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Mapping 1-0..1 Relationship with Navigation Property Without FK

I've got 2 entities with a 1-0..1 relationship between them, but restrictions on what the generated DB schema can look like.
So 1 Vehicle to 0 or 1 RecVehicle entity
I need to be able to have a navigation property from Vehicle to RecVehicle, but without the DB Schema for the Vehicles table having a FK to RecVehicle. The PK of the RecVehicle table should be the Id of the Vehicle entity it relates to.
We are using EF code first
public class Vehicle
{
[Key]
public int Id { get; set; }
public virtual RecVehicle RecVehicle { get; set; } // Need to be able to use as navigation
}
public class RecVehicle
{
[Key]
public int VehicleId { get; set; }
[ForeignKey("VehicleId")]
public Vehicle Vehicle { get; set; }
}
The generated schema needs to be something like this:
Vehicles
[ Id(int, pk, not null), ...] <-- no FK column to RecVehicles
RecVehicles
[ VehicleId(int, pk, fk, not null), ...]
Originally what I had tried something like this:
public class Vehicle
{
[Key]
public int Id { get; set; }
[InverseProperty("Vehicle")]
public virtual RecVehicle RecVehicle { get; set; } // Need to be able to use as navigation
}
but this causes this exception:
Unable to determine the principal end of an association between the types 'Contract.Entities.Vehicle' and 'Contract.Entities.RecVehicle'. The principal end of this association must be explicitly configured using either the relationship fluent API or data annotations.
I'm not sure what fluent API relationships to setup to make this work, nor the correct set of data annotations to make this work, or if it's even possible.
Reasoning
The reason there is strict limitations on the DB schema is our Data team has a migration/data import process that we can not alter
We have an existing code base that uses the navigation property in many places (2 teams, desync in schema) so changing to use a lookup in code requires many changes in the code base that we are trying to avoid.

The RecVehicle can be connected to multiple Vehicles
Can you try the following navigation property?
public virtual ICollection<RecVehicle> RecVehicle { get; set; }
instead of
public virtual RecVehicle RecVehicle { get; set; }
Due to the RecVehicle primary key this list only maximum contains one element

Ended up being able to get this relationship to work like this:
public class Vehicle
{
[Key]
public int Id { get; set; }
public virtual RecVehicle RecVehicle { get; set; }
}
public class RecVehicle
{
[Key]
public int VehicleId { get; set; }
[ForeignKey("VehicleId"), Required] //<--- Required attr fixed the principal/dependent confusion EF was having
public virtual Vehicle Vehicle { get; set; }
}

EF 6 code-first generating extra table for many-many table?

Referencing from the #Ogglas answer of this post,
I would like to ask if it is normal for EF to generate another table?
If the additional table should not be there, then what am I doing wrong here? Please enlighten me. TIA!
Sample code:
public class Aggregate
{
public Aggregate()
{
Episodes = new HashSet<Episode>();
}
[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
[Key]
public Guid Id { get; set; }
...
public virtual ICollection<Episode> Episodes { get; set; }
}
public class Episode
{
public Episode()
{
Aggregates = new HashSet<Aggregate>();
}
[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
[Key]
public Guid Id { get; set; }
...
public virtual ICollection<Aggregate> Aggregates { get; set; }
}
public class EpisodeAggregate
{
[Key]
[Column(Order = 1)]
[ForeignKey("Episode")]
public Guid EpisodeId { get; set; }
[Key]
[Column(Order = 2)]
[ForeignKey("Aggregate")]
public Guid AggregateId { get; set; }
public virtual Episode Episode { get; set; }
public virtual Aggregate Aggregate { get; set; }
public DateTime Timestamp { get; set; }
}
In my DbContext.cs:
public DbSet<EpisodeAggregate> EpisodeAggregates { get; set; }

You are right. In a relational database, a many-to-many relation is solved using a junction table.
For a standard many-to-many relationship, you don't need to mention this junction table; entity framework recognizes the many-to-many by your use of the virtual ICollection<...> on both sides of the many-to-many relation, and will automatically create the tables for you.
To test my database theories and entity framework, I quite often use a simple database with Schools, Students and Teachers. One-to-many for School-Student and School-Teacher and many-to-many for Teacher-Student. I always see that the Teacher-Student junction table is created automatically, without ever having to mention it.
However!
Your junction table is not standard. A standard junction table has only two columns: the EpisodeId and the AggregateId. It doesn't even have an extra primary key. The combination [EpisodeId, AggregateId] is already unique and can be used as a primary key.
You have in table EpisodeAggregate an extra column: TimeStamp. Apparently you want to know when an Episode and an Aggregate got related.
"Give me the TimeStamp when Episode[4] got related with Aggregate[7]"
This makes that this table is not a standard junction table. There is no many-to-many relation between Episodes and Aggregates. You made a one-to-many relation between Episode and its Relations with the Aggregates, and similarly a one-to-many relation between Aggregates and its Relations with the Episodes.
This makes that you have to change your many-to-many into one-to-many:
class Episode
{
public Guid Id { get; set; }
// every Episode has zero or more Relations with Aggregates (one-to-many)
public virtual ICollection<EpisodeAggregateRelation> EpisodeAggregateRelations { get; set; }
...
}
class Aggregate
{
public Guid Id { get; set; }
// every Episode has zero or more Relations with Episodes(one-to-many)
public virtual ICollection<EpisodeAggregateRelation> EpisodeAggregateRelations { get; set; }
...
}
class EpisodeAggregateRelation
{
// Every Relation is the Relation between one Episode and one Aggregate
// using foreign key:
public Guid EpisodeId { get; set; }
public Guid AggregateId { get; set; }
public virtual Episode Episode { get; set; }
public virtual Aggregate Aggregate { get; set; }
public DateTime Timestamp { get; set; }
}
If you are certain the there will always be at utmost one relation between an Episode and an Aggregate, you can use the combination [EpisodeId, AggregateId] as a primary key. If you think these two might have several relations, you need to add a separate primary key.
I often use my classes in different databases, hence I don't like attributes, I solve it in fluent API in OnModelCreating:
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Entity<Episode>()
.HasKey(episode => episode.Id)
// define the one-to-many with EpisodeAggregateRelations:
.HasMany(episode => episode.EpisodeAggregateRelations)
.WithRequired(relation => relation.Episode)
.HasForeignKey(relation => relation.EpisodeId);
modelBuilder.Entity<Aggregate>()
.HasKey(aggregate => aggregate.Id)
// define the one-to-many with EpisodeAggregateRelations:
.HasMany(aggregate => aggregate .EpisodeAggregateRelations)
.WithRequired(relation => relation.Aggregate)
.HasForeignKey(relation => relation.aggregateId);
The above is not needed!
Because you followed the entity framework code first conventions, you can omit these two statements. Entity framework will recognize the primary key and the one-to-many relation. Only if you want to deviate from the conventions, like a non-standard table name, or if you want to define the column order:
modelBuilder.Entity<Episode>()
.ToTable("MySpecialTableName")
.Property(episode => episode.Date)
.HasColumnName("FirstBroadcastDate")
.HasColumnOrder(3)
.HasColumnType("datetime2");
But again: you followed the conventions, all those attributes like Key, ForeignKey, DatabaseGenerated are not needed. And the column order: who cares? Let your database management system decide about the most optimum column order.
My advice would be: try to experiment: leave out this fluent API and check whether your unit tests still pass. Checked in five minutes.
The EpisodeAggregateRelation has something non-standard: it has a composite primary key. Hence you need to define this. See Configuring a composite primary key
modelBuilder.Entity<EpisodeAggregateRelation>()
.HasKey(relation => new
{
relation.EpisodId,
relation.AggregateId
});
If you already defined the one-to-many in Episodes and Aggregates, or if that was not needed because of the conventions, you don't have to mention this relation here again.
If you want, you can put the one-to-many in the fluent API part of EpisodeAggregateRelation, instead of in the fluent API part of Episode / Aggregate:
// every EpisodeAggregateRelation has one Episode, using foreign key
modelBuilder.Entity<EpisodeAggregateRelation>()
.HasRequired(relation => relation.Episode(
.WithMany(episode => episode.EpisodeAggregateRelations)
.HasForeignKey(relation => relation.EpisodeId);
// similar for Aggregate
One final tip
Don't create a HashSet in the constructor. It is a waste of processing power if you fetch data: you create the HashSet, and it is immediately replaced by the ICollection<...> that entity framework creates.
If you don't believe me: just try it out, and see that your unit tests pass, with the possible exception of the unit test that checks for an existing ICollection<...>

Configure One-None/One Relationship with Multiple Tables using Entity

I'm in a situation where one table has two One-None/One Relationships. How do I implement this using Entity Framework Code-First?
I've seen the following links
https://www.safaribooksonline.com/library/view/programming-entity-framework/9781449317867/ch04s07.html
https://cpratt.co/0-1-to-1-relationships-in-entity-framework/
https://www.tektutorialshub.com/one-to-one-relationship-entity-framework/
Where essentially it's said that the dependent end needs to have a primary key that is the same as that of the principal end. But I'm weary of implementing this with more than one One-None/One Relationship without confirmation and proper knowledge of what's going on. Furthermore I am not sure how to construct statements as it does not have a conventional Foreign Key.
I've also seen Configuring multiple 1 to 0..1 relationships between tables entity framework which confused me beyond recognition.
See below for the relevant part of my DB Diagram:
So Essentially, a Player shouldn't be saved without a DKImage, similarly a Product shouldn't be saved without a DKImage.
Below is the code for Models: Players, Products, DKImages (I know it's not correct, I only implemented it this way so I can generate the database and show the diagram)
Player
public enum Positions { PG, SG, SF, PF, C }
public class Player
{
[Key]
[ForeignKey("Images")]
public int PlayerID { get; set; }
[Required]
public string PlayerName { get; set; }
[Required]
public string PlayerLastName { get; set; }
[Required]
public int PlayerAge { get; set; }
[Required]
public Positions Position { get; set; }
[Required]
public bool Starter { get; set; }
[Required]
[Display(Name = "Active / Not Active")]
public bool Status { get; set; }
//Foreign Keys
public int PlayerStatsID { get; set; }
//Navigation Properties
[ForeignKey("PlayerStatsID")]
public virtual IQueryable<PlayerStats> PlayerStats { get; set; }
public virtual DKImages Images { get; set; }
}
DKImages
public class DKImages
{
[Key]
public int ImageID { get; set; }
[Required]
public string ImageURL { get; set; }
[Required]
public DateTime DateUploaded { get; set; }
//Foreign Keys
[Required]
public int CategoryID { get; set; }
//Navigation Properties
public virtual Products Products { get; set; }
public virtual Category Category { get; set; }
public virtual Player Player { get; set; }
}
Products
public class Products
{
[ForeignKey("Images")]
[Key]
public int ProductID { get; set; }
[Required]
public string ProductName { get; set; }
[Required]
public DateTime DateAdded { get; set; }
//Foreign Keys
[Required]
public int ProductTypeID { get; set; }
//Navigation Properties
[ForeignKey("ProductTypeID")]
public virtual ProductType ProductType { get; set; }
public virtual DKImages Images { get; set; }
}
Edit
I have been told that the code above is correct. If so then how do I create CRUD LINQ Statements (Or any method of constructing CRUD statements for that matter) with the above code.

What you want here is referred to as polymorphic associations: several entities having child entities of one type. They're typically used for comments, remarks, files etc. and usually applied to 1:n associations. In your case there are polymorphic 1:1 associations. Basically these associations look like this (using a bit more generic names):
How to implement them?
Entity Framework 6
In EF6 that's problem. EF6 implements 1:1 associations as shared primary keys: the child's primary key is also a foreign key to its parent's primary key. That would mean that there should be two FKs on Image.ID , one pointing to Person.ID and another one pointing to Product.ID. Technically that's not a problem, semantically it is. Two parent entities now own the same image or, stated differently, an image should always belong to two different parents. In real life, that's nonsense.
The solution could be to reverse the references:
But now there's another problem. The entity that's referred to is named the principal, the other entity is dependent. In the second diagram, Image is the principal, so in order to create a Person, its image must be inserted first and then the person copies its primary key. That's counter-intuitive and most likely also impractical. It's impossible if images are optional.
Nevertheless, since in your case you want images to be required let me show how this association is mapped in EF6.
Let's take this simple model:
public class Person
{
public int ID { get; set; }
public string Name { get; set; }
public virtual Image Image { get; set; }
}
public class Product
{
public int ID { get; set; }
public string Name { get; set; }
public virtual Image Image { get; set; }
}
public class Image
{
public int ImgID { get; set; } // Named for distinction
public string Url { get; set; }
}
The required mapping is:
modelBuilder.Entity<Image>().HasKey(pd => pd.ImgID);
modelBuilder.Entity<Person>().HasRequired(p => p.Image).WithRequiredDependent();
modelBuilder.Entity<Product>().HasRequired(p => p.Image).WithRequiredDependent();
As you see, Image has two required dependents. Perhaps that's better than two required parents, but it's still weird. Fortunately, in reality it's not a problem, because EF doesn't validate these associations. You can even insert an image without a "required" dependent. I don't know why EF doesn't validate this, but here it comes in handy. The part WithRequiredDependent might as well have been WithOptional, it doesn't make a difference for the generated data model, but at least this mapping conveys your intentions.
An alternative approach could be inheritance. If Person and Product inherit from one base class this base class could be the principal in a 1:1 association with Image. However, I think this is abusing a design pattern. People and products have nothing in common. From a design perspective there's no reason for them to be part of one inheritance tree.
Therefore, in EF6 I think the most feasible solution is to use the third alternative: separate image tables per entity.
Entity Framework Core
In EF-core 1:1 associations can be implemented the EF6 way, but it's also possible to use a separate foreign key field in the dependent entity. Doing so, the polymorphic case looks like this:
The Image class is different:
public class Image
{
public Image()
{ }
public int ImgID { get; set; }
public int? PersonID { get; set; }
public int? ProductID { get; set; }
public string Url { get; set; }
}
And the mapping:
modelBuilder.Entity<Person>().Property(p => p.ID).UseSqlServerIdentityColumn();
modelBuilder.Entity<Person>()
.HasOne(p => p.Image)
.WithOne()
.HasForeignKey<Image>(p => p.PersonID);
modelBuilder.Entity<Product>().Property(p => p.ID).UseSqlServerIdentityColumn();
modelBuilder.Entity<Product>()
.HasOne(p => p.Image)
.WithOne()
.HasForeignKey<Image>(p => p.ProductID);
modelBuilder.Entity<Image>().HasKey(p => p.ImgID);
Watch the nullable foreign keys. They're necessary because an image belongs to either a Person or a Product. That's one drawback of this design. Another is that you need a new foreign key field for each new entity you want to own images. Normally you want to avoid such sparse columns. There's also an advantage as compared to the EF6 implementation: this model allows bidirectional navigation. Image may be extended with Person and Product navigation properties.
EF does a pretty good job translating this into a database design. Each foreign key has a filtered unique index, for example for Person:
CREATE UNIQUE NONCLUSTERED INDEX [IX_Image_PersonID] ON [dbo].[Image]
(
[PersonID] ASC
)
WHERE ([PersonID] IS NOT NULL)
This turns the association into a genuine 1:1 association on the database side. Without the unique index it would be a 1:n association from the database's perspective.

An exemple in your Player table would be this :
public class Player
{
// All the rest you already coded
[Required]
public int ImageID
[ForeignKey("ImageID")]
public virtual DKImage DKImage {get;set;}
}
This would force a player to have a DKImage, but as said in the comments, this create a one to many relationship.
Another way out would be to put all Player fields into the DKImage table, those fields would be null if there is no player associated to this DKImage.
Edit for 1 to 1..0
Ivan Stoev's link got some pretty interesting insight on how to accomplish this :
https://weblogs.asp.net/manavi/associations-in-ef-4-1-code-first-part-3-shared-primary-key-associations
It seems like you will have to put a bit more code in your class :
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Entity<DKImage>().HasOptional(t => t.Player).WithRequired();
}
If the tutorial is correct, this would read as :
"DKImage entity has an optional association with one Player object but this association is required for Player entity".
I have not tested it yet.

Table with foreign keys missing members in generated class in edmx

I have a table in the database that has 4 foreign keys referencing to it. When I add the table to the edmx, the table and navigational properties are there. However, the foreign key ids from this table is missing and only the virtual objects are there.
This is the following table that is generated in the .tt file:
public partial class Device
{
public int SolutionId { get; set; }
public string SiteId { get; set; }
public string Name { get; set; }
public int SysId { get; set; }
public Nullable<int> SysType { get; set; }
public string SerialNumber { get; set; }
public Nullable<int> ParentId { get; set; }
public virtual DeviceModel DeviceModel { get; set; }
public virtual DeviceType DeviceType { get; set; }
public virtual SolutionApplication SolutionApplication { get; set; }
public virtual SolutionType SolutionType { get; set; }
}
There are a few members missing:
DeviceModelId, DeviceTypeId, SolutionApplicationId, and SolutionTypeId
Why is it missing? Is there any way to get those keys actually be part of the partial class?
using EntityFrameworks v6.0.2. Lazy Loading

In short, Entity Framework 'abstracts that away'.
Its clever enough to recognise that your FKs represent relationships and so allows you to work with the objects themselves. So instead of having you worry about checking the FK constraint, etc. for, say, SolutionTypeId - you just need to add a SolutionType object to your Device object and let Entity Framework sort it out. (Of course, this causes problems if you try to add a new SolutionType that violates the SolutionType PK so maybe you need to first find an existing object from the SolutionTypes table).
So, instead of thinking of it as a Device table linked to a SolutionType table via a FK - just think of it as a Device object with a SolutionType object as a property. EF sorts out the db for you when you save changes (assuming your model is accurate!)

Using fluent API to set up composite foreign key in one-to-one relation in legacy database

I'm just a beginner in EF code first model. Given two POCO classes mapped to current legacy MS SQL database. They are associated with a composite foreign key setting up one to many relation. Since it's actually one-to-one relation I'd like to have corresponding navigation properties in my POCO objects and do mapping in fluent API. Here is my example:
public partial class Answer
{
//primary key
public int id { get; set; }
//foreign keys
public int question { get; set; }
public int assignedForm { get; set; }
//simple fields
public short state { get; set; }
public int author { get; set; }
//navigation property
public virtual AssignedQuestion AssignedQuestion { get; set; }
}
public partial class AssignedQuestion
{
// primary keys
public int id { get; set; }
public int assignedForm { get; set; }
//simple field
public string content { get; set; }
//navigation property
//public virtual ICollection<Answer> Answers { get; set; }
public virtual Answer Answer { get; set; }
}
If I wanted to do one-to-many relation I would simply uncomment "Answers" collection and have Fluent API mapping:
modelBuilder.Entity<AssignedQuestion>()
.HasKey(q => new { q.id, q.assignedForm });
modelBuilder.Entity<Answer>()
.HasRequired(a => a.AssignedQuestion)
.WithMany(aq=>aq.Answers)
.HasForeignKey(a => new { a.question,a.assignedForm});
My goal is to go with one-to-one relation and use "Answer" property in AssignedQuestion with such Fluent API as:
modelBuilder.Entity<AssignedQuestion>()
.HasKey(q => new { q.id, q.assignedForm });
modelBuilder.Entity<Answer>()
.HasRequired(a => a.AssignedQuestion)
.WithOptional(aq => aq.Answer);
//.HasForeignKey(a => new { a.question, a.assignedForm });
The problem is I can't specify exactly foreign key fields (as in previous example) and uncomment HasForeignKey call. In this case EF tries to join tables using conventional field names "AssignedQuestion_ID" and "AssignedQuestion_AssignedForm" instead of "question" and "assignedForm" in Answer table. Is there a walkaround in Fluent API other than changing field names?

It is not one-to-one relationship so your first mapping is correct. The reason why it is one-to-many is that EF understands one-to-one only when build on PKs on both sides. If AssignedQuestion has PK id and assignedForm your Answer will need to have FK and PK on its id and assignedForm otherwise EF doesn't see it as one-to-one relation. Even if you mark your question and assignedForm with unique constaint in database (to make it one-to-one in the database) EF will still not be able to handle it as one-to-one because it doesn't support unique constraints yet (except PK).