Error:
Unhandled Exception: System.Data.SqlClient.SqlException: The operation failed because an index or statistics with name 'IX_ID' already exists on table 'PrivateMakeUpLessons'.
Model (Simplified, building in a separate test project for debugging):
public abstract class Lesson
{
public Guid ID { get; set; }
public string Room { get; set; }
public TimeSpan Time { get; set; }
public int Duration { get; set; }
}
public abstract class RecurringLesson : Lesson
{
public int DayOfWeek { get; set; }
public DateTime StartDate { get; set; }
public DateTime EndDate { get; set; }
public string Frequency { get; set; }
}
public class PrivateLesson : RecurringLesson
{
public string Student { get; set; }
public string Teacher { get; set; }
public virtual ICollection<Cancellation> Cancellations { get; set; }
}
public class Cancellation
{
public Guid ID { get; set; }
public DateTime Date { get; set; }
public virtual PrivateLesson Lesson { get; set; }
public virtual MakeUpLesson MakeUpLesson { get; set; }
}
public class MakeUpLesson : Lesson
{
public DateTime Date { get; set; }
public string Teacher { get; set; }
public virtual Cancellation Cancellation { get; set; }
}
Configuration:
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Entity<Lesson>().ToTable("Lessons");
modelBuilder.Entity<RecurringLesson>().ToTable("RecurringLessons");
modelBuilder.Entity<PrivateLesson>().ToTable("PrivateLessons");
modelBuilder.Entity<MakeUpLesson>().ToTable("PrivateMakeUpLessons");
modelBuilder.Entity<Cancellation>()
.HasOptional(x => x.MakeUpLesson)
.WithRequired(x => x.Cancellation);
base.OnModelCreating(modelBuilder);
}
Notes:
This worked fine in EF 4.2. Is there something wrong with my model? The actual model is much more complicated which is why I have all the classes abstracted out. Also, I am working against an existing database so I need to use Table-Per-Type inheritance.
If I change the relationship of Cancellation to PrivateMakeUpLesson from 1 to 0..1 to 0..1 to 0..1 it works. This is undesirable because you can't have a PrivateMakeUpLesson without a Cancellation.
Also, if I make PrivateMakeUpLesson NOT inherit from Lesson then it also works, but it IS a lesson and needs to remain so for existing business logic.
I'd appreciate any guidance. Thank you!
Edit:
Starting a bounty. I can't find any documentation on what changed between EF 4.2 and EF 4.3 with regard to the index generation for code first. It's clear that EF 4.3 is creating more indexes and that the naming scheme has changed but I want to know if there's a bug in EF or if there is something fundamentally wrong with my model or fluent API configuration.
As of EF 4.3, indexes are added for freign key columns during database creation. There is a bug that can cause an index to be created more than once. This will be fixed in a future EF release.
Until then, you can work around the issue by creating your database using Migrations instead of database initializers (or the Database.Create() method).
After generating the initial migration, you will need to delete the redundant call to Index().
CreateTable(
"dbo.PrivateMakeUpLessons",
c => new
{
ID = c.Guid(nullable: false),
...
})
.PrimaryKey(t => t.ID)
.ForeignKey("dbo.Lessons", t => t.ID)
.ForeignKey("dbo.Cancellations", t => t.ID)
.Index(t => t.ID)
.Index(t => t.ID); // <-- Remove this
To continue creating your database at run-time, you can use the MigrateDatabaseToLatestVersion initializer.
In my opinion this is clearly a bug.
The problem starts with the observation that EF creates an index IX_ID at all. If you strip down the model to the following...
public abstract class Lesson
{
public Guid ID { get; set; }
}
public class RecurringLesson : Lesson
{
}
public class MyContext : DbContext
{
public DbSet<Lesson> Lessons { get; set; }
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Entity<RecurringLesson>().ToTable("RecurringLessons");
}
}
... and let EF create the database schema you get two tables Lessons and RecurringLessons as expected for a TPT inheritance mapping. But I am wondering why it creates two indices for the table RecurringLessons:
Index PK_RecurringLessons (clustered, unique) with Index column ID
Index IX_ID (not clustered, not unique) with Index column ID again
I don't know if there is any benefit for the database to have a second index on the same column. But for my understanding it doesn't make sense 1) to create an index on the same column that is already covered in the PK clustered index, and 2) to create a not unique index on a column which is the primary key and therefore necessarily unique.
Moreover due to the one-to-one relationship EF tries to create an index on the table of the dependent of this association which is PrivateMakeUpLessons. (It's the dependent (and not the principal) because Cancellation is required in entity MakeUpLesson.)
ID is the foreign key in this association (and primary key at the same time because one-to-one relationships are always shared primary key associations in Entity Framework). EF apparently always creates a index on the foreign key of a relationship. But for one-to-many relationships this is not a problem because the FK column is different from the PK column. Not so for one-to-one relatonships: The FK and PK are the same (that is ID), hence EF tries to create an index IX_ID for this one-to-one relationship which already exists due to the TPT inheritance mapping (which leads to a one-to-one relationship as well from database perspective).
The same consideration as above applies here: The table PrivateMakeUpLessons has a clustered PK index on column ID. Why is a second index IX_ID on the same column required at all?
In addition EF doesn't seem to check that it already wants to create an Index with name IX_ID for the TPT inheritance, leading finally to the exception in the database when the DDL is sent to create the database schema.
EF 4.2 (and before) didn't create any indices (except PK indices) at all, this was introduced in EF 4.3, especially indices for FK columns.
I didn't find a workaround. In the worst case you have to create the database schema manually and avoid that EF tries to create it (= disable database initialization). In the best case there is a way to disable automatic FK index creation, but I don't know if it's possible.
You can submit a bug report here: http://connect.microsoft.com/VisualStudio
Or maybe someone from EF development team will see your question here and provide a solution.
I got a very similar error to this one in my code a while back. Try putting the cancellation list inside the Lesson class. That's what solved my problem.
Below I describe 2 scenarios what is probably going wrong. Please read in depth by clicking the links I provided to know more about my explanation.
First
Lesson and RecurringLesson are abstract classes (so you want to have it as the base classes).
You are creating a table of the Lesson and the RecurringLesson entities which will result in a Table per hierarchy structure.
brief description
Creating a class of the base table will result in one big table which contains the columns of all inherited tables. So all properties of PrivateLesson, MakeUpLesson and all others inherited entities will be stored in the Lessons table. EF will add also a Discriminator column. The value of this column defaults to the persistent class name (like "PrivateLesson" or "MakeUpLesson") only the column matching to that particular entity (matching the Discriminator value) will be used in that particular row.
BUT
You are also mapping the inherited classes like PrivateLesson and MakeUpLesson. This will force EF to use the Table per Type structure which results in one table per class. This can cause conflicts you are facing right now.
Second
Your example shows you have an one-to-one relationship (Cancellation -> MakeUpLesson) and a one-to-many relationship (Cancellation -> PrivateLesson) because PrivateLesson and MakeUpLessonare both (indirect) inherited from Lesson in combination with the first described scenario can cause problems because it will result in 2 foreign key relationships in the database per entity. (one using Table per hierarchy structure and one using the Table per Type structure).
Also this post can help you defining a correct one-to-one definition.
Please verify by performing the following steps:
I assume you have your own test environment so you can create new test databases
1.
Delete the relationships to the Cancellation by commenting out all properties to this class:
public class PrivateLesson : RecurringLesson
{
public string Student { get; set; }
public string Teacher { get; set; }
//public virtual ICollection<Cancellation> Cancellations { get; set; }
}
public class Cancellation
{
public Guid ID { get; set; }
public DateTime Date { get; set; }
//public virtual PrivateLesson Lesson { get; set; }
//public virtual MakeUpLesson MakeUpLesson { get; set; }
}
public class MakeUpLesson : Lesson
{
public DateTime Date { get; set; }
public string Teacher { get; set; }
//public virtual Cancellation Cancellation { get; set; }
}
And remove the configuration to it:
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Entity<Lesson>().ToTable("Lessons");
modelBuilder.Entity<RecurringLesson>().ToTable("RecurringLessons");
modelBuilder.Entity<PrivateLesson>().ToTable("PrivateLessons");
modelBuilder.Entity<MakeUpLesson>().ToTable("PrivateMakeUpLessons");
//modelBuilder.Entity<Cancellation>()
// .HasOptional(x => x.MakeUpLesson)
// .WithRequired(x => x.Cancellation);
base.OnModelCreating(modelBuilder);
}
2.
Create a new empty database
3.
Let EF generate the table structure for you in this empty database.
4.
Verify the first scenario. If that's true this need to be fixed first by using the Table per hierarchy structure OR the Table per Type structure. Probably you want to use the Table per hierarchy structure because (if I understand your question well) there is already an production environment.
When my project was updated from EF 6.0.2 to EF 6.1.1, I had such a problem, then back to 6.0.2, after the return of an older version, the error disappeared
Related
We use Entity Framework 6 with CodeFirst and an Oracle.ManagedDataAccess.
I just created a 1 to 0..1 (one to zero or one) relation between two tables, and it works like a charm. But when adding a second relation, I got into trouble, because the original primary key column was demoted to just a data column, and a surrogate (sequence) primary key column was added. The foreign key constraint is still on the old field.
Code:
public class Node
{
[Key, Column("ID"), Required]
public int Id { get; set; }
[Column("POINT_CODE"), Required, StringLength(10)]
public string PointCode { get; set; }
// ...columns left out...
[ForeignKey("PointCode"), Required]
public NetworkPoint PointCodeFk { get; set; }
}
public class Point
{
[Key, Column("POINT_CODE"), Required, StringLength(10)]
public string PointCode { get; set; }
// ...columns left out...
[ForeignKey("PointCode")]
public Node NodeFk { get; set; }
}
public class MyDbContext : EntityContext
{
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Entity<Point>()
.HasOptional(m => m.NodeFk)
.WithRequired(o => o.PointCodeFk);
}
// ...stuff left out...
}
The difference with the working example is, the property Node.PointCode has the attribute Key, and there is no Node.Id column.
When running this example, I get the error message:
Multiplicity is not valid in Role 'Point_NodeFk_Target' in relationship 'Point_NodeFk'. Because the Dependent Role properties are not the key properties, the upper bound of the multiplicity of the Dependent Role must be '*'.
So I guess EntityFramework now thinks it should be an 1:N relation.
How can this be solved? I can't change the database (although I would like to very much).
Entity Framework requires a primary key on each table. You need to add a primary without which EF will complain. So looking at your code Point class require ID, Id ,PointId or PointID property so EF will know it has a primary key. Any of the conventions I showed here as a class property will be acceptable to EF.
I've read as many posts as I can on this topic but none of the solutions I have tried seem to work. I have an existing database and created a new Code First From Existing Database project.
I have a base table called Thing. Every object has a record in this table using Id as the Unique Primary Key. Each other object inherits from this but they use the same Id in the child tables without using a new Identity column in the sub tables. Effectively giving each 'Thing' a unique Id:
public class Thing
{
public int Id { get; set; }
public string Name { get; set; }
}
public class Car
{
public int Id { get; set; }
//other properties
}
public class Person
{
public int Id { get; set; }
//other properties
}
public class Color
{
public int Id { get; set; }
//other properties
}
Every new record first creates an item in 'Thing' and then using that Id value creates a new record in its respective table, creating multiple 1 to 0..1 relationships where the Id field on the derived tables is also the FK to Thing.
Thing 1 to 0..1 Car
Thing 1 to 0..1 Person
Thing 1 to 0..1 Color
and so on
I have tried many different Data Annotation and Fluent API combinations but it always comes back to the same error:
'Unable to retrieve metadata for Model.Car'. Unable to determine the principal end of association between the types 'Model.Thing' and 'Model.Car'. The principal end of this association must be explicitly configured using either the relationship fluent API or data annotations.'
I did manage to get past this error by using virtual with the inverse annotation and setting the Id field to be Key and ForeignKey, but then the message jumps to Person. If you then set it up the same as Car the message reverts back to Car.
It seems I could go back and create a normal Foreign Key to each child table, but that is a lot of work and I am sure it is possible to get this working somehow. Preferably using fluent API.
If you are going to use Data Annotations, you need to declare the PK of the dependent entity as FK too:
public class Thing
{
public int Id { get; set; }
public string Name { get; set; }
public virtual Car Car{get;set;}
}
public class Car
{
[Key,ForeignKey("Thing")]
public int ThingId { get; set; }
//other properties
public virtual Thing Thing{get;set;}
}
And if you are going to use Fluent Api (remove the attributes from your model), the configuration would be like this:
modelBuilder.Entity<Car>().HasRequired(c=>c.Thing).WithOptional(t=>t.Thing);
Based on the multiplicity that is specified, it only makes sense for Thing to be the principal and Car to be the dependent, since a Thing can exist without a Car but a Car must have a Thing.
As you can see you don't need to specify that ThingId is the FK of this relationship.This is because of Entity Frameworkâs requirement that the primary key of the dependent be used as the foreign key. Since there is no choice, Code First will just infer this for you.
Update
Reading again your question I think you are trying to create a hierarchy. In that case you could use the Table per Type (TPT) approach.
I have two POCO classes:
Order Class:
public class Order
{
public int Id { get; set; }
public int? QuotationId { get; set; }
public virtual Quotation Quotation { get; set; }
....
}
Quotation Class:
public class Quotation
{
public int Id { get; set; }
public virtual Order Order { get; set; }
....
}
Each Order may be made from one or zero quotation, and
each quotation may cause an order.
So I have an "one or zero" to "one or zero" relation, how can I implement this, in EF Code first by Fluent API?
By changing pocos to:
public class Order
{
public int OrderId { get; set; }
public virtual Quotation Quotation { get; set; }
}
public class Quotation
{
public int QuotationId { get; set; }
public virtual Order Order { get; set; }
}
and using these mapping files:
public class OrderMap : EntityTypeConfiguration<Order>
{
public OrderMap()
{
this.HasOptional(x => x.Quotation)
.WithOptionalPrincipal()
.Map(x => x.MapKey("OrderId"));
}
}
public class QuotationMap : EntityTypeConfiguration<Quotation>
{
public QuotationMap()
{
this.HasOptional(x => x.Order)
.WithOptionalPrincipal()
.Map(x => x.MapKey("QuotationId"));
}
}
we will have this DB(that means 0..1-0..1):
with special thanks to (Vahid Nasiri)
#Masoud's procedure was:
modelBuilder.Entity<Order>()
.HasOptional(o => o.Quotation)
.WithOptionalPrincipal()
.Map(o => o.MapKey("OrderId"));
modelBuilder.Entity<Quotation>()
.HasOptional(o => o.Order)
.WithOptionalPrincipal()
.Map(o => o.MapKey("QuotationId"));
It gives:
By changing the code to:
modelBuilder.Entity<Order>()
.HasOptional(o => o.Quotation)
.WithOptionalPrincipal(o=> o.Order);
It gives:
See http://msdn.microsoft.com/en-us/data/jj591620 EF Relationships
An excellent Book
http://my.safaribooksonline.com/book/-/9781449317867
Here is a post from developer from Dec 2010. But still relevant
http://social.msdn.microsoft.com/Forums/uk/adonetefx/thread/aed3b3f5-c150-4131-a686-1bf547a68804
The above article is a nice summary or the possible combinations here.
A solution where dependant Table has key from Primary table is possible.
If you Want Independent Keys where both are Principals in a PK/FK scenario, i dont think you can do it in Code first with Fluent API. If they share a Key, You are OK.
1:1 optional assumes the dependent uses the key from Primary.
But since you need to save one of the tables before the other. You can check one of the Foreign Keys with code. OR add teh second Foreign to Database after Code first has created it.
You will get close. But EF will complain about conflicting Foreign keys if you want both to be Foreign keys. Essentially the A depends on B depends A EF doesnt like, even if the columns are nullable and technically possible on the DB.
Here use this test program to try it. Just comment in an out the Fluent API stuff to try some options.
I could NOT get EF5.0 to work with INDEPENDENT PK/FK 0:1 to 0:1
But of course there are reasonable compromises as discussed.
using System.Data.Entity;
using System.Linq;
namespace EF_DEMO
{
class Program
{
static void Main(string[] args) {
var ctx = new DemoContext();
var ord = ctx.Orders.FirstOrDefault();
//. DB should be there now...
}
}
public class Order
{
public int Id {get;set;}
public string Code {get;set;}
public int? QuotationId { get; set; } //optional since it is nullable
public virtual Quotation Quotation { get; set; }
//....
}
public class Quotation
{
public int Id {get;set;}
public string Code{get;set;}
// public int? OrderId { get; set; } //optional since it is nullable
public virtual Order Order { get; set; }
//...
}
public class DemoContext : DbContext
{
static DemoContext()
{
Database.SetInitializer(new DropCreateDatabaseIfModelChanges<DemoContext>());
}
public DemoContext()
: base("Name=Demo") { }
public DbSet<Order> Orders { get; set; }
public DbSet<Quotation> Quotations { get; set; }
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Entity<Order>().HasKey(t => t.Id)
.HasOptional(t => t.Quotation)
.WithOptionalPrincipal(d => d.Order)
.Map(t => t.MapKey("OrderId")); // declaring here via MAP means NOT declared in POCO
modelBuilder.Entity<Quotation>().HasKey(t => t.Id)
.HasOptional(q => q.Order)
// .WithOptionalPrincipal(p => p.Quotation) //as both Principals
// .WithOptionalDependent(p => p.Quotation) // as the dependent
// .Map(t => t.MapKey("QuotationId")); done in POCO.
;
}
}
}
Adapted from this answer, try this.
First, fix your classes:
public class Order
{
public int Id {get; set;}
public virtual Quotation Quotation { get; set; }
// other properties
}
public class Quotation
{
public int Id {get; set;}
public virtual Order Order { get; set; }
// other properties
}
Then use the fluent API like that:
modelBuilder.Entity<Quotation>()
.HasOptional(quote => quote.Order)
.WithRequired(order=> order.Quotation);
Basically, for 1:1 or [0/1]:[0/1] relationships, EF needs the primary keys to be shared.
public class OfficeAssignment
{
[Key]
[ForeignKey("Instructor")]
public int InstructorID { get; set; }
[StringLength(50)]
[Display(Name = "Office Location")]
public string Location { get; set; }
public virtual Instructor Instructor { get; set; }
}
The Key Attribute
There's a one-to-zero-or-one relationship between the Instructor and the OfficeAssignment entities. An office assignment only exists in relation to the instructor it's assigned to, and therefore its primary key is also its foreign key to the Instructor entity. But the Entity Framework can't automatically recognize InstructorID as the primary key of this entity because its name doesn't follow the ID or classnameID naming convention. Therefore, the Key attribute is used to identify it as the key:
https://www.asp.net/mvc/overview/getting-started/getting-started-with-ef-using-mvc/creating-a-more-complex-data-model-for-an-asp-net-mvc-application
using DataAnnotations:
public class Order
{
[Key]
public int Id {get; set;}
public virtual Quotation Quotation { get; set; }
}
public class Quotation
{
[Key, ForeignKey(nameof(Order))]
public int Id {get; set;}
public virtual Order Order { get; set; }
}
(Note that this is using EF 6.4.4.)
It's fairly straightforward to specify, as long as you don't want foreign key properties:
modelBuilder
.Entity<Order>()
.HasOptional(o => o.Quotation)
.WithOptionalPrincipal(q => q.Order);
modelBuilder
.Entity<Quotation>()
.HasOptional(q => q.Order)
.WithOptionalDependent(o => o.Quotation);
Notice here the usage of both WithOptionalPrincipal and WithOptionalDependent. This should give you a single foreign key column on the dependent side (Quotation in the example), but with no foreign key properties. If you want the foreign key on the other side, switch "Dependent" and "Principal" around.
(Note that it is not necessary to have both definitions above; WithOptionalDependent will imply the other side is the principal and vice-versa, so you can use only one of them if you wanted, but I find specifying the relationships from both sides helps prevent errors by double declaring things; any conflict will result in a model error to let you know you missed something.)
While there is an index on the foreign key column, the index does not have a unique constraint. While it is possible to add your own unique constraint (which would require a Key IS NOT NULL filter), it doesn't seem to work and you will get exceptions when updating relationships in some cases. I think this is related to the "swapping problem" where EF will perform its updates in separate queries, so enforcing uniqueness would prevent EF from "moving" a key in two steps.
EF seems to handle the association itself internally, without a unique DB constraint:
On either side, assigning an already used reference results in the other usage of the reference being removed automatically. (So if it is already the case that A1 <=> B1 when you opened the context, and then you write A1 => B2, then A1 <=> B1 is removed and A1 <=> B2 is added, regardless of which side you're on.)
If you try to create a duplicate key by assigning the same reference more than once, EF will throw an exception saying "multiplicity constraint violation". (So in the same context, you wrote both A1 => B1 and A2 => B1, or some similar conflicting mapping.)
If you update the DB manually to create a duplicate key situation, when EF encounters this it will throw an exception saying "A relationship multiplicity constraint violation occurred...this is a non-recoverable error."
It does not seem possible in EF6 to map a property to the foreign key column (at least with Fluent API). Attempting to do so results in a non-unique column name exception since it tries to use the same name for both the property and the association separately.
Note also that it is technically incorrect to have two foreign keys (ie: one on both sides). Such an arrangement would actually be two 0..1 to 0..1 associations since there would be nothing to say that keys on both ends should match. This could maybe work if you enforce the relationship some other way, through the UI and/or possibly a database constraint of some kind.
I also notice that there may be a misunderstanding/miscommunication of exactly what a 0..1 to 0..1 association is. What this means, from my understanding and the way EF seems to consider it as well, is that it is a 1 to 1 association that is optional on both sides. So, you can have objects on either side with no relationship. (Whereas a 1 to 0..1 assocation, objects on one side could exist without a relationship, but objects on the other side would always need an object to relate to.)
But 0..1 to 0..1 does not mean that you can have the association travel in one direction and not the other. If A1 => B1, then B1 => A1 (A1 <=> B1). You cannot assign B1 to A1 without also making A1 relate to B1. This is why it is possible for this association to use only a single foreign key. I think some people may be trying to have an association in which this is not true (A1 relates to B1 but B1 does not relate to A1). But that is really not one association but two 0..1 to 0..1 associations.
This goes for both Entity Framework 4 (4.3.1) and 5.
I have a User class (to go with my Entity Framework MembershipProvider). I've removed some of the properties to simplify. The actual User is from the MVCBootstrap project, so it's not part of the same assembly as the other classes.
public class User {
[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public Guid Id { get; set; }
[Required]
[StringLength(256)]
public String Username { get; set; }
}
And then I have this class:
public class NewsItem {
public Int32 Id { get; set; }
[Required]
[StringLength(100)]
public String Headline { get; set; }
[Required]
public virtual User Author { get; set; }
[Required]
public virtual User LastEditor { get; set; }
}
Then I create the database context (The DbSet for the user is in the MembershipDbContext):
public class MyContext : MVCBootstrap.EntityFramework.MembershipDbContext {
public MyContext(String connectString) : base(connectString) { }
public DbSet<NewsItem> NewsItems { get; set; }
}
Running this code will give me this exception when the database is being created:
Introducing FOREIGN KEY constraint 'FK_dbo.WebShop_dbo.User_LastEditor_Id' on table 'WebShop' may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints.
Could not create constraint. See previous errors.
So I change the database context:
public class MyContext : MVCBootstrap.EntityFramework.MembershipDbContext {
public MyContext(String connectString) : base(connectString) { }
public DbSet<NewsItem> NewsItems { get; set; }
protected override void OnModelCreating(DbModelBuilder modelBuilder) {
base.OnModelCreating(modelBuilder);
modelBuilder.Configurations.Add(new NewsItemConfiguration());
}
}
And this configuration:
public class NewsItemConfiguration : EntityTypeConfiguration<NewsItem> {
public NewsItemConfiguration() {
HasRequired(n => n.Author).WithOptional();
HasRequired(n => n.LastEditor).WithOptional();
}
}
Or is this wrong?
Anyway, when I run the code, the database get's created, and the database seems okay (looking at foreign key constraints etc.).
But, then I get the 10 latest NewsItems from the context, and start loading them into view models, part of this is accessing the Author property on the NewsItem. The controller doing this takes forever to load, and fails after a long, long time. When running in debug mode, I get an exception in this piece of code: this.AuthorId = newsItem.Author.Id;, then exception I get is this:
A relationship multiplicity constraint violation occurred: An EntityReference can have no more than one related object, but the query returned more than one related object. This is a non-recoverable error.
It's probably something simple and stupid I'm doing wrong, I'm sure I've get similar code running on several sites, so .. what is causing this? Are my models wrong, is it the database context, or?
This part
Introducing FOREIGN KEY constraint 'FK_dbo.WebShop_dbo.User_LastEditor_Id' on table 'WebShop' may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints. Could not create constraint. See previous errors.
is actually a SQL Server issue (and an issue of many other RDBMS's). It is a complex issue resolving multiple cascade paths, and SQL Server decides just to punt and not try. See
Foreign key constraint may cause cycles or multiple cascade paths?
You were trying to configure your model to delete the child Author and LastEditor objects when the NewsItem is deleted. SQL Server won't do that.
Come to think of it... is that what you want? It seems you would want to disassociate the Author and LastEditor from the NewsItem, not delete them from the database.
Your object model requires a 1:1 relationship between NewsItem and Author, and between NewsItem and LastEditor. I'm not sure what this refers to in the code
this.AuthorId = newsItem.Author.Id;
but it seems to me, you should be making the assignment the other way around, e.g.
newsItem.Author = myAuthorInstance;
or if you include foreign key properties in your model and if you have previously saved your author instance and have an Id:
newsItem.AuthorId = myAuthorInstance.Id;
If you share the generated DB schema (relevant parts) that would make it easier to diagnose the issue.
User can be an author of several news items. Also, user can be editor of several news items.
Hence, relationship have to be "one-to-many":
public class NewsItemConfiguration : EntityTypeConfiguration<NewsItem> {
public NewsItemConfiguration() {
HasRequired(n => n.Author).WithMany();
HasRequired(n => n.LastEditor).WithMany();
}
}
I have a Video class and a MediaContent class that are linked by a 1-1, required:required relationship: each Video must have exactly 1 associated MediaContent. Deleting a MediaContent object must result in the deletion of the associated Video object.
Using the fluent API, the relationship can be modeled as follows:
modelBuilder.Entity<Video.Video>()
.HasRequired(v => v.MediaContent).WithRequiredPrincipal(mc => mc.Video)
.WillCascadeOnDelete(true);
When adding a migration to reflect this change in the database, this is how the relationship gets transcribed in terms of foreign keys:
AddForeignKey("MediaContents", "MediaContentId", "Videos", "VideoId", cascadeDelete: true);
Updating the database, I get the following error:
Cascading foreign key 'FK_MediaContents_Videos_MediaContentId' cannot be created where the referencing column 'MediaContents.MediaContentId' is an identity column.
Dropping the WillCascadeOnDelete(true) property removes the error, but I'm not sure I understand why. Shouldn't the error appear whether or not cascading is turned on? The way I understand the problem, the error comes from the fact that the generation of VideoId and MediaContentId is handled by auto-increment (or by whatever the id generation strategy is), potentially contradicting the foreign key constraint. But I can't see what this has to do with delete-cascading...
What am I missing? More generally, how would you go about modeling a cascadable one-to-one, required:required relationship with EF?
I avoid the modelBuilder cruft approach and use simple POCOs and attributes generally - which you can use to accomplish your goals like so:
public class Video
{
public int Id { get; set; }
// Adding this doesn't change the db/schema, but it is enforced in code if
// you try to add a Video without a MediaContent.
[Required]
public MediaContent MediaContent { get; set; }
}
public class MediaContent
{
[ForeignKey("Video")]
public int Id { get; set; }
public Video Video { get; set;}
}