I have 2 Entities, each of which is managed by EF Code First, and each happily sitting in its own table. Entity_A has a property, called "SumTotal", which should be the sum of a specific column in Entity_B that matches a certain criteria.
SumTotal should not be persisted in the DB, but rather calculated each time an instance of Entity_A is retrieved.
I have looked at ComputedColumns, but it appears that the computedcolumn can only be defined relative to columns in the same table.
I also have a feeling that I need to set SumTotal to NotMapped (or something similar with AutoGenerated), but dont know how to get the actual value into SumTotal.
Hope this question makes sense, thanks in advance
You can project the results to an anonymous object and transform that it to your entity
var projection = db.EntityAs.Where(/* */)
.Select(a => new {A = a, Sum = a.Bs.Sum(b => b.Total)})
foreach(p in projection)
{
p.A.SumTotal = p.Sum;
}
var As = projection.Select(p => p.A);
Related
guys, I've got very strange problem, that nobody from my company knows.
I have a Collection inside one object, which I created manualy:
Cost cost = new Cost
{
...
};
cost.Formulas.Add(new Formula()
{
CostID = cost.ID,
FieldName = "Distance",
Formula1 = "12+2",
});
Next, I got another object with the same type and ID from context:
Cost updatingCost = context.Costs.Include("Formulas").FirstOrDefault(c => c.ID == cost.ID);
And finally I do:
updatingCost.Formulas.Add(cost.Formulas.ToList()[0]);
And here is magic! After that line cost.Formulas doesn't contains that Formula any more!!
cost.Formulas.Count = 0
What does this method iCollection.Add() do? Why it removes an object from other collection? It blows my mind, please help to understand it!
In fact it is pretty simple. By adding the new formula to another Cost object, updatingCost, EF changes the foreign key value CostID to the value of updatingCost.ID.
This is because EF executes relationship fixup many times when entites are manipulated in code. This process makes sure that primitive ID values and object references and collection contents match.
Sorry for this being so long, but at least I think I got all info to be able to understand and maybe help?
I would like to load data from my database using eager loading.
The data is set up in five tables, setting up two Levels of m:n relations. So there are three tables containing data (ordered in a way of hierarchy top to bottom):
CREATE TABLE [dbo].[relations](
[relation_id] [bigint] NOT NULL
)
CREATE TABLE [dbo].[ways](
[way_id] [bigint] NOT NULL
)
CREATE TABLE [dbo].[nodes](
[node_id] [bigint] NOT NULL,
[latitude] [int] NOT NULL,
[longitude] [int] NOT NULL
)
The first two really only consist of their own ID (to hook other data not relevant here into).
In between these three data tables are two m:n tables, with a sorting hint:
CREATE TABLE [dbo].[relations_ways](
[relation_id] [bigint] NOT NULL,
[way_id] [bigint] NOT NULL,
[sequence_id] [smallint] NOT NULL
)
CREATE TABLE [dbo].[ways_nodes](
[way_id] [bigint] NOT NULL,
[node_id] [bigint] NOT NULL,
[sequence_id] [smallint] NOT NULL
)
This is, essentially, a part of the OpenStreetMap data structure. I let Entity Framework build it's objects from this database and it set up the classes exactly as the tables are.
The m:n tables do really exist as class. (I understand in EF you can build your objects m:n relation without having the explicit in-between class - should I try to change the object model in this way?)
What I want to do: My entry point is exactly one item of relation.
I think it would be best to first eager load the middle m:n relation, and then in a loop iterate over that and eager load the lowest one. I try to do that in the following way
IQueryable<relation> query = context.relations;
query = query.Where( ... ); // filters down to exactly one
query = query.Include(r => r.relation_members);
relation rel = query.SingleOrDefault();
That loads the relation and all it's 1:n info in just one trip to the database - ok, good. But I noticed it only loads the 1:n table, not the middle data table "ways".
This does NOT change if I modify the line like so:
query = query.Include(r => r.relation_members.Select(rm => rm.way));
So I cannot get the middle level loaded here, it seems?
What I cannot get working at all is load the node level of data eagerly. I tried the following:
foreach (relation_member rm in rel.relation_members) {
IQueryable<way_node> query = rm.way.way_nodes.AsQueryable();
query = query.Include(wn => wn.node);
query.Load();
}
This does work and eagerly loads the middle level way and all 1:n info of way_node in one statement for each iteration, but not the Information from node (latitude/longitude). If I access one of these values I trigger another trip to the database to load one single node object.
This last trip is deadly, since I want to load 1 relation -> 300 ways which each way -> 2000 nodes. So in the end I am hitting the server 1 + 300 + 300*2000... room for improvment, I think.
But how? I cannot get this last statement written in valid syntax AND eager loading.
Out of interest; is there a way to load the whole object graph in one trip, starting with one relation?
Loading the whole graph in one roundtrip would be:
IQueryable<relation> query = context.relations;
query = query.Where( ... ); // filters down to exactly one
query = query.Include(r => r.relation_members
.Select(rm => rm.way.way_nodes
.Select(wn => wn.node)));
relation rel = query.SingleOrDefault();
However, since you say that the Include up to ...Select(rm => rm.way) didn't work it is unlikely that this will work. (And if it would work the performance possibly isn't funny due to the complexity of the generated SQL and the amount of data and entities this query will return.)
The first thing you should investigate further is why .Include(r => r.relation_members.Select(rm => rm.way)) doesn't work because it seems correct. Is your model and mapping to the database correct?
The loop to get the nodes via explicit loading should look like this:
foreach (relation_member rm in rel.relation_members) {
context.Entry(rm).Reference(r => r.way).Query()
.Include(w => w.way_nodes.Select(wn => wn.node))
.Load();
}
Include() for some reason sometimes gets ignored when there is sorting/grouping/joining involved.
In most cases you can rewrite an Include() as a Select() into an anonymous intermediary object:
Before:
context.Invoices
.Include(invoice => invoice .Positions)
.ToList();
After:
context.Invoices
.Select(invoice => new {invoice, invoice.Positions})
.AsEnumerable()
.Select(x => x.invoice)
.ToList();
This way the query never should loose Include() information.
//get an associate book to an author
var datatable = _dataContext.Authors
.Where(x => authorids.Contains(x.AuthorId))
.SelectMany(x => x.Books)
.Distinct();
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.
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.
I implemented inheritance with a discriminator field so all my records are in the same table. My basetype is Person (also the name of the table) and Driver and Passenger inherit from it. I receive instances of the correct type (Driver and Passenger) when I perform a query on the object context to Person. example:
var q = from d in ctx.Person
select d;
But I also create a function that calls a stored procedure and mapped the output of the function to the type Person. But now I get a list of Person and not Drivers or Passengers when I execute this method.
Anybody an idea how to solve this or is this a bug in EF4?
AFAIK, you can't use discriminator mapping (e.g TPH) when dealing with stored procedure mappings.
The stored procedure must be mapped to a complex type or custom entity (e.g POCO), the mapping cannot be conditional.
What you could do is map it to a regular POCO, but then project that result set into the relevant derived type (manual discrimination).
E.g:
public ICollection<Person> GetPeople()
{
var results = ExecuteFunction<Person>(); // result is ObjectResult<Person>
ICollection<Person> people = new List<Person>();
foreach (var result in results)
{
if (result.FieldWhichIsYourDiscriminator == discriminatorForDriver)
{
people.Add((Driver)result);
}
// other discriminators
}
}
If your always expecting a collection of one type (e.g only Drivers), then you wouldn't need the foreach loop, you could just add the range. The above is in case you are expecting a mixed bag of different people types.
Would be interested to see other answers, and if there is a better way though - but the above should work.