Develop Reference

In Entity Framework Core, is it possible to map this type of relationship?

I have a database table that stores user entered localization strings for many other tables in the database.
PK
TableName
RecordID
Field
Value
1
TableA
1
Name
2
TableA
1
Category
3
TableB
1
Make
4
TableB
1
Model
I don't know if there is a term for this type of table so sorry for the vague title. When querying for this data in SQL we would simply join on TableName and RecordID.
How would I go about mapping this in Entity Framework? Is a navigation property possible with a table like this?
I was hoping that something could be done in OnModelCreating to configure navigation properties on the base entity to only include records where TableName == "Table_". If possible I'd like to avoid having to add a Where clause in an .Include statement every time a record is queried.
public class TableA
{
// This should only include PK 1-2
public List<LocalizedString> LocalizedStrings { get; set; }
}
public class TableB
{
// This should only include PK 3-4
public List<LocalizedString> LocalizedStrings { get; set; }
}
The only possible solution I can think of is to try using TPH inheritance. With the TableName as the discriminator I believe I would need to create a class for every type. TableALocalizedString, TableBLocalizedString, etc. If possible I'd like to avoid that, this table references a lot of other tables.

C# Include, ThenInclude Select specific fields

I have the following classes:
public class Teacher
{
Guid ID;
string Name;
string Address;
string Tel;
ICollection<Student> Students;
}
public class Student
{
Guid id;
string Name;
string Address;
string Tel;
ICollection<Teacher> Teachers;
}
As this is a Many-to-Many relationship, EF would create a Junction Table TeacherStudent. I want to find all teachers and the list of the teacher's students (name only), eg:
Teacher1 name:
Student1 name,
Student3 name,
Student5 name,
Student9 name
Teacher2 name:
Student1 name,
Student2 name,
Student3 name,
Student9 name
I've tried:
_context.Teacher.Include(x=>x.TeacherStudent).ThenInclude(y=>y.Student);
This works and Student info is a list under each Teacher. However, I don't want all the info, I only want the name. I've tried SelectMany but it flattened out the data and I get the Teacher's name repeated for each Student. How can get a list of Student names under the Teacher's name and not retrieve the fields I don't need ?
Thanks.

Include is used to eager load related entities. Select is used to project an entity query into a desired data structure, or otherwise retrieve pieces of information about an entity.
So you can get a list of teachers and students' names:
var teachers = _context.Teacher
.Select(t => new
{
t.Id,
t.Name,
Students = t.Students.Select(s => new
{
s.Id,
s.Name
}).ToList()
}).ToList();
What this would return is an anonymous type containing a list of teachers, pulling back just their ID and name, and a collection of Student anonymous types with the student ID and Name.
Typically when reading data with projections it is a good idea to read IDs since you will often want to use this information to either bring up details, remove, or otherwise alter entries.
If I were to use projection, does it mean that EF still would
retrieve all the fields before projecting
Projection builds an SQL query to pull back just the fields needed to populate the data structure you request. It doesn't load entities so this is somewhat mutually exclusive to Include. You do not need to "include" related entities to use projection.
Eager Loading with Include is typically something you will use when performing an update. for instance if you are going to be adding or removing student associations to a Teacher, you will want to load the Teacher and associated Students to ensure that the change tracking understands what you are doing when you go to add or remove students from a Teacher.

Entity framework set navigation property

We have database which does not have proper foreign keys set. We are now generating edmx using this database. What we want is to set navigation property so that we can get corresponding details from other table. Here is example what exactly we are looking for.
Lets say There is a table Employee and Department. Now in database there is no relation between these tables but Employee has DepartmentId which is taken from Department table.
When we fetch Employee we get only DepartmentID but we also want to get Department as property along with it so that we can get information like "DepartMentName", "Location" which is stored in Department table.
We tried adding Navigation property in EDMX file but it fails and keeps giving error related to relation.
Please help

You can go with something like this. Create a wrapper class for Employee and Department.
public class EmpDept
{
public Employee Employee {get; set;}
public Department Department {get; set;}
}
public IEnumberable<EmpDept> GetEmployeesWithDeptpartment()
{
var result = from e in context.Employee
where e.Id == somevalue
select new EmpDept()
{
Employee = e,
Department = context.Department.Where(d => d.Id == e.DepartmentId)
};
return result.ToList();
}
It means you have an extra class, but it's quick and easy to code, easily extensible, reusable and type-safe.
Hope this helps

How to access data in foreign key table from EntityDataSource?

I have a table defined in my Entity model. I also have the foreign key Navigation properties defined on the tables in the model.
Users
- UserID
- Username
- UserGroupID
Groups
- GroupID
- GroupName
I have a grid connected to an EntityDataSource, which retrieves and displays the Users table. Instead of displaying the UserGroupID identity column for each user, I need to display the corresponding GroupName. Is there an easy built in way to grab the GroupName from the User object since they are connected in the entity model?
Thanks!
Kevin

In this tutorial, the GridView that displays the Instructors table does what you are trying to do with EntityDataSource and GridView (display a value from a navigation property):
http://www.asp.net/web-forms/tutorials/getting-started-with-ef/the-entity-framework-and-aspnet-getting-started-part-4

You should setup an association on each entity. For Code First it should look something like this:
class User {
// user properties...
[ForeignKey("UserGroupID")]
public virtual Group Group { get; set; }
}
class Group {
// group properties...
public virtual ICollection<User> Users { get;set; }
}
Then you can just call:
user.Group.GroupName

Linq To Sql One To Many

Say I have 3 tables, Cars, Persons, CarOwners.
Cars is just a table of cars and Persons are different people that can own cars. CarOwners has a CarId and a PersonId relationship and a CarOwnerId PK.
If I just create the relationship and then drag them to the linq context, a property of the Person class will be generated called CarOwners. To get the cars that this person owns I would have to do the following query:
var cars = person.CarOwners.Select(c=> c.Car);
But I would like to be able to do:
var cars = person.Cars;
Is that at all possible? The extra step is quite annoying.

You can just create a new property on Person:
public partial class Person
{
public IEnumerable<Car> Cars
{
get { return this.CarOwners.Select(c => c.Car); }
}
}
Or upgrade to Entity Framework where you can separate logical model from physical model.

Try to remove the CarOwnerId from CarOwners in your sql database. Then select CarId and PersonId in designer mode in SSMS (using shift key), right click and select `Set Primary Key.
In EF 4, you'll get only navigation keys in your model this way (you can use: Person.Cars and Car.Persons). Not sure about L2S.