I have the fallowing 2 classes:
[Table("People")]
public class Person : IPerson
{
[Key]
[DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]
public int Id { get; set; }
public string FirstName { get; set; }
public Person()
{
Results = new Collection<Result>();
}
public string LastName { get; set; }
public string Name
{
get
{
return FirstName + " " + LastName;
}
set{}
}
public string Email { get; set; }
public DateTime? LastModified { get; set; }
public virtual ICollection<Result> Results { get; set; }
}
and
[Table("UserProfile")]
public class UserProfile : Person
{
public UserProfile()
{
Faculty = new Faculty();
Projects = new Collection<Project>();
}
public string UserName { get; set; }
public string CNP { get; set; }
public virtual Faculty Faculty { get; set; }
public virtual ICollection<Project> Projects { get; set; }
}
Every time i use EF CodeFirst to generate my DB, and try too run the seed method i get an error. An error occurred while updating the entries. See the inner exception for details. System.Data.SqlClient.SqlException: Cannot insert explicit value for identity column in table 'UserProfile' when IDENTITY_INSERT is set to OFF. You can find more about it here Seeding Membership with custom data Entity framework
I found out that to fix it after running update-database from the package manager console i have to open the table in server explorer and set Idendtity Specification to false for the Id in the UsersProfile table, my question is can i do something to my model so that i won't have to this every time i regenerate my Db.
I tried the answer from here Identity specification set to false but because of the inheritance (i think) i get
Conflicting configuration settings were specified for property 'Id' on type 'Prometheus.Models.Person':
DatabaseGeneratedOption = None conflicts with DatabaseGeneratedOption = Identity
Thank you.
I'm not totally sure exactly what is happening here but I do know the UserProfile table is often created with a call to WebSecurity.InitializeDatabaseConnection rather than the migrations code and that's going to put in an IDENTITY field. So there is a conflict between WebSecurity and Entity Framework here.
Then you are adding inheritance into the picture and because you have specified table names it is Table per Type - and Entity Framework wants that to use a shared primary key. So it probably does not want the UserProfile table to have an IDENTITY field.
I think you hit the nail on the head when you said
UserProfile inherits from People, that's my problem
I would change that relationship so that Person has a UserProfile instead. I think that models the real world more accurately and if you ever have any people who aren't users it will be easier to make the relationship optional. Like this:
[Table("People")]
public class Person : IPerson
{
//avoids virtual call in constructor
private ICollection<Result> _Results;
public Person()
{
_Results = new Collection<Result>();
}
//no annotations required.
//An int field named Id is a database generated key by convention
public int Id { get; set; }
//Person has a UserProfile (optional)
public int? UserProfileID { get; set; }
public UserProfile UserProfile { get; set; }
//etc
public virtual ICollection<Result> Results
{
get { return _Results; }
set { _Results = value; }
}
}
public class UserProfile : ModelBase
{
//UserProfile is always related to a Person
public int PersonID { get; set; }
public UserProfile Person { get; set; }
//etc
}
You will also find loads of stuff about preferring composition over inheritance.
Otherwise you need to dig in to the migrations to get the tables created in the way that supports TPT - but you should also be aware that switching identity on/off in migrations does not happen automatically.Perhaps you could create the UserProfile table in a migration instead of with WebSecurity.InitializeDatabaseConnection
Related
I'm having issues when I try and add a record to my DB. The view is registration and the following error occurs on _db.SaveChanges() when trying to add to Secretaries/Student.
SqlException: Invalid column name 'UsersUserID'.
Screenshot of error(1). Screenshot of Error(2)
I manually added changes to the Migration/Migration Snapshot and removed the column UsersUserID as it duplicated FK UserID. I've tried searching for the column but there are no occurrences. Deleting migrations and creating a new DB does not solve the problem either. Does anyone have any idea on how to solve this problem?
Here is my Users Model.
public class Users
{
[Key]
public int UserID { get; set; }
[Required]
public string Name { get; set; }
[Required]
public string Surname { get; set; }
public string Password { get; set; }
public string Email { get; set; }
public string AddressLine1 { get; set; }
public string AddressLine2 { get; set; }
public int SuburbID { get; set; }
public string Title { get; set; }
public int RoleID { get; set; }
public string ErrorMessage { get; set; }
public virtual ICollection<Secretaries> Secretaries { get; set; }
public virtual ICollection<Student> Student { get; set; }
public virtual Suburb Suburb { get; set; }
public virtual Role Role { get; set; }
}
Here is my Student Model
public class Student
{
[Key]
public int StudentID { get; set; }
public int UserID { get; set; }
[Required]
public DateTime DateJoined { get; set; } = DateTime.Now;
[StringLength(1)]
[Required]
public char Is_Active { get; set; } = 'T';
public virtual Users Users { get; set; }
}
Here is my secretaries model
public class Secretaries
{
[Key]
public int SecretaryID { get; set; }
public int UserID { get; set; }
[Required]
public DateTime DateJoined { get; set; } = DateTime.Now;
[StringLength(1)]
[Required]
public char Is_Active { get; set; } = 'T';
public virtual Users Users { get; set; }
}
This is the section where I get the error
public IActionResult Registration(Users users)
{
try
{
//users.Role.RoleID;
_db.Users.Add(users);
//_db.SaveChanges();
int role = users.RoleID;
int id = users.UserID;
if (role == 1)
{
Student student= new Student();
student.UserID = id;
_db.Student.Add(student);
_db.SaveChanges();
}
else if (role == 2)
{
Secretaries secretary = new Secretaries();
secretary.UserID = id;
_db.Secretaries.Add(secretary);
_db.SaveChanges();
}
return RedirectToAction("Index", "Home");
}
catch
{
return View(users);
}
}
To start, naming convention would help around losing the Plural for the entity. "Users" implies multiple, and your naming is inconsistent given "Student". EF can manage details like having a table name "Users" with an entity named "User". Worst case you can use the [Table("Users")] attribute if there are naming issues. The entity can use a meaningful name even if it doesn't match what the table is called.
The issue you are likely running into is that while EF can auto-resolve some relationships between entities and things like FKs by convention, unless you follow the known conventions closely, EF can miss some details requiring you to provide them. Personally I opt to keep configuration pretty deliberate and consistent, not to rely on convention and the occasional surprise when it doesn't work. In your case two likely issues is marrying the User to their declare FK, and then also the bi-directional mapping.
With the entity and property named "Users", EF likely cannot match this to the UserId by convention, at best it would probably be looking for "UsersId", so when it didn't find that it would look for TableNamePKName so "UsersUserID" where "Users" is the table name and "UserID" is the PK in that table. To resolve that:
[ForeignKey("UserID")]
public virtual Users Users { get; set; }
This tells EF where to look. If this is a later version of C# then:
[ForeignKey(nameof(UserID))]
... to avoid magic strings if available.
The next detail that may trip up the mapping will be the bi-directional references where a Secretary has a User and User has a collection of secretaries. Bi-directional references should be avoided unless they serve a very clear and useful purpose. In most cases they are not needed, but where you do you them it helps to declare the inverse side of the relationship:
[ForeignKey("UserID")]
[InverseProperty("Secretaries")]
public virtual Users Users { get; set; }
This tells EF to expect a navigation property called "Secretaries" on the Users entity which will link via this User reference.
Bi-directional references can cause issues and require more attention when updating entities, especially if you have code that does something like serialize an entity. They often aren't needed as you can typically query data from one side of the relationship.
For instance to get all Secretaries associated to a given user you might think you need something like:
var user = context.Users.Include(u => u.Secretaries).Single(u => u.UserID == userId);
var secretaries = user.Secretaries;
... when instead you can use:
var secretaries = context.Secretaries.Where(s => s.User.UserID == userId).ToList(); // or (s => s.UserID == userId) where the UserID FK is exposed.
Depending on the purpose of the association you schema may better be served by inheritance where instead of a "has a" relationship, the Secretary/Student to User becomes an "is a" relationship. Does a User "have" Secretaries/Students, or is it more that a User "Is a" Secretary vs. a Student? Alternatively, is "Student" vs. "Secretary" more of a "Role" that can be held by a User? (Which suits a case where one user might be both a Student and a Secretary)
With the schema you currently have defined, it would allow one user to be potentially associated with many Students, as well as many Secretaries which may, or may not be the desired and intended use. (Also known as Many-to-One relationships)
Edit: To implement a schema where a User can be one of these Types or Roles there are a couple of options.
Inheritance: Where a User "is a" Teacher or "is a" Student etc. EF can support inheritence where you would have something like:
public abstract class User
{
public int UserId { get; set; }
// other common fields...
}
public class Student : User
{
// Student-specific fields...
}
public class Secretary : User
{
// Secretary-specific fields...
}
From here you can configure EF to either use a single table (Users) with a discriminator column (Think "UserType" or "Role" to indicate Student vs. Secretary, etc.) or to use a User Table plus Student and Secretary tables. You can learn more about inheritance options by looking up "Table-per-Hierarchy" or "Table-per-Concrete Type" in relation to EF. If the different types of users have different fields then Table-per-concrete type is generally a better option. Table-Per-Hierarchy can work but you end up with several null-able-columns as the table needs to cater to the lowest denominator.
If the different types of users consist of the exact same data then you could use something called a Many-to-one relationship between a User table, and a UserType or a UserRole table which would identify whether the user was a Student, Teacher, etc. This can be either Many-to-One, (many users can be the same role, but each user can have only 1 role) or it can be adapted to many-to-many if users could later hold 2 or more roles.
Many-to-one:
public class User
{
public int UserId { get; set; }
public int RoleId { get; set; }
[ForeignKey(nameof(RoleId))]
public virtual Role Role { get; set; }
}
public class Role
{
public int RoleId { get; set; }
public string Name { get; set; }
}
Many-to-Many
public class User
{
public int UserId { get; set; }
public int RoleId { get; set; }
public virtual ICollection<Role> Roles { get; set; } = new List<Role>();
}
public class Role
{
public int RoleId { get; set; }
public string Name { get; set; }
}
This would need to be configured with a .HasMany(x => x.Roles).WithMany() relationship nominating a joining table (I.e. UserRoles) consisting of a UserId and RoleId. Optionally in a Many-to-Many relationship you can define the joining table as an entity if there are details about the relationship you want to access. (beyond just the keys relating the entities)
The other advantage of using a Many-to-One or Many-to-Many vs. inheritance is that it's easy to express optional roles, where a user might not be any of the roles. You can still adapt something like a Many-to-One implementation to have Student-specific data vs. Teacher-specific data by introducing a One-to-zero-or-one relationship between User and a table like StudentUserDetails on the UserId shared by both tables.
Ultimately there are a number of ways you can manage relational relationships for data in a database where EF can be configured to understand and map those relationships either within, or between entities.
Based on the solution provided in this question : How to update foreign key in EF 6 - Code First, I'm able to update my foreign key using the id field.
But now, I get an exception when getting entities from the database. Using this code :
// Retrieve data first
using (var db = new TestDbContext())
{
var p2 = db.Persons.First();
}
I get the following SqlException : "Invalid column name 'Country_Id1'."
Does anyone have any clues to be able to retrieve data and to update the foreign key ?
Asked in another way, is it possible to use both the navigation property to ease the use of my entity and the id of the foreign key to be able to update my dependent entity ?
My entities
public class Person
{
public int Id { get; set; }
public string Name { get; set; }
public virtual Country Country { get; set; }
public int Country_Id { get; set; }
}
public class Country
{
public int Id { get; set; }
public string Name { get; set; }
}
That might be because entity framework is trying to create new foreign key based on navigation property Country in Person entity.
I think you should annotate Country_Id property with ForeignKey attribute as below.
public class Person
{
public int Id { get; set; }
public string Name { get; set; }
[ForeignKey("Country_Id")]
public virtual Country Country { get; set; }
public int Country_Id { get; set; }
}
However if you follow the ef naming convention for naming property as below, you don't need to annotate it.
Any property with the same data type as the principal primary key
property and with a name that follows one of the following formats
represents a foreign key for the relationship: '', '', or ''
You may read more from here
public class Person
{
public int Id { get; set; }
public string Name { get; set; }
public virtual Country Country { get; set; }
public int CountryId { get; set; }
}
Note: you might need to run database migration or need to recreate database.
My problem looks simple. I need to implement a relationships between items in the database. For example: relationship between entities like computer and software shows users that computer stores a specific software and similarly - a software is installed in the specific computer. I think I should implement an entity with source id and target id or something similar. I wrote some code using code first in EntityFramework 6. Here are two classes:
public class ConfigurationItem
{
public int Id { get; set; }
public String Name { get; set; }
public String DeploymentState { get; set; }
public String IncidentState { get; set; }
[DataType(DataType.MultilineText)]
public String Description { get; set; }
[DataType(DataType.MultilineText)]
public String Note { get; set; }
public virtual ICollection<Relationship> Relationship { get; set; }
}
public class Relationship
{
[Key]
public int RelationshipId { get; set; }
[ForeignKey("ConfigurationItem")]
public int SourceId { get; set; }
[ForeignKey("ConfigurationItem")]
public int TargetId { get; set; }
public String Type { get; set; }
public virtual ConfigurationItem Source { get; set; }
public virtual ConfigurationItem Target { get; set; }
}
This solution doesn't work. I need a tip or something what should I try to make it work properly. EF throws an error about foreign key:
The ForeignKeyAttribute on property 'SourceId' on type 'cms_1.Models.Relationship' is not valid. The navigation property 'ConfigurationItem' was not found on the dependent type 'cms_1.Models.Relationship'. The Name value should be a valid navigation property name.
When I try to resolve it EF throws an error about cascade deleting. I know how to disable it but I just don't want to. I need a proper solution with that feature but I think I don't know how to do a model representing given scenario.
Simply - I need to store two foreign keys from entity "A" in the entity "B". How is it possible?
from a quick review , I can tell that you need 3 tables :
first : Computer
second : Software
third : a table , lets call it ComputerSoftware which tell which software has in what computer ( or you can also see it - which computer use what software ), which has ComputerID column and SoftwareID column.
example (source)
class Country
{
public int Id { get; set; }
public virtual ICollection<CountryCurrency> CountryCurrencies { get; set; }
}
class Currency
{
public int Id { get; set; }
}
class CountryCurrency
{
[Key, Column(Order=0)]
public virtual int CountryId { get; set; }
[Key, Column(Order=1)]
public virtual int CurrencyId { get; set; }
public virtual Country Country { get; set; }
public virtual Currency Currency { get; set; }
}
Your issue could be that in the migration file creating those tables, it will have something like
.ForeignKey("dbo.Relationship", t => t.Id, cascadeDelete: true)
This will be set on both tables, ConfigurationItem and Relationship of their Primary Key fields. When you delete one, that config tells SQL Server to delete the relationships as well and the relationship probably has a cascadeDelete: true to the parent. This will cause the cyclical cascading delete issue you are experiencing.
After the migration has been generated, go in and change one or all to cascadeDelete: false and this will fix that issue. This is what EF generates by default if I recall.
I am apparently having a real devil of a time understanding Entity Framework 6 which I am using with ASP.NET MVC 5.
The core of the matter is that I have a really quite simple data model that is typical of any real world situation where I have various business objects that have other business objects as properties (and of course they child objects may in turn have other child business objects) and also various types of lookup/type data (Country, State/Province, LanguageType, StatusType etc.) and I cannot figure out how to save/update it properly.
I keep going back and forth between two error states:
1) I either run into the situation where saving a parent business object results in unwanted duplicate values being inserted into my lookup/type tables (for example saving a business object that has been assigned an existing LanguageType of 'English' will result in another LanguageType for 'English' being inserted into the LanguageType table), or
2) I use some of the suggestions I've seen here and elsewhere on the net (e.g. Saving Entity causes duplicate insert into lookup data, Prevent Entity Framework to Insert Values for Navigational Properties ) to solve issue 1 and then find myself fighting against this same issue: An object with the same key already exists in the ObjectStateManager. The ObjectStateManager cannot track multiple objects with the same key .
I will now provide a few code snippets to help build the picture of what I am trying to do and what I am using to do it. First, an example of the entities involved:
public class Customer : BaseEntity
{
public string Name { get; set; }
[LocalizedDisplayName("Contacts")]
public virtual List Contacts { get; set; }
}
public class Contact : BaseEntity
{
[Required]
public string FirstName { get; set; }
[Required]
public string LastName { get; set; }
public int? LanguageTypeID { get; set; }
[Required]
[ForeignKey("LanguageTypeID")]
public virtual LanguageType Language { get; set; }
}
public class LanguageType : Lookup
{
[LocalizedDisplayName("CultureName")]
public string CultureName { get; set; }
}
public class Lookup : BaseEntity
{
public string DisplayName { get; set; }
public int DisplayOrder { get; set; }
public string Name { get; set; }
public string Description { get; set; }
}
public class BaseEntity
{
public int ID { get; set; }
public DateTime? CreatedOn { get; set; }
public DateTime? UpdatedOn { get; set; }
public DateTime? DeletedOn { get; set; }
public bool Deleted { get; set; }
public bool Active { get; set; }
public ApplicationUser CreatedByUser { get; set; }
public ApplicationUser UpdatedByUser { get; set; }
}
In my controller, I have some code like the following:
foreach(Contact contact in lstContacts)
{
customer.Contacts.Add(contact);
}
if (ModelState.IsValid)
{
repository.Add(customer);
}
Let us suppose that each of the contacts has the same LanguageType of 'English' assigned (and in this example it is the fact that I am trying to save multiple contacts that have the same LanguageType that triggers the ObjectStateManager error). Initially, the repository.Add() code just did a context.SaveChanges() which did not work as expected, so now it looks something like this (Entity variable is a Customer):
try
{
if(Entity.Contacts != null)
{
foreach(Contact contact in Entity.Contacts)
{
var entry = this.context.Entry(contact.Language);
var key = contact.Language.ID;
if (entry.State == EntityState.Detached)
{
var currentEntry = this.context.LanguageTypes.Local.SingleOrDefault(l => l.ID == key);
if (currentEntry != null)
{
var attachedEntry = this.context.Entry(currentEntry);
//attachedEntry.CurrentValues.SetValues(entityToUpdate);
attachedEntry.State = EntityState.Unchanged;
}
else
{
this.context.LanguageTypes.Attach(contact.Language);
entry.State = EntityState.Unchanged;
}
}
}
}
context.Customers.Add(Entity);
context.SaveChanges();
}
catch(Exception ex)
{
throw;
}
Is it fundamentally wrong to expect this to have worked? How am I supposed to save and example like this? I have similar problems saving similar object graphs. When I look at tutorials and examples for EF, they are all simple and they all just call SaveChanges() after doing something very similar to what I am doing here.
I've just recently been using the ORM capabilities of ColdFusion (which is hibernate under the covers) and there are would simply load the LanguageType entity, assign it to the Contact entity, save the Contact entity, assign it to the Customer and then save the Customer.
In my mind, this is the most basic of situations and I cannot believe that it has caused me so much pain - I hate to say it, but using plain old ADO.NET (or heaven forbid, ColdFusion which I really don't enjoy) would have been MUCH simpler. So I am missing SOMETHING. I apparently have a key flaw in my understanding/approach to EF and If somebody could help me to make this work as expected and help me to figure out just where my misunderstanding lies, I would greatly appreciate it. I have spend too many hours and hours on this and it is a waste of time - I have/will have countless examples just like this one in the code I am building so I need to adjust my thinking with respect to EF right now so I can be productive and do approach things in the expected way.
Your help will mean so much and I thank you for it!
Let's consider the following object graph in which a teacher instance is the root object,
Teacher --[has many]--> courses
Teacher --[Has One]--> Department
In entity framework's DbContext, each instance of an object has a State indicating whether the object is Added, Modified, Removed or Unchanged. What happens apparently is the following :
Creating the root object for the first time
In this case, in addition to the newly created root object Teacher, ALL the child objects in the graph will have the State Added as well even if they're already created. The solution for this problem is to include the foreign key property for each child element and use it instead, i.e. Teacher.DepartmentId = 3 for example.
Updating the root object and one of its child elements' properties
Suppose you fetch a teacher object from the db, and you change the Teacher.Name property as well as the Teacher.Department.Name property; in this case, only the teacher root object will have the State marked as Modified, the department's State on the other hand remains Unchanged and the modification won't be persisted into DB; Silently without any warning.
EDIT 1
I used your classes as follows and I don't have a problem with persisting the objects :
public class Customer : BaseEntity
{
public string Name { get; set; }
public virtual List<Contact> Contacts { get; set; }
}
public class Contact : BaseEntity
{
public string FirstName { get; set; }
public string LastName { get; set; }
public int? LanguageTypeID { get; set; }
public Customer Customer { get; set; }
[ForeignKey("LanguageTypeID")]
public LanguageType Language { get; set; }
}
public class LanguageType : Lookup
{
public string CultureName { get; set; }
}
public class Lookup : BaseEntity
{
public string DisplayName { get; set; }
public int DisplayOrder { get; set; }
public string Name { get; set; }
public string Description { get; set; }
}
public class BaseEntity
{
public int ID { get; set; }
public DateTime? CreatedOn { get; set; }
public DateTime? UpdatedOn { get; set; }
public DateTime? DeletedOn { get; set; }
public bool Deleted { get; set; }
public bool Active { get; set; }
public ApplicationUser CreatedByUser { get; set; }
public ApplicationUser UpdatedByUser { get; set; }
}
public class ApplicationUser
{
public int ID { get; set; }
public string UserName { get; set; }
public string Password { get; set; }
}
And used the following Context :
public class Context : DbContext
{
public Context() : base("name=CS") { }
public DbSet<Customer> Customers { get; set; }
public DbSet<Contact> Contacts { get; set; }
public DbSet<LanguageType> LanguageTypes { get; set; }
public DbSet<ApplicationUser> ApplicationUsers { get; set; }
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
//I'm generating the database using those entities you defined;
//Here we're demanding not add 's' to the end of table names
modelBuilder.Conventions.Remove<PluralizingTableNameConvention>();
}
}
Then I created a unit tests class with the following :
[TestMethod]
public void TestMethod1()
{
//our context
var ctx = new Infrastructure.EF.Context();
//our language types
var languageType1 = new LanguageType { ID = 1, Name = "French" };
var languageType2 = new LanguageType { ID = 2, Name = "English" };
ctx.LanguageTypes.AddRange(new LanguageType[] { languageType1, languageType2 });
//persist our language types into db before we continue.
ctx.SaveChanges();
//now we're about to start a new unit of work
var customer = new Customer
{
ID = 1,
Name = "C1",
Contacts = new List<Contact>() //To avoid null exception
};
//notice that we're assigning the id of the language type and not
//an object.
var Contacts = new List<Contact>(new Contact[] {
new Contact{ID=1, Customer = customer, LanguageTypeID=1},
new Contact{ID=2, Customer = customer, LanguageTypeID=2}
});
customer.Contacts.AddRange(Contacts);
//adding the customer here will mark the whole object graph as 'Added'
ctx.Customers.Add(customer);
//The customer & contacts are persisted, and in the DB, the language
//types are not redundant.
ctx.SaveChanges();
}
It all worked smoothly without any problems.
As far as i know there is no build in support for reattaching modified graphs (like the SaveOrUpdate method of nHibernate). Perhaps this or this can help you.
I'm trying to create a simple entity framework code first application. I have these classes:
public class User
{
public int UserId { get; set; }
public string Username { get; set; }
public virtual ActivationTicket ActivationTicket { get; set; }
}
public class ActivationTicket
{
public int ActivationTicketId { get; set; }
public virtual User User { get; set; }
public string Ticket { get; set; }
}
When I try to create a new user and save it to the database (a user without a ActivationTicket that is) I receive an exception
The INSERT statement conflicted with the FOREIGN KEY constraint "ActivationTicket_User". The conflict occurred in database "Test", table "dbo.ActivatioTickets", column 'ActivationTicketId'. The statement has been terminated.
I assume EF treats the mapping between User and ActivationTicket as 1-1 but it should be 1-0..1
What do I have to do to get this to work?
You will need a mapping rule like this:
modelBuilder
.Entity<User>()
.HasOptional<ActivationTicket>(u => u.ActivationTicket)
.WithOptionalPrincipal();
This will give you an ActivationTickets table with a UserId that is nullable.
#b3n
It should be enough to do this, at least with VS 2013:
public class User
{
public int UserId { get; set; }
public string Username { get; set; }
public int? ActivationTicketId { get; set;}
public virtual ActivationTicket ActivationTicket { get; set; }
}
This is the important part:
public int? ActivationTicketId { get; set;}
This will specify the foreignkey in your "User" table for the ActivasionTicket and define that it's optional.
credits go to:
http://forums.asp.net/t/1948351.aspx?Change+Foreign+Key+to+nullable+using+Code+First#5554732
I had the same problem and it instantly worked for me.
Also as a note i marked all my primary keys with the data annotation "[Key]". This might be necessary in order to make this work.