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EF: manual (non autogenerated) keys require ad hoc handling of new entities

In a MVVM application with EF Core as ORM I decided to model a table with a manually inserted, textual primary key.
This is because in this specific application I'd rather use meaningful keys instead of meaningless integer ids, at least for simple key-value tables like the table of the countries of the world.
I have something like:
Id | Description
-----|--------------------------
USA | United States of America
ITA | Italy
etc. etc.
So the entity is:
public class Country
{
[Key]
[DatabaseGenerated(DatabaseGeneratedOption.None)]
public string Id { get; set; }
public string Description { get; set; }
}
Here is my viewmodel. It's little more than a container for the ObservableCollection of Countries. Actually it gets loaded from a repository. It's trivial and I inlcuded the entire code at the end. It's not really relevant and I could do with just the DbContext as well. But I wanted to show all the layers to see where the solution belongs to. Oh yes, then it contains the synchronizing code that actually offends EF Core.
public class CountriesViewModel
{
//CountryRepository normally would be injected
public CountryRepository CountryRepository { get; set; } = new CountryRepository(new AppDbContext());
public ObservableCollection<Country> Countries {get; set;}
public CountriesViewModel()
{
Countries = new ObservableCollection<Country>();
Countries.CollectionChanged += Countries_CollectionChanged;
}
private void Countries_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
{
foreach (Country c in e.NewItems)
{
CountryRepository.Add(c);
}
}
}
In my MainWindow I just have:
<Window.DataContext>
<local:CountriesViewModel/>
</Window.DataContext>
<DockPanel>
<DataGrid ItemsSource="{Binding Countries}"/>
</DockPanel>
Problem and question
Now this doesn't work. When we try to insert a new record, in this case I do it using the automatic feature of DataGrid I get a:
System.InvalidOperationException: 'Unable to track an entity of type 'Country'
because primary key property 'Id' is null.'
Each time i add a new record to the ObservableCollection I also try to add it back to the repository, that in turn adds it on the EF DbContext that doesn't accept entities with null key.
So what are my options here?
One is postponing the addition of the new record till the Id has been inserted. This is not trivial as the collection handling that I've shown, but this is not the problem. The worst is that this way I would have some record that are tracked by EF (the updated and the deleted and the new with pk assigned) and some that are tracked by the view model (the new ones with the key not yet assigned).
Another is using alternate keys; I would have an integer, autogenerated primary key and the ITA,USA etc code would be an alternate key that would be used also in relations. It's not so bad from as simplicity, but I'd like a application-only solution.
What I'm looking for
I'm looking for a neat solution here, a pattern to be used whenever this problem arises and that plays well in the context of a MVVM/EF application.
Of course I could also look in the direction of the view events, that is force the user to insert the key before of a certain event that triggers the insertion. I would consider it a second-class solution because it is sort of view dependent.
Remaining code
Just for completeness, in case that you want to run the code, here is the remaining code.
DbContext
(Configured for postgres)
public class AppDbContext : DbContext
{
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
optionsBuilder.UseNpgsql("Host=localhost;Database=WpfApp1;Username=postgres;Password=postgres");
}
public DbSet<Country> Countries { get;set; }
}
Repository
The reason why I implemented the repository for such a simple example is because I think that a possible solution may be to include the new-without-key records managment in the Repository instead of in the viewmodel. I still hope that someone comes out with a simpler solution.
public class CountryRepository
{
private AppDbContext AppDbContext { get; set; }
public CountryRepository(AppDbContext appDbContext) => AppDbContext = appDbContext;
public IEnumerable<Country> All() => AppDbContext.Countries.ToList();
public void Add(Country country) => AppDbContext.Add(country);
//ususally we don't have a save here, it's in a Unit of Work;
//for the example purpose it's ok
public int Save() => AppDbContext.SaveChanges();
}

Probably the cleanest way to address the aforementioned issue in EF Core is to utilize temporary value generation on add. In order to do that, you would need a custom ValueGenerator like this:
using Microsoft.EntityFrameworkCore.ChangeTracking;
using Microsoft.EntityFrameworkCore.ValueGeneration;
public class TemporaryStringValueGenerator : ValueGenerator<string>
{
public override bool GeneratesTemporaryValues => true; // <-- essential
public override string Next(EntityEntry entry) => Guid.NewGuid().ToString();
}
and fluent configuration similar to this:
modelBuilder.Entity<Country>().Property(e => e.Id)
.HasValueGenerator<TemporaryStringValueGenerator>()
.ValueGeneratedOnAdd();
The potential drawbacks are:
In pre EF Core 3.0 the generated temporary value is set onto entity instance, thus would be visible in the UI. This has been fixed in EF Core 3.0, so now Temporary key values are no longer set onto entity instances
Even though the property looks empty (null) and is required (default for primary/alternate keys), if you don't provide explicit value, EF Core will try to issue INSERT command and read the "actual" value back from database similar to identity and other database generated values, which in this case will lead to non user friendly database generated runtime exception. But EF Core in general does not do validations, so this won't be so different - you have to add and validate property required rule in the corresponding layer.

.NET Core Entity Framework InvalidOperationException

I have a simple model
[Table("InterfaceType")]
public class InterfaceType
{
[Key]
public int InterfaceTypeId { get; set; }
public string Description { get; set; }
}
and in my DbContext
public DbSet<InterfaceType> InterfaceTypes { get; set; }
and in my controller
List<InterfaceType> types = _context.InterfaceTypes.FromSql(
"SELECT * FROM [Interfaces].[Control].[InterfaceType]").ToList();
Which is returning the error:
InvalidOperationException: The required column 'InterfaceID' was not present in the results of a 'FromSql' operation.
I am using FromSql in other methods similar to this with no issue although those models do contain an InterfaceId. Why does this operation expect an InterfaceId when it is not in the model. I have also tried the below with the same result.
List<InterfaceType> types = _context.InterfaceTypes.FromSql(
"SELECT InterfaceTypeId, Description FROM [Interfaces].[Control].[InterfaceType]").ToList();
I have also tried:
interfacesOverview.SelectedInterface.InterfaceTypes = _context.InterfaceTypes.ToList();
After declaring via the fluent api:
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.Entity<InterfaceType>().ToTable("InterfaceType", "Control");
}
with the same result.
For clarity here is the table in MSSQL:
CREATE TABLE [Control].[InterfaceType](
[InterfaceTypeId] [tinyint] NOT NULL,
[Description] [varchar](25) NULL,
CONSTRAINT [PK_InterfaceType] PRIMARY KEY CLUSTERED
(
[InterfaceTypeId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
UPDATE
I've looked at the SQL that EF is generating:
SELECT [i].[InterfaceTypeId], [i].[Description], [i].[InterfaceID] FROM [Control].[InterfaceType] AS [i]
Where is it getting InterfaceID from?

Where is it getting InterfaceID from?
First, it should be clear that it's not coming from the shown "simple" (but apparently incomplete) model.
The EF generated SQL clearly indicates that you didn't rename the PK property generated column, also there is no Discriminator column, so it cannot be coming from inheritance. And the chance that you have explicitly defined a shadow property called InterfaceID and not noticing it is small.
All this, along with the fact that the name InterfaceID matches one of the EF Core conventional names for FK property/column name for me is a clear indication of a conventional FK introduced by a relationship. For instance having a second model like this:
public class Interface
{
public int ID { get; set; }
// or
// public int InterfaceID { get; set; }
public ICollection<InterfaceType> InterfaceTypes { get; set; }
}
As explained in the Relationships - Single Navigation Property EF Core documentation topic:
Including just one navigation property (no inverse navigation, and no foreign key property) is enough to have a relationship defined by convention.
and the accompanying example shows Blog / Post model with only public List<Post> Posts { get; set; } property in Blog highlighted.
All EF Core runtime behaviors are based on model metadata. It doesn't matter what is the structure of your database, the more important is what EF Core thinks it is base on your model classes, data annotations and fluent configuration, and if that matches the database schema. The easier way to check that is to generate migration and check if it matches the database schema or not.
So if the relationship is intentional, then you have to update your database to match your model. Otherwise you need to update your model to match the database - by removing or ignoring the collection navigation property (or correcting the invalid data annotation / fluent configuration causing the discrepancy).

My understanding of this problem, is that EF created a Shadow Property
inside your model class, possibly by partially discovered relationship in your Interface model.
Also I feel there is a mismatch between your ModelSnapshot used by EFCore and real state of tables in Database (possibly by pending migration). Double check, how your InterfaceType in <YourDbContext>ModelSnapshot.cs, and check if there's a property you are missing.

My guess is that you also have an "Interface" table registered in the context that holds a reference to the InterfaceType. Interface would have an InterfaceTypeId field declared, however with EF, if you are using HasOne with a ForeignKey, check that you haven't accidentally assigned something like:
.HasOne(x => x.InterfaceType).WithOne().HasForeignKey<InterfaceType>("InterfaceId");
In the case of an Interface having an InterfaceType it would be mapped more like:
.HasOne(x => x.InterfaceType).WithMany();
This might have crept into one of your other associated entities. Often these are typos where the autocomplete picked the wrong type without you noticing. If that mapping exists on any of your classes, EF will be expecting to find an InterfaceId column on InterfaceType. Do a search on HasForeignKey<InterfaceType> and see if that turns up anything out of the ordinary.

First why not use
List<InterfaceType> types = _context.InterfaceTypes.ToList();
Secondly did you apply any changes to the model and forget to persist this to the database, as it could be that the column is correct in your class but not in your database. This is often something i forget to do when using a Code-FirstModel.
Here is some additional info on FromSQL :- https://learn.microsoft.com/en-us/ef/core/querying/raw-sql
More detail on migration here:- https://learn.microsoft.com/en-us/ef/core/managing-schemas/migrations/
I hope this helps.

Maybe try to add DatabaseGeneratedAttribute
[Table("InterfaceType")]
public class InterfaceType
{
[DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity),Key()]
public int InterfaceTypeId { get; set; }
...

Just to check if there is a way I can reproduce, I created a sample .NET Core Console application to check this and in my case, I am able to retrieve the data from DB without any exception.
I understand you have other models where the same code is working,
and if you move the problematic code outside your original solution, you might be
able to figure out if there is something obvious you are missing.
I tried to follow your code as closely as possible in attempt to reproduce this issue, where some of the things I had to change.
I don't know which .NET Core and EF Core versions you have used. In my sample, I used:
.NET Core 2.2
Microsoft.EntityFrameworkCore 2.2
Microsoft.EntityFrameworkCore.Design 2.2
Microsoft.EntityFrameworkCore.SqlServer 2.2
Microsoft.EntityFrameworkCore.Tools 2.2
I created:
Model Class as per your sample
Context Class with OnModelCreating per your sample
Executed following dotnet core commands in the same order as listed below:
dotnet restore
dotnet build
dotnet ef migrations add InitMgr
dotnet ef database update
Added few test records in the table
Copied your records retrieval code, removed "[Interfaces]" from the query and debugged the code below.
var _context = new InterfaceTypeContext ();
List<InterfaceType> types = _context.InterfaceTypes.FromSql ("SELECT * FROM [Control].[InterfaceType]").ToList ();
I was able to retrieve the data from DB.
It would also help if you share Minimal, Complete, and Verifiable
example for someone to debug and help you find a solution for
this.

The following has worked for me:
Insert some data:
insert into [Control].[InterfaceType] values (1, 'Desc1'), (2, 'Desc2');
C#:
class SOContext : DbContext
{
public DbSet<InterfaceType> InterfaceTypes { get; set; }
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
var conn_string = #"Server=(localdb)\mssqllocaldb;Database=Interfaces;Trusted_Connection=Yes;";
optionsBuilder.UseSqlServer(conn_string);
}
}
[Table("InterfaceType", Schema = "Control")]
public class InterfaceType
{
[DatabaseGenerated(DatabaseGeneratedOption.None), Key]
public byte InterfaceTypeId { get; set; }
public string Description { get; set; }
public override string ToString() =>
$"Id: {InterfaceTypeId} | Description: {Description}";
}
// Output:
// Id: 1 | Description: Desc1
// Id: 2 | Description: Desc2

Load object from table with id as foreign key

i have define users with UserRole type Role as foreing key of another table. Entiti framework auto transformed name UserRole to UserRoleId with type integer (Id). Its ok. But i have UserRoleId = 2 in table User table preview
and when i trying get user by Id:
var user = await _userManager.FindByIdAsync(id);
so parameter UserRole is null user preview from debuging.
Application user:
public class ApplicationUser : IdentityUser
{
public int PriorityLevel { get; set; }
public Role UserRole { get; set; }
public string ForName { get; set; }
public string LastName { get; set; }
public string Mobile { get; set; }
public string WorkPlace { get; set; }
public string Notice { get; set; }
}
Must i use some entity framework command for load UserRole as type Role? Or can i get UserRoleId from table?
Thanks for any advice.

You tagged your question with Entity Framework, but just in case, I'll cover Entity Framework Core, as well, since they diverge slightly on this.
Entity Framework supports lazy-loading, but importantly, the property must be virtual to enable that, i.e.:
public virtual Role UserRole { get; set; }
EF adds the logic necessary to do lazy-load the property by deriving a dynamic "proxy" class from your entity and overriding the property. Since non-virtual properties cannot be overridden, this logic is not added to properties without virtual. Without virtual, navigation properties will not be loaded automatically, and will remain null, whether there's associations in the database or not.
Entity Framework Core does not support lazy-loading at all, so adding virtual will not help you.
In either case, you should really be eager-loading, instead. This is done by adding Include to your query. However, you have two problems there:
UserManager<TUser> doesn't support including related entities. You would need to use the DbSet, i.e. context.Users.
You cannot use Include with Find (or any of the Find-related methods), as it has logic that is incompatible with doing joins. Namely, Find attempts to pull the entity from the context cache without doing a query at all, if possible, and as such, cannot guarantee that navigation properties can be included. As a result, you will have to switch to something like SingleOrDefault.
var user = await context.Users.Include(x => x.UserRole).SingleOrDefault(x => x.Id == id);
With that, UserRole should now actually have a value.
Alternatively, you should be able to explicitly-load UserRole:
var user = await _userManager.FindByIdAsync(id);
context.Entry(user).Reference(s => s.UserRole).Load();
This methodology is the same in both Entity Framework and Entity Framework Core. However it was only introduced into EF Core in 1.1, so be cognizant of the version you're running. The one downside here is that this will result in (potentially) two queries. Again, since the Find methods attempt to retrieve from the context cache, it's possible you'll avoid a query there, but the entity is not in the cache, it will take two queries: one for the user and one for the related role. However, that would have been the case as well with lazy-loading. Only eagerly-loading allows the queries to be combined using JOINs.

You can get all the roles of the user using this method.
_userManager.GetRolesAsync(id);

Can Entity Framework automatically extend models with datetime fields to keep the changing date

Can you tell Entity Framework to add an extra field for each field of a certain type? For example: Is it possible to generate a ChangedAt datetime field for each boolean field defined in the model, so this
public bool Confirmed { get; set; }
could result in a table with an additional field ConfirmedChangedAt where the value is updated each time the boolean value is changed.

Usually behavior like this should be implemented directly into your business logic and not automatically into the data layer. So I suggest to write something like this:
// entity
public class Order
{
public bool Confirmed { get; set; }
public DateTime? ConfirmedAt { get; set; }
}
// business logic
public class OrderManager
{
.................
public void Confirm( Order order )
{
// changing of entity status
order.Confirmed = true;
order.ConfirmedAt = DateTime.Now;
// storing new entity status
_orderRepository.Update( order );
................
}
}

I think if i understand you correctly, You are expecting the Entity Framework to be able to add columns to the database automatically so that you don't have to add them manually, Well you have 2 cases:
if you are using the database first approach you could achieve this
by using a query that's specific to your needs to add these columns
for you based on the conditions you have.
If you are using the code first approach and you have an existing database you may reverse engineer the database using the Entity Framework Power Tools and you could customize the T4 Templates to generate the entities with the extra properties that you need.

Plain answer no.
But it's depend on way how you interact with EF (code first, model first,database first).
If you using EF 6 and code first approach you can use idea of base Entity class
public class BaseEntity
{
public DateTime ChangedAt {get;set;}
}
public class ConcreteEntity : BaseEntity
{
public string Name {get;set;}
}
Now ConcreteEntity has ChangedAt by inheritance.
If this solution not for you, please explain question with more details.

Entity Framework associations with multiple (separate) keys on view

I'm having problems setting up an Entity Framework 4 model.
A Contact object is exposed in the database as an updateable view. Also due to the history of the database, this Contact view has two different keys, one from a legacy system. So some other tables reference a contact with a 'ContactID' while other older tables reference it with a 'LegacyContactID'.
Since this is a view, there are no foreign keys in the database, and I'm trying to manually add associations in the designer. But the fluent associations don't seem to provide a way of specifying which field is referenced.
How do I build this model?
public class vwContact
{
public int KeyField { get; set; }
public string LegacyKeyField { get; set; }
}
public class SomeObject
{
public virtual vwContact Contact { get; set; }
public int ContactId { get; set; } //references vwContact.KeyField
}
public class LegacyObject
{
public virtual vwContact Contact { get; set; }
public string ContactId { get; set; } //references vwContact.LegacyKeyField
}
ModelCreatingFunction(modelBuilder)
{
// can't set both of these, right?
modelBuilder.Entity<vwContact>().HasKey(x => x.KeyField);
modelBuilder.Entity<vwContact>().HasKey(x => x.LegacyKeyField);
modelBuilder.Entity<LegacyObject>().HasRequired(x => x.Contact).???
//is there some way to say which key field this reference is referencing?
}

EDIT 2: "New things have come to light, man" - His Dudeness
After a but more experimentation and news, I found using a base class and child classes with different keys will not work by itself. With code first especially, base entities must define a key if they are not explicitly mapped to tables.
I left the suggested code below because I still recommend using the base class for your C# manageability, but I below the code I have updated my answer and provided other workaround options.
Unfortunately, the truth revealed is that you cannot accomplish what you seek without altering SQL due to limitations on EF 4.1+ code first.
Base Contact Class
public abstract class BaseContact
{
// Include all properties here except for the keys
// public string Name { get; set; }
}
Entity Classes
Set this up via the fluent API if you like, but for easy illustration I've used the data annotations
public class Contact : BaseContact
{
[Key]
public int KeyField { get; set; }
public string LegacyKeyField { get; set; }
}
public class LegacyContact : BaseContact
{
public int KeyField { get; set; }
[Key]
public string LegacyKeyField { get; set; }
}
Using the Entities
Classes that reference or manipulate the contact objects should reference the base class much like an interface:
public class SomeCustomObject
{
public BaseContact Contact { get; set; }
}
If later you need to programmatically determine what type you are working with use typeof() and manipulate the entity accordingly.
var co = new SomeCustomObject(); // assume its loaded with data
if(co.Contact == typeof(LegacyContact)
// manipulate accordingly.
New Options & Workarounds
As I suggested in comment before, you won't be able to map them to a single view/table anyway so you have a couple options:
a. map your objects to their underlying tables and alter your "get/read" methods on repositories and service classes pull from the joined view -or-
b. create a second view and map each object to their appropriate view.
c. map one entity to its underlying table and one to the view.
Summary
Try (B) first, creating a separate view because it requires the least amount of change to both code and DB schema (you aren't fiddling with underlying tables, or affecting stored procedures). It also ensures your EF C# POCOs will function equivalently (one to a view and one to table may cause quirks). Miguel's answer below seems to be roughly the same suggestion so I would start here if it's possible.
Option (C) seems worst because your POCO entities may behave have unforseen quirks when mapped to different SQL pieces (tables vs. views) causing coding issues down the road.
Option (A), while it fits EF's intention best (entities mapped to tables), it means to get your joined view you must alter your C# services/repositories to work with the EF entities for Add, Update, Delete operations, but tell the Pull/Read-like methods to grab data from the joint views. This is probably your best choice, but involves more work than (B) and may also affect Schema in the long run. More complexity equals more risk.

Edit I'm not sure this is actually possible, and this is why:
The assumption is that a foreign key references a primary key. What you've got is two fields which are both acting as primary keys of vwContact, but depending on which object you ask it's a different field that's the primary key. You can only have one primary key at once, and although you can have a compound primary key you can't do primary key things with only half of it - you have to have a compound foreign key with which to reference it.
This is why Entity Framework doesn't have a way to specify the mapping column on the target side, because it has to use the primary key.
Now, you can layer some more objects on top of the EF entities to do some manual lookup and simulate the navigation properties, but I don't think you can actually get EF to do what you want because SQL itself won't do what you want - the rule is one primary key per table, and it's not negotiable.
From what you said about your database structure, it may be possible for you to write a migration script which can give the contact entities a consistent primary key and update everything else to refer to them with that single primary key rather than the two systems resulting from the legacy data, as you can of course do joins on any fields you like. I don't think you're going to get a seamlessly functional EF model without changing your database though.
Original Answer That Won't Work
So, vwContact contains a key KeyField which is referenced by many SomeObjects and another key LegacyKeyField which is referenced by many LegacyObjects.
I think this is how you have to approach this:
Give vwContact navigation properties for SomeObject and LegacyObject collections:
public virtual ICollection<SomeObject> SomeObjects { get; set; }
public virtual ICollection<LegacyObject> LegacyObjects { get; set; }
Give those navigation properties foreign keys to use:
modelBuilder.Entity<vwContact>()
.HasMany(c => c.SomeObjects)
.WithRequired(s => s.Contact)
.HasForeignKey(c => c.KeyField);
modelBuilder.Entity<vwContact>()
.HasMany(c => c.LegacyObjects)
.WithRequired(l => l.Contact)
.HasForeignKey(c => c.LegacyKeyField);
The trouble is I would guess you've already tried this and it didn't work, in which case I can't offer you much else as I've not done a huge amount of this kind of thing (our database is much closer to the kinds of thing EF expects so we've had to do relatively minimal mapping overrides, usually with many-to-many relationships).
As for your two calls to HasKey on vwContact, they can't both be the definitive key for the object, so it's either a compound key which features both of them, or pick one, or there's another field you haven't mentioned which is the real primary key. From here it's not really possible to say what the right option there is.

You should be able to do this with two different objects to represent the Contact view.
public class vwContact
{
public int KeyField { get; set; }
public string LegacyKeyField { get; set; }
}
public class vwLegacyContact
{
public int KeyField { get; set; }
public string LegacyKeyField { get; set; }
}
public class SomeObject
{
public virtual vwContact Contact { get; set; }
public int ContactId { get; set; } //references vwContact.KeyField
}
public class LegacyObject
{
public virtual vwLegacyContact Contact { get; set; }
public string ContactId { get; set; } //references vwLegacyContact.LegacyKeyField
}
ModelCreatingFunction(modelBuilder)
{
// can't set both of these, right?
modelBuilder.Entity<vwContact>().HasKey(x => x.KeyField);
modelBuilder.Entity<vwLegacyContact>().HasKey(x => x.LegacyKeyField);
// The rest of your configuration
}

I have tried everything that you can imagine, and found that most solutions won't work in this version of EF... maybe in future versions it supports referencing another entity by using an unique field, but this is not the case now. I also found two solutions that work, but they are more of a workaround than solutions.
I tried all of the following things, that didn't work:
Mapping two entities to the same table: this is not allowed in EF4.
Inheriting from a base that has no key definitions: all root classes must have keys, so that inherited classes share this common key... that is how inheritance works in EF4.
Inheriting from base class that defines all fields, including keys, and then use modelBuilder to tell wich base-properties are keys of the derived types: this doesn't work, because the methos HasKey, Property and others that take members as parameters, must reference members of the class itself... referencing properties of a base class is not allowed. This cannot be done: modelBuilder.HasKey<MyClass>(x => x.BaseKeyField)
The two things that I did that worked:
Without DB changes: Map to the table that is source of the view in question... that is, if vwContact is a view to Contacts table, then you can map a class to Contacts, and use it by setting the key to the KeyField, and another class mapping to the vwContacts view, with the key being LegacyKeyField. In the class Contacts, the LegacyKeyField must exist, and you will have to manage this manually, when using the Contacts class. Also, when using the class vwContacts you will have to manually manage the KeyField, unless it is an autoincrement field in the DB, in this case, you must remove the property from vwContacts class.
Changing DB: Create another view, just like the vwContacts, say vwContactsLegacy, and map it to a class in wich the key is the LegacyKeyField, and map vwContacts to the original view, using KeyField as the key. All limitations from the first case also applies: the vwContacts must have the LegacyKeyField, managed manually. And the vwContactsLegacy, must have the KetField if it is not autoincrement idenitity, otherwise it must not be defined.
There are some limitations:
As I said, these solutions are work-arounds... not real solutions, there are some serious implications, that may even make them undesirable:
EF does not know that you are mapping two classes to the same thing. So when you update one thing, the other one could be changed or not, it depends if the objects is cached or not. Also, you could have two objects at the same time, that represents the same thing on the backing storage, so say you load a vwContact and also a vwContactLegacy, changes both, and then try to save both... you will have to care about this yourself.
You will have to manage one of the keys manually. If you are using vwContacts class, the KeyFieldLegacy is there, and you must fill it. If you want to create a vwContacts, and associate is with a LegacyObject, then you need to create the reference manually, because LegacyObject takes a vwContactsLegacy, not a vwContacts... you will have to create the reference by setting the ContactId field.
I hope that this is more of a help than a disillusion, EF is a powerfull toy, but it is far from perfect... though I think it's going to get much better in the next versions.

I think this may be possible using extension methods, although not directly through EF as #Matthew Walton mentioned in his edit above.
However, with extension methods, you can specify what to do behind the scenes, and have a simple call to it.
public class LegacyObject
{
public virtual vwContact Contact { get; set; }
public string ContactId { get; set; } //references vwContact.LegacyKeyField
}
public class LegacyObjectExtensions
{
public static vwContact Contacts(this LegacyObject legacyObject)
{
var dbContext = new LegacyDbContext();
var contacts = from o in legacyObject
join c in dbContext.vwContact
on o.ContactId == c.LegacyKeyField
select c;
return contacts;
}
}
and
public class SomeObject
{
public virtual vwContact Contact { get; set; }
public int ContactId { get; set; } //references vwContact.KeyField
}
public class SomeObjectExtensions
{
public static vwContact Contacts(this SomeObject someObject)
{
var dbContext = new LegacyDbContext();
var contacts = from o in someObject
join c in dbContext.vwContact
on o.ContactId == c.KeyField
select c;
return contacts;
}
}
Then to use you can simply do like this:
var legacyContacts = legacyObject.Contacts();
var someContacts = someObject.Contacts();

Sometimes it makes more sense to map it from the other end of the relationship, in your case:
modelBuilder.Entity<LegacyObject>().HasRequired(x => x.Contact).WithMany().HasForeignKey(u => u.LegacyKeyField);
however this will require that u.LegacyKeyField is marked as a primary key.
And then I'll give my two cents:
if the Legacy db is using LegacyKeyField, then perhaps the legacy db will be read only. In this case we can create two separate contexts Legacy and Non-legacy and map them accordingly. This can potentially become a bit messy as you'd have to remember which object comes from which context. But then again, nothing stops you from adding the same EF code first object into 2 different contexts
Another solution is to use views with ContactId added for all other legacy tables and map them into one context. This will tax performance for the sake of having cleaner context objects, but this can be counteracted on sql side: indexed views, materialized views, stored procs, etc. So than LEGACY_OBJECT becomes VW_LEGACY OBJECT with CONTACT.ContactId brought over, then:
modelBuilder.Entity<LegacyObject>().ToTable("VW_LEGACY_OBJECT");
modelBuilder.Entity<LegacyObject>().HasRequired(x => x.Contact).WithMany().HasForeignKey(u => u.ContactId);
I personally would go with creating "mapper views" with CustomerId on legacy tables, as it's cleaner from c# layer perspective and you can make those views look like real tables. It is also difficult to suggest a solution without knowing what exactly is the scenario that you have a problem with: querying, loading, saving, etc.