represents a collection of entity objects on the 'Many' side of One to Many or Many to Many relationship.