Entity Framework - Many to many question - entity-framework

I have a table called ASB and a table called PeopleInvolved. There is a junction table called PeopleInvolved_ASB which simply contains an ASBID and a PeopleInvolvedID column. The columns act as a composite primary key.
The designer does not show the junction table (as expected). I want to retrieve a list of PeopleInvolved based on an ASBID.
To retrieve the people, I'm doing this:
// This top line gets the ASB record from the Case
var asbRecord = (from c in dd.Case
where c.CaseID == caseID
select c.ASB).First();
var asbID = asbRecord.Select(asb => asb.ASBID).First();
var people = (from asb in dd.ASB
where asb.ASBID == asbID
select asb.PeopleInvolved);
Now, what I want to do is add each PeopleInvolved record to a simple list of type PeopleInvolved. I can't do this though. I keep getting:
Error 4 Cannot convert type 'System.Data.Objects.DataClasses.EntityCollection' to 'Dynamic.PeopleInvolved'
How can I get a simple list of PeopleInvolved into a generic list that I can pass back to my controller?
Thanks,

Based on the error you report I'm guessing PeopleInvolved is a collection. So try this:
var people = (from asb in dd.ASB
where asb.ASBID == asbID
from pi in asb.PeopleInvolved
select pi).ToList();
I think it's extremely confusing to have both a type named PeopleInvolved and a collection of that type with exactly the same name. Similarly, it seems you have a type called ASB and an Entity Set containing many instances of that type by the same name,
It's much clearer to make entity set names plural and type names singular.
If I've guessed the types wrong, please clarify the layout of ASB and Dynamic.PeopleInvolveed.

This is what you want to do?
List<PeopleInvolved> genericPeopleInvolvedList = (from asb in dd.ASB
where asb.ASBID == asbID
select asb.PeopleInvolved).ToList();
[Updated: answered bad before]
Just realised that asb.PeopleInvolved is collection not single entity (damn!). So,previous linq query is returning CollectionS of PeopleInvolved entities. Since you are selecting by ASPID there should be only one asb.ASBID that fullfill where clause asb.ASBID == asbID, and you can do as follows:
var listWithCollectionOfPeopleInvolved = (from asb in dd.ASB
where asb.ASBID == asbID
select asb.PeopleInvolved).ToList();
List<PeopleInvolved> peopleInvolved = listWithCollectionOfPeopleInvolved.First().ToList();
But it's much nicer if you do it using Include:
var asbInstance = (from asb in dd.ASB.Include("PeopleInvolved")
where asb.ASBID == asbID
select asb).FirstOrDefault();
foreach(PeopleInvolved pi in asbInstance.PeopleInvolved)
{
//do your stuff
}
With Include automatically load associated properties.

List<PeopleInvolved> = new List<PeopleInvolved>(people);

Related

EF1: Filtering derived types of entity class using .OfType<> by passing a string value

I have a situation where I'm trying to filter a LINQ select using a derived sub class.
ctx.BaseEntity.OfType<SubClass>() - this works fine.
However I'd like to do this using a string value instead. I've come across a performance barrier when I have lots (>20) Sub Classes and selecting an Entity without using OfType just isn't an option. I have a generic UI that renders from the base class, so I don't know what Class Type will be returned at compile time.
So what I'd like to do is this:
Perform a projected Select where I
return just the SubClassType from
the database
Perform a second select
using this value as the OfType to
only select the relevant related
entity from the database (No mass
unions generated)
int id = 1;
var classType = (from c in ctx.BaseClass.Include("ClassType")
where c.id == id
select new
{
c.ClassType.TypeName
}).First();
BaseClass caseQuery = ctx.BaseClass.OfType<classType.TypeName>()
.Include("ClassType")
.Include("ChildEntity1")
.Include("ChildEntity2")
.Where(x => x.id== id);
But obviously this won't work because OfType requires a Type and not a string.
Any ideas on how I can achieve this?
Update:
As a side note to the original question, it turns out that the moment you project a query that uses a Navigation Property - it builds the monster SQL too, so I've ended up using a stored procedure to populate my ClassType entity from the BaseClass Id.
So I've just got it to work using eSQL, which I'd never used before. I've posted the code here just in case it helps someone. Has anyone else got a more strongly typed solution they can think of?
BaseClass caseQuery = ctx.BaseClass.CreateQuery<BaseClass>("SELECT VALUE c FROM OFTYPE(Entities.[BaseClass],namespace.[" + classType.TypeName + "]) as c")
.Include("ClassType")
.Include("ChildEntity1")
.Include("ChildEntity2")
.Where(x => x.id== id).FirstOrDefault();
To answer the headline question about calling OfType with a string / runtime type, you can do the following:
// Get the type, assuming the derived type is defined in the same assembly
// as the base class and you have the type name as a string
var typeToFilter = typeof(BaseClass)
.Assembly
.GetType("Namespace." + derivedTypeName);
// The use reflection to get the OfType method and call it directly
MethodInfo ofType = typeof(Queryable).GetMethod("OfType");
MethodInfo ofTypeGeneric = method.MakeGenericMethod(new Type[] { typeToFilter });
var result = (IQueryable<Equipment>)generic.Invoke(null, new object[] { equipment });
Combine this with your stored procedure to get the class name and you (should?) avoid the massive join - I don't have table-per-type implementation to play with so I can't test.

EF builds EntityCollection, but I (think I) want IQueryable

I have an entity A with a simple navigation property B. For any given instance of A, we expect several related thousand instances of B.
There is no case where I call something like:
foreach(var x in A.B) { ... }
Instead, I'm only interested in doing aggregate operations such as
var statY = A.B.Where(o => o.Property == "Y");
var statZ = A.B.Where(o => o.CreateDate > DateTime.Now.AddDays(-1));
As far as I can tell, EF instantiates thousands of references to B and does these operations in memory. This is because navigation properties use EntityCollection. Instead, I'd like it to perform these queries at the SQL level if possible.
My current hunch is that Navigation Properties may not be the right way to go. I'm not attached to EF, so I am open to other approaches. But I'd be very interested to know the right way to do this under EF if possible.
(I'm using EF4.)
CreateSourceQuery seems to do the trick.
So my examples would now be:
var statY = A.B.CreateSourceQuery().Where(o => o.Property == "Y");
var statZ = A.B.CreateSourceQuery().Where(o => o.CreateDate > DateTime.Now.AddDays(-1));
There's one thing you should know. Members that derives from IQueryable<> are executed on the server, not in memory. Members which are derived from IEnumerable<> is executed in memory.
for example
var someEntities = db.SomeEntities; <-- returns an IQueryable<> object. no data fetched. SomeEntities table may contain thousands of rows, but we are not fetching it yet, we are just building a query.
someEntities = someEntities.Where(s => s.Id > 100 && s.Id < 200); <-- creates expression tree with where statement. The query is not executed yet and data is not fetched on the client. We just tell EF to perform a where filter when query will execute. This statement too returns an IQueryable<> object.
var entities = someEntities.AsEnumerable(); <-- here we tell EF to execute query. now entities will be fetched and any additional linq query will be performed in memory.
you can also fetch the data using foreach, calling ToArray() or ToList<>.
Hope you understand what I mean, and sorry for my english :)

Entity Framework Foreign Key Queries

I have two tables in my entity framework, objects, and parameters which have a foreign key pointing to the object to which they belong. I want to populate a tree with all the attributes of a certain object. So in order to find those I want to do this:
String parentObject = "ParentObjectName";
var getAttributes = (from o in myDB.ATTRIBUTE
where o.PARENT_OBJECT == parentObject
select o);
However when I try to do this I get an error saying it cannot convert from type OBJECT to string, even though in the database this value is stored as a string. I have a workaround where I get an instance of the parentObject, then go through every attribute and check whether it's parent_object == parentObjectInstance, but this is much less efficient than just doing 1 query. Any help would be greatly appreciate, thanks!
Well, PARENT_OBJECT.ToString() can't be called (implicitly or explicitly) in L2E, but if it just returns a property, you can look at that directly:
String parentObject = "ParentObjectName";
var getAttributes = (from o in myDB.ATTRIBUTE
where o.PARENT_OBJECT.NAME == parentObject
select o);
...note the .NAME
Try this:
String parentObject = "ParentObjectName";
var getAttributes = (from o in myDB.ATTRIBUTE
where o.PARENT_OBJECT.ToString() == parentObject
select o);

Eager loading with Include()

Imagine this case:
var locations = from Locations in this.LocationDataContext.Locations
.Include("ChildLocations")
where
(Locations.LocationType.ID == 3)
select
Locations;
This query will load all locations with type == 3 and all the related child locations, ok. But what i'm trying to figure out is how to filter the child locations that are being loaded. What if location have 3milion child locations?
Maybe something like this? (doesnt work because ChildLocations is a set of entities)
var locations = from Locations in this.LocationDataContext.Locations
.Include("ChildLocations")
where
(Locations.LocationType.ID == 3) &&
(Locations.ChildLocations.LocationType.ID == 2)
select
Locations;
Thank you.
The Entity Framework will never materialize a partially-complete instance. You cannot, in other words, materialize a Location with only some of its ChildLocations. This would be an "incomplete" object, and the Entity Framework does not allow this.
However, there are workarounds. If you only need the information from the ChildLocations and not from the Location itself, just select that:
from Locations in this.LocationDataContext.Locations
where Locations.LocationType.ID == 3
from ChildLocation in Locations
where ChildLocation.LocationType.ID == 2
select ChildLocation;
In this case, since we are only selecting the ChildLocations, it is OK to only select a few of them, since they can be materialized completely. It is only when materializing the Location that we need all of the children.
Another workaround is to materialize partial Location information into an anonymous type. This allows you to get information about both the Location and some of the ChildLocations without violating the rule that instances can only be materialized in their complete form. Since you're not actually materializing a real Location, there is no requirement to materialize the entire thing:
from Locations in this.LocationDataContext.Locations
where Locations.LocationType.ID == 3
select new
{
ID = Locations.ID,
LocationType= Locations.LocationType
ChildLocations = from ChildLocation in Locations
where ChildLocation.LocationType.ID == 2
select ChildLocation
}

ADO.NET Mapping From SQLDataReader to Domain Object?

I have a very simple mapping function called "BuildEntity" that does the usual boring "left/right" coding required to dump my reader data into my domain object. (shown below) My question is this - If I don't bring back every column in this mapping as is, I get the "System.IndexOutOfRangeException" exception and wanted to know if ado.net had anything to correct this so I don't need to bring back every column with each call into SQL ...
What I'm really looking for is something like "IsValidColumn" so I can keep this 1 mapping function throughout my DataAccess class with all the left/right mappings defined - and have it work even when a sproc doesn't return every column listed ...
Using reader As SqlDataReader = cmd.ExecuteReader()
Dim product As Product
While reader.Read()
product = New Product()
product.ID = Convert.ToInt32(reader("ProductID"))
product.SupplierID = Convert.ToInt32(reader("SupplierID"))
product.CategoryID = Convert.ToInt32(reader("CategoryID"))
product.ProductName = Convert.ToString(reader("ProductName"))
product.QuantityPerUnit = Convert.ToString(reader("QuantityPerUnit"))
product.UnitPrice = Convert.ToDouble(reader("UnitPrice"))
product.UnitsInStock = Convert.ToInt32(reader("UnitsInStock"))
product.UnitsOnOrder = Convert.ToInt32(reader("UnitsOnOrder"))
product.ReorderLevel = Convert.ToInt32(reader("ReorderLevel"))
productList.Add(product)
End While
Also check out this extension method I wrote for use on data commands:
public static void Fill<T>(this IDbCommand cmd,
IList<T> list, Func<IDataReader, T> rowConverter)
{
using (var rdr = cmd.ExecuteReader())
{
while (rdr.Read())
{
list.Add(rowConverter(rdr));
}
}
}
You can use it like this:
cmd.Fill(products, r => r.GetProduct());
Where "products" is the IList<Product> you want to populate, and "GetProduct" contains the logic to create a Product instance from a data reader. It won't help with this specific problem of not having all the fields present, but if you're doing a lot of old-fashioned ADO.NET like this it can be quite handy.
Although connection.GetSchema("Tables") does return meta data about the tables in your database, it won't return everything in your sproc if you define any custom columns.
For example, if you throw in some random ad-hoc column like *SELECT ProductName,'Testing' As ProductTestName FROM dbo.Products" you won't see 'ProductTestName' as a column because it's not in the Schema of the Products table. To solve this, and ask for every column available in the returned data, leverage a method on the SqlDataReader object "GetSchemaTable()"
If I add this to the existing code sample you listed in your original question, you will notice just after the reader is declared I add a data table to capture the meta data from the reader itself. Next I loop through this meta data and add each column to another table that I use in the left-right code to check if each column exists.
Updated Source Code
Using reader As SqlDataReader = cmd.ExecuteReader()
Dim table As DataTable = reader.GetSchemaTable()
Dim colNames As New DataTable()
For Each row As DataRow In table.Rows
colNames.Columns.Add(row.ItemArray(0))
Next
Dim product As Product While reader.Read()
product = New Product()
If Not colNames.Columns("ProductID") Is Nothing Then
product.ID = Convert.ToInt32(reader("ProductID"))
End If
product.SupplierID = Convert.ToInt32(reader("SupplierID"))
product.CategoryID = Convert.ToInt32(reader("CategoryID"))
product.ProductName = Convert.ToString(reader("ProductName"))
product.QuantityPerUnit = Convert.ToString(reader("QuantityPerUnit"))
product.UnitPrice = Convert.ToDouble(reader("UnitPrice"))
product.UnitsInStock = Convert.ToInt32(reader("UnitsInStock"))
product.UnitsOnOrder = Convert.ToInt32(reader("UnitsOnOrder"))
product.ReorderLevel = Convert.ToInt32(reader("ReorderLevel"))
productList.Add(product)
End While
This is a hack to be honest, as you should return every column to hydrate your object correctly. But I thought to include this reader method as it would actually grab all the columns, even if they are not defined in your table schema.
This approach to mapping your relational data into your domain model might cause some issues when you get into a lazy loading scenario.
Why not just have each sproc return complete column set, using null, -1, or acceptable values where you don't have the data. Avoids having to catch IndexOutOfRangeException or re-writing everything in LinqToSql.
Use the GetSchemaTable() method to retrieve the metadata of the DataReader. The DataTable that is returned can be used to check if a specific column is present or not.
Why don't you use LinqToSql - everything you need is done automatically. For the sake of being general you can use any other ORM tool for .NET
If you don't want to use an ORM you can also use reflection for things like this (though in this case because ProductID is not named the same on both sides, you couldn't do it in the simplistic fashion demonstrated here):
List Provider in C#
I would call reader.GetOrdinal for each field name before starting the while loop. Unfortunately GetOrdinal throws an IndexOutOfRangeException if the field doesn't exist, so it won't be very performant.
You could probably store the results in a Dictionary<string, int> and use its ContainsKey method to determine if the field was supplied.
I ended up writing my own, but this mapper is pretty good (and simple): https://code.google.com/p/dapper-dot-net/