I have a statement:
var items = from e in db.Elements
join a in db.LookUp
on e.ID equals a.ElementID
where e.Something == something
select new Element
{
ID = e.ID,
LookUpID = a.ID
// some other data get populated here as well
};
As you can see, all I need is a collection of Element objects with data from both tables - Elements and LookUp. This works fine. But then I need to know the number of elements selected:
int count = items.Count();
... this call throws System.NotSupportedException:
"The entity or complex type 'Database.Element' cannot be constructed in a LINQ to Entities query."
How am I supposed to select values from multiple tables into one object in Entity Framework? Thanks for any help!
You are not allowed to create an Entity class in your projection, you have to either project to a new class or an anonymous type
select new
{
ID = e.ID,
LookUpID = a.ID
// some other data get populated here as well
};
Your code doesn't work at all. The part you think worked has never been executed. The first time you executed it was when you called Count.
As exception says you cannot construct mapped entity in projection. Projection can be made only to anonymous or non mapped types. Also it is not clear why you even need this. If your class is correctly mapped you should simply call:
var items = from e in db.Elements
where e.Something == something
select e;
If LookupID is mapped property of your Element class it will be filled. If it is not mapped property you will not be able to load it with single query to Element.
Related
I'm trying to select objects from the database with Entity Framework into an anonymous type. When using Union and Selecting a Subcollection, I get an exception:
System.ArgumentException: The 'Distinct' operation cannot be applied to the collection ResultType of the specified argument.
My model contains several types derived from BaseType. This base type has a reference to RefType which contains a collection of ItemType. The types derived from BaseType are stored in separate tables, thus the Union.
The query looks like this:
var q1 = ctx.Set<Type1>().Select(x => new { x.Id, x.Ref.Items });
var q2 = ctx.Set<Type2>().Select(x => new { x.Id, x.Ref.Items });
q1.Union(q2).ToList();
But to reproduce the error, you can even union queries of the same type, as long as you select a collection.
I would do the select after the union, but to union Type1, Type2, etc. I must cast them to BaseType, which is not allowed in LINQ-to-SQL.
Any way to do this in the same query?
The exception emerges from Entity Framework's query generation pipeline when the ExpressionConverter tries to translate the expression q1.Union(q2) into SQL.
In a valid query you'll see that EF adds a DISTINCT clause to the SQL query. A type with collection properties (x.Ref.Items) doesn't pass as a valid argument for a Distinct operation and EF throws the exception you see.
Unfortunately, using Concat instead of Union is not a valid work-around. EF will also throw an exception:
The nested query is not supported. Operation1='UnionAll' Operation2='MultiStreamNest'
Which means that it's simply not supported to concat nested queries containing types with collection properties.
So you have to do the Union in memory:
var result = q1.AsEnumerable() // Continues the query in memory
.Union(q2).ToList();
C# doesn't have any problem with equating anonymous types containing collections: it simply considers any collection member as unequal to another collections member. This does mean that the query can produce a collection containing non-unique results (same Id, same Items) which may not be expected when relying on Union's implicit Distinct.
I am not sure why, for some reason distinct is failing, maybe because it is anonymous type, and it is still IQuerable, I would suggest firing the query something like this
var q1 = ctx.Set<Type1>().Select(x => new { x.Id, x.Ref.Items }).ToList<object>();
var q2 = ctx.Set<Type2>().Select(x => new { x.Id, x.Ref.Items }).ToList<object>();
q1.Union(q2).ToList();
Note that in this case, Distinct will check for all properties equality, meaning if 2 objects have the same id but different items, both will be there.
if you don't care about distinct values, you can also use concat
if you care about distinct and first option didn't work, you can use group by and implement your own distinct,
something like this
var q1 = ctx.Set<Type1>().Select(x => new { Id = x.Id, Items =x.Ref.Items });
var q2 = ctx.Set<Type2>().Select(x => new { Id = x.Id, Items = x.Ref.Items });
//this will group by id, and select the first object items
var qFinal = q1.concat(q2).GroupBy(e => e.id)
.select(e => new {e.key, e.First().Items})
.ToList();
maybe you don't want First(), you can use whatever you want
I have a method:
public List<Timetable> getTimetableTableForRegion(String id) {
List<Timetable> timetables;
TypedQuery<Timetable> query = em_read.createQuery("SELECT ..stuff.. where R.id = :id", Timetable.class).setParameter("id", Long.parseLong(id));
timetables = query.getResultList();
return timetables;
}
which returns this:
so, what am I missing in order to return a list of Timetable's?
ok, so, ..stuff.. part of my JPQL contained an inner join to other table. Even through in SELECT there were selected fields just from one table, which was used as type - Timetable, Eclipslink was unable to determine if this fields are part of that entity and instead of returning list of defined entity returned list of Object[].
So in conclusion: Use #OneToMany/#ManyToOne mappings (or flat table design) and query just for ONE table in your JPQL to be able to typize returned entities.
Not sure it might be something is looking for, but I had similar problem and converted Vector to ArrayList like this:
final ArrayList<YourClazz> results = new ArrayList<YourClazz>();;
for ( YourClazzkey : (Vector<YourClazz>) query.getResultList() )
{
results.add(key);
}
i have faced the same problem. and my entity has no one to one or one to many relationship. then also jpql was giving me queryresult as vector of objects. i changed my solution to query to criteria builder. and that worked for me.
code snippet is as below:
CriteriaBuilder builder = this.entityManager.getCriteriaBuilder();
CriteriaQuery<Timetable> criteria = builder.createQuery(Timetable.class);
Root<Enumeration> root = criteria.from(Timetable.class);
criteria.where(builder.equal(root.get("id"), id));
List<Timetable> topics = this.entityManager.createQuery(criteria) .getResultList();
return topics;
I am in the process of converting an application that uses LINQ to SQL over to LINQ to Entities. I use a repository pattern and I have run in a problem that works in LINQ to SQL but not Entities.
In my data layer, I use LINQ statements to fill my object graph so that none of my database entities are exposed anywhere else. In this example, I have a Lookup Respository that returns a list of Categories. It looks like this:
public IQueryable<Entities.DomainModels.Category> getCategories()
{
return (from c in Categories
where !c.inactive
orderby c.categoryName
select new Entities.DomainModels.Category
{
id = c.categoryID,
category = c.categoryName,
inactive = c.inactive
});
}
Later, I want to put the categories into a sub query and it looks like this:
var d = from p in Programs
let categories = (from pc in p.Categories
join c in getCategories() on pc.categoryID equals c.id
select c)
select new
{
id = p.id,
title = p.title
categories = categories.ToList()
};
When I run this, I get the following error:
LINQ to Entities does not recognize the method 'System.Linq.IQueryable`1[Entities.DomainModels.Category] getCategories()' method, and this method cannot be translated into a store expression.
For reference, the following works though it doesn't return the data I need (it's basically a join):
var q = from p in Programs
from pc in p.Categories
join c in getCategories() on pc.categoryID equals c.id
select new
{
id = p.id,
category = c
};
I understand what the error means in concept however LINQ to SQL would make it work. I have this pattern throughout my data layer and I really want to keep it. Should this be working? If not, how can I modify it without mixing my layers.
You cant pass getCategories() to EF.
The query must be destructible to expression tree.
Calculate getCategories() first.
eg
var simpleList = getCategories().Select(id).Tolist;
then use a contains
where(t=> simpleList.Contains(t.CatId) // or the query syntax equivalent
I've set up a many-to-many association between two tables based on a third table that just holds a pair of key values. Now I'd like to do a query that groups the right tables key values by the lefts without needing other data.
LeftTable { LeftID, LeftField1, LeftField2 }
JoinTable { LeftID, RightID}
RightTable { RightID, RightField1, RightField2 }
Is there any way to essentially just query the JoinTable and get all the 'RightIDs' grouped by the 'LeftIDs' without the SQL trying to fetch the fields from either side?
The JoinTable is not an entity in its own right in the model, but is mapped to the association.
I've experimented a bit with both using ObjectQuery and EntityCommand (ESQL) and both seem to still load in the other fields by joining to RightTable which I don't need.
My ESQL looks something like:
SELECT lt.LeftID, (SELECT rt.RightID
FROM NAVIGATE(lt, MyModel.LeftToRightAssoc, RightTable) as rt)
FROM MyEntities.LeftTable as lt;
but the generated SQL is still fetching in RightField1 and RightField2.
Surely there must be a simpler way to do this?
Assuming that your class Left has a navigation property Rights (a collection of Right entities) you could try this:
var list = context.Lefts.Select(l => new
{
LeftId = l.LeftId,
RightIds = l.Rights.Select(r => r.RightId)
});
foreach (var item in list)
{
Console.WriteLine("LeftId = {0}", item.LeftId);
foreach (var rightId in item.RightIds)
{
Console.WriteLine("RightId = {0}", rightId);
}
}
You would get a collection of anonymous type objects where each element has the LeftId and a collection of corresponding RightIds. This query should not touch the other fields like RightField1, etc. Instead of an anonymous type you could also create your own custom type and then project into this type in the query above.
In EF, it is possible to write the following line:
orderLine.OrderReference.EntityKey.EntityKeyValues[0].Value
Which results in the ID of the associated OrderReference.
What would be the solution if I wanted to know the ID's of the orderLines associated with an order?
The point of the line you show is to get the ID without loading orderLine.Order. But you can't get the IDs of a collection without loading. So just look at the ID property, either directly or from the context.
// from context
var lineIds = (from o in Context.Orders
where o.Id = someId
from l in o.Lines
select l.Id).AsEnumerable();
// from loaded order
if (!order.Lines.IsLoaded) order.Lines.Load();
var lineIds = from l in order.Lines
select l.Id;