How to manually set entity primary key in Entity Framework code first database?

Question

Well, I have the following model structure: I have one class - DatabaseEntity which is basically

public class DatabaseEntity
{
    public int Id { get; set; }
}

so each entity like product, category etc will inherit DatabaseEntity and have Id property. Also I have typical EntityFramework repository class with InsertOrUpdate method:

private readonly DbContext _database;

public void InsertOrUpdate<TObject>(TObject entity) where TObject : DatabaseEntity
{
    if(entity.Id == default(int))
    {
         // New entity
         DbSet<TObject>().Add(entity);
    }
    else
    {
         // Existing entity
         _database.Entry(entity).State = EntityState.Modified;
    }
    _database.SaveChanges();
}

Then I download from eBay via eBay api list of categoies I have to add to database. Basically category is:

public class EbayCategory : DatabaseEntity
{
    // It has Id since it inherits DatabaseEntity
    public string Name { get; set; }      
    // ... some other properties
}

But, the problem is, when I download those categories I download and their Id properties, which, of course, already have values. And when I try to save them to database like:

public void UpdateCategories(IEnumerable<EbayCategory> newCategories)
{
    foreach (var newCategory in newCategories)
    {
        _repository.InsertOrUpdate(newCategory);
    }
}

I face some issues... First of all, entity.Id != default(int) because it has value, so repository tries to update this entity, instead of adding, but it is not in the database or context so it throws the following exception:

System.Data.Entity.Infrastructure.DbUpdateConcurencyException
"Store update, insert, or delete statement affected an unexpected number of rows (0). Entities may have been modified or deleted since entities were loaded. Refresh ObjectStateManager entries."

... because it thinks that someone else deleted entity which I am trying to update. How can I save this InsertOrUpdate logic, since a lot of projects are based on it, and be able to add items (EbayCategories) with primary key (Id) to database and then update/delete them like other entities without discarding EbayCategory.Id value?

Answer 1

To allow you to manually generate Ids you need a class that has a manually generated ID - so it cannot inherit from DatabaseEntity

public class EbayCategory
{
    [DatabaseGenerated(DatabaseGeneratedOption.None)]
    public int Id { get; set; }

    public string Name { get; set; }      
    // ... some other properties
}

Now you will need a different InsertOrUpdate to handle entities that have manually generated keys:

public void InsertOrUpdate(EbayCategory entity)
{
    if(Find(entity.ID == null)
    {
         // New entity
         DbSet<EbayCategory>().Add(entity);
    }
    else
    {
         // Existing entity
         _database.Entry(entity).State = EntityState.Modified;
    }
    _database.SaveChanges();
}

Answer 2

Colin's answer above quite correctly shows how to achieve this setting using data annotations. But in the presented problem the entity is a subclass so you can't add the annotation without changing the entity class.

There is an alternative configuration method: Fluent Configuration. Here's my example using an EntityTypeConfiguration class:

public class LookupSchoolsConfiguration : EntityTypeConfiguration<LookupSchools>
    {
        public LookupSchoolsConfiguration()
        {
            Property(l => l.Id).HasDatabaseGeneratedOption(DatabaseGeneratedOption.None);
        }
    }

You can also add configuration directly to the modelBuilder as per this post: https://stackoverflow.com/a/4999894/486028