"principal end of an association" - EF - asp.net-mvc

Unable to determine the principal end of an association between the
types 'XYZ.Models.Attachment' and 'XYZ.Models.Accounts.User'. The
principal end of this association must be explicitly configured using
either the relationship fluent API or data annotations.
Exception has been thrown by the target of an invocation.
That error I get, when I try to update-database with my EF Models.
Part of User.cs:
[Table("Users")]
public class User
{
[Key]
[DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]
public int UserId { get; set; }
public int MainPhotoId { get; set; }
[ForeignKey("MainPhotoId")]
public virtual Attachment Photo { get; set; }
}
Attachment.cs
[Table("Attachments")]
public class Attachment
{
[Key]
[DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]
public int AttachmentId { get; set; }
public string name { get; set; }
public int UserId { get; set; }
public DateTime AddDate { get; set; }
public bool del { get; set; }
[ForeignKey("UserId")]
public virtual User Author { get; set; }
}
Why I get this error? And how to resolve it?
Regards

Mapping conventions detect a one-to-one relationship between User.Photo and Attachment.Author and cannot infer what the principal and what the dependent end is. Hence the exception.
Actually, according to your comments, you want two relationships and not a single one-to-one relationship. You can achieve that only by overriding the convention with Fluent API and you probably need to make one of the relationships optional because otherwise you have a circular mutual dependency between User and Attachment. You can, for example, make the User.Photo property optional by choosing a nullable foreign key:
public int? MainPhotoId { get; set; }
Then the mapping would look like this:
modelBuilder.Entity<User>()
.HasOptional(u => u.Photo)
.WithMany()
.HasForeignKey(u => u.MainPhotoId);
modelBuilder.Entity<Attachment>()
.HasRequired(a => a.Author)
.WithMany()
.HasForeignKey(a => a.UserId);
With this mapping you can remove the [ForeignKey] attributes because the definition of the FK properties is part of the Fluent API mapping (HasForeignKey).

Related

Implementing Many-To-Many relationship in Asp.Net (confused?)

My current aim is to build a database structure using classes in Entity Framework & ASP MVC.
I currently have a Users table and a Posts table. What I would like to do is create a many to many relationship for Users who have liked Posts (whilst conserving who created the post). And be able to access for each user all of the posts they have liked. Currently I have these classes but I'm unsure of how to link them as all of the online examples are linking Primary Keys from different databases where I just want to use the Username Parameter. Any help would be great. I have this so far.
public class Posts
{
[Key]
public virtual int PostId { get; set; }
public virtual string Title { get; set; }
public virtual string URL { get; set; }
[DisplayName("Main Text")]
public virtual string TextBody { get; set; }
public int PostLikes { get; set; }
private DateTime Datedata = DateTime.Now;
public DateTime PostDate { get { return Datedata; } set { Datedata = value; } }
public virtual Users User { get; set; }
public ICollection<PostLikes> UsersWhoHaveSigned { get; set; }
}
{
public class Users
{
[Key]
public virtual int UserId { get; set; }
public virtual string Username { get; set; }
public virtual string FirstName { get; set; }
public virtual string LastName { get; set; }
public virtual List<Posts> Post { get; set; }
}
}
I have not built the UsersWhoHaveSigned table yet. Early experimentation caused me so much backtracing it was painful. Any help would be great.
Edit: I was hoping to ask for help and then appropriate that informtaion to fit my example which utilises the individual accounts add-on. This produces some addition files that are now causing interference with the code you've provided.
Here is the IdentityModels.cs file.
using System.Data.Entity;
using System.Security.Claims;
using System.Threading.Tasks;
using Microsoft.AspNet.Identity;
using Microsoft.AspNet.Identity.EntityFramework;
namespace Coursework2.Models
{
// You can add profile data for the user by adding more properties to your ApplicationUser class, please visit https://go.microsoft.com/fwlink/?LinkID=317594 to learn more.
public class ApplicationUser : IdentityUser
{
public async Task<ClaimsIdentity> GenerateUserIdentityAsync(UserManager<ApplicationUser> manager)
{
// Note the authenticationType must match the one defined in CookieAuthenticationOptions.AuthenticationType
var userIdentity = await manager.CreateIdentityAsync(this, DefaultAuthenticationTypes.ApplicationCookie);
// Add custom user claims here
return userIdentity;
}
}
public class ApplicationDbContext : IdentityDbContext<ApplicationUser>
{
public ApplicationDbContext()
: base("DefaultConnection", throwIfV1Schema: false)
{
}
public static ApplicationDbContext Create()
{
return new ApplicationDbContext();
}
}
}
I believe that the assembly directives at the top are preventing system.data.entity from being used so when I try to implement ApplicationDbContext : DbContext I get error messages :/
Ideally I'm looking to use the IdentityModels.cs file as a replacement for the users class. But still very lost.
First of all, I recommend that you use the singular form for your class names, as EF will automatically pluralize table names.
Second, for a key property, you can just use the term Id, without any annotations, and EF will pick it up as the principal key.
Finally, I'll assume you are looking to use a Code-First approach. Consider the following classes (yours, but refactored for clarity purpose):
public class Post
{
public virtual Guid Id { get; set; }
public virtual string UserName { get; set; }
public virtual User User { get; set; }
public virtual ICollection<PostLike> Likes { get; set; }
}
public class PostLike
{
public virtual Guid Id { get; set; }
public virtual Guid PostId { get; set; }
public virtual Post Post { get; set; }
public virtual string UserName { get; set; }
public virtual User User { get; set; }
}
public class User
{
public virtual Guid Id { get; set; }
public virtual string UserName { get; set; }
public virtual ICollection<Post> Posts { get; set; }
public virtual ICollection<PostLike> Likes { get; set; }
}
To make it work, you'd need a DbContext such as the following. Pay attention to the OnModelCreating method, which is where the magic happens:
public class ApplicationDbContext
: DbContext
{
public ApplicationDbContext(DbContextOptions<ApplicationDbContext> options)
: base(options)
{
}
public DbSet<Post> Posts { get; set; }
public DbSet<PostLike> PostLikes { get; set; }
public DbSet<User> Users { get; set; }
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
base.OnModelCreating(modelBuilder);
modelBuilder.Entity<User>()
.HasAlternateKey(u => u.UserName);
modelBuilder.Entity<User>()
.HasMany(u => u.Posts)
.WithOne(p => p.User);
modelBuilder.Entity<Post>()
.HasOne(p => p.User)
.WithMany(u => u.Posts)
.HasForeignKey(p => p.UserName)
.HasPrincipalKey(u => u.UserName);
modelBuilder.Entity<Post>()
.HasMany(p => p.Likes)
.WithOne(pl => pl.Post);
modelBuilder.Entity<PostLike>()
.HasOne(pl => pl.Post)
.WithMany(p => p.Likes);
modelBuilder.Entity<PostLike>()
.HasOne(pl => pl.User)
.WithMany(u => u.Likes)
.HasForeignKey(pl => pl.UserName)
.HasPrincipalKey(u => u.UserName);
}
}
Voila! I hope it answers your question ;)
If so, please don't forget to mark my post as the answer!
Edit:
I'll provide some explanations, that I had left out to answer your question ASAP.
So, first thing you need to do, is to declare the UserName as an alternate key, because you want to create relationships depending on it, and you already have the 'Id' principal key declared.
Second, on each object that should own a User reference base on the UserName alternate key, you need to declare the object's UserName property as the foreign key of the relationship, and the User's UserName property as the principal key.
In other words, the foreign key is the property that a referencing object uses for the relationship, and the principal key is the property based on which the referenced object is bound to the referencing one.
Note that principal keys must have a key or alternate key constraint, or it won't work.
Just to comment on your answer. I found that I had to use
using Microsoft.EntityFrameworkCore and remove System.Data.Entity - This was causing the program to be confused as to which DbContext I wanted to use. Thanks!

Cannot create foreign key constraint on self-joining Many-to-Many relationship

I have created the following classes:
public class Character
{
public int ID { get; set; }
public string Title { get; set; }
public ICollection<Relationship> RelatedTo { get; set; }
public ICollection<Relationship> RelatedFrom { get; set; }
}
public class Relationship
{
public int ToID { get; set; }
public int FromID { get; set; }
public Character CharacterFrom { get; set; }
public Character CharacterTo { get; set; }
public string Details { get; set; }
}
In my Context I have this:
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.Entity<Relationship>()
.HasKey(r => new { r.ToID, r.FromID });
modelBuilder.Entity<Relationship>()
.HasOne(r => r.CharacterFrom)
.WithMany(r => r.RelatedTo)
.HasForeignKey(r => r.FromID)
.OnDelete(DeleteBehavior.ClientSetNull);
modelBuilder.Entity<Relationship>()
.HasOne(r => r.CharacterTo)
.WithMany(r => r.RelatedFrom)
.HasForeignKey(r => r.ToID)
.OnDelete(DeleteBehavior.ClientSetNull);
}
I think that it is right but I cannot apply the migration due to the following error:
Cannot create the foreign key "FK_Relationship_Character_FromID" with the SET NULL referential action, because one or more referencing columns are not nullable.
I've tried every combination of DeleteBehaviour for OnDelete. None of them work. I don't believe I can make the ICollections nullable and it doesn't seem right that I'd want to. I've spent two hours on this searching for answers. Every tutorial or explanation on EF Core that I've tried to follow seems to take a slightly different approach and be subtly incompatible with every other one. Please help!
The error is telling you that you cannot use DeleteBehavior.ClientSetNull (or DeleteBehavior.SetNull) because the corresponding FK property is not nullable - both ToID and FromID are of type int, hence does not allow setting to null (neither client nor server).
To turn off the cascade delete (in order to break the multiple cascade paths I guess) for required FK relationships, use DeleteBehavior.Restrict instead.

Entity Framework Code First - map same entity twice for different purpose

I have two models, One ApplicationUser which holds all users in the system and I have a Quotation model which will hold all Quotations made. now I want to store two mappings to ApplicationUser inside Quotations. So that I can map to created User as well as cancelled User. My model looks like this
public class Quotation
{
public int QuotationID { get; set; }
public DateTime QuotationDate { get; set; }
public DateTime QuotationCancelDate { get; set; }
public int ApplicationUserID { get; set; }
public virtual ApplicationUser CreatedUser { get; set; }
[ForeignKey("ApplicationUserID")]
public ApplicationUser CancelledUser { get; set; }
}
But this throws an error
Quotation_CancelledUser_Target_Quotation_CancelledUser_Source: : The types of all properties in the Dependent Role of a referential constraint must be the same as the corresponding property types in the Principal Role. The type of property 'ApplicationUserID' on entity 'Quotation' does not match the type of property 'Id' on entity 'ApplicationUser' in the referential constraint 'Quotation_CancelledUser'.
So I guess , The approach I am taking is wrong. Can anyone point out the correct way to achieve this?
The problem you are observing is called "Multiple Cascade Path". A Multiple Cascade Path happens when a cascade path goes from column col1 in table A to table B and also from column col2 in table A to table B.
The exception is caused by SQL Server when code first attempted to add table that has columns appearing more than once of another table.
In SQL Server, a table cannot appear more than one time in a list of all the cascading referential actions that are started by either a DELETE or an UPDATE statement. For example, the tree of cascading referential actions must only have one path to a particular table on the cascading referential actions tree.
You will need to use FluentAPI to configure the relationship. I am using EF5 currently and do not know if this can be accomplished in EF6/7.
So modifying your code sample, it would look like:
public class Quotation
{
public int QuotationID { get; set; }
public DateTime QuotationDate { get; set; }
public DateTime QuotationCancelDate { get; set; }
public int CreatedUserID { get; set; }
// Navigation property
public virtual ApplicationUser CreatedUser { get; set; }
public int CancelledUserID { get; set; }
// Navigation property
public virtual ApplicationUser CancelledUser { get; set; }
}
// Created a simple class for example
public class ApplicationUser
{
[Key]
public int Id { get; set; }
public string Name { get; set; }
}
Now in you context class you can write:
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
// Disable the default PluralizingTableNameConvention
modelBuilder.Conventions.Remove<PluralizingTableNameConvention>();
modelBuilder.Conventions.Remove<OneToManyCascadeDeleteConvention>();
// Add configuration here
modelBuilder.Entity<Quotation>()
.HasKey(e => e.QuotationID);
modelBuilder.Entity<ApplicationUser>()
.HasKey(e => e.Id);
modelBuilder.Entity<Quotation>()
.HasRequired(a => a.CreatedUser)
.WithMany()
.HasForeignKey(u => u.CreatedUserID);
modelBuilder.Entity<Quotation>()
.HasRequired(a => a.CancelledUser)
.WithMany()
.HasForeignKey(u => u.CancelledUserID);
}
For more information with example refer this link.

Validation error during model generation in one-to-many relationship

When I run application I have this error:
PossibleAnswer_Question_Source: : Multiplicity is not valid in Role
'PossibleAnswer_Question_Source' in relationship
'PossibleAnswer_Question'. Because the Dependent Role properties are
not the key properties, the upper bound of the multiplicity of the
Dependent Role must be '*'.
How to resolve it?
Model classes for Question and PossibleAnswer:
public class Question
{
public int ID { get; set; }
public string Text { get; set; }
public bool IsAssociatedWithProfessor { get; set; }
public bool IsAssociatedWithAssistant { get; set; }
public virtual ICollection<PossibleAnswer> PossibleAnswers { get; set; }
}
public class PossibleAnswer
{
public int ID { get; set; }
public string Text { get; set; }
public int QuestionID { get; set; }
[ForeignKey("QuestionID")]
public virtual Question Question { get; set; }
}
And I put this in OnModelCreating(DbModelBuilder modelBuilder):
modelBuilder.Entity<PossibleAnswer>()
.HasRequired(f => f.Question)
.WithRequiredDependent()
.WillCascadeOnDelete(false);
The problem is you are not configuring a one-to-many relationship in the OnModelCreating method (that is a one-to-one configuration). To achieve what you want, you could do this:
modelBuilder.Entity<PossibleAnswer>()
.HasRequired(pa => pa.Question)
.WithMany(q=>q.PossibleAnswers)
.HasForeignKey(pa=>pa.QuestionID)
.WillCascadeOnDelete(false);
This way, you don't need to use the ForeignKey attribute on the Question navigation property. Is a good practice try to not merge Fluent Api with Data Annotations

Fluent API mapping other side if a 1-1 relationship

I have a relationship set up between 2 tables using code first and Fluent API. This works and the schema is generated with the correct fields and key assignments. However, my problem is I need to be able to say SupplyPoint.SupplyPointPricing in my resultant Model. What I have below only gives me the other way around.
I figure there must be a way to keep the structure I have but just MAP SupplyPoint.
public partial class SupplyPoint
{
[Key]
public int SupplyPointId { get; set; }
public string SupplyPointName { get; set; }
}
public class SupplyPointPricing
{
public int SupplyPointPricingId { get; set; }
public int? SupplyPointId { get; set; }
[ForeignKey("SupplyPointId")]
public virtual SupplyPoint SupplyPoint { get; set; }
}
Then I use Fluent API and this gives me the 1-1 between the tables and the Schema I expect
modelBuilder.Entity<SupplyPointPricing>()
.HasOptional(a => a.SupplyPoint)
.WithMany()
.HasForeignKey(u => u.SupplyPointId);
Last time I had this problem I had to change the design around and have a foreign key in the SupplyPoint table. On that previous occasion that was OK since it was a 1-1 required where there are always a matching record. This time around I need to keep this structure since there is 1-0 between SupplyPoint and SupplyPointPricing.
This is how I have always done 1-to-1 relationships with my models, without using Fluent mapping:
public partial class SupplyPoint
{
[Key]
public int SupplyPointId { get; set; }
public virtual SupplyPointPricing SupplyPointPricing { get; set; }
}
public class SupplyPointPricing
{
[Key, ForeignKey("SupplyPoint")]
public int SupplyPointId { get; set; }
public virtual SupplyPoint SupplyPoint { get; set; }
}
As you can see, the SupplyPointPricing does not have an Id of its own, but only the Id of the SupplyPoint, which acts as a key in the 1-to-1 relatioship.
The only thing I'm not sure of is if the SupplyPointId can be a nullable int.
This will not create a foreign key on the SupplyPoint, so if you don't have a Pricing, the SupplyPoint.SupplyPointPricing property will be null.

Resources