Question

I am using Entity Framework 4.3 and I am trying to reference an existing entity by setting the navigation property when creating a new entity, however when i call save EF complains that there is a PK violation in the table for which i set the navigation property to (i.e. it is creating a new record as opposed to a reference!).

How can i attach to an existing POCO as opposed to referencing it and having EF trying to create a new database record (but not simply use an ID, ideally i would like to reference an actual entity that came from another query)?

Thanks in advance,

Chris

public class BusinessUnit
{
    public int BusinessUnitID { get; set; }
    public ExternalPlugin AccountsDataSourceModule { get; set; }
    public ExternalPlugin OptionalContactsDataSourceModule { get; set; }
}

public BusinessUnit NewBusinessUnit(string name, ExternalPlugin accountsModuleId = null, ExternalPlugin contactsModuleId = null)
{
    IUnitOfWork unitOfWork = UnitOfWorkFactory.CreateUnitOfWork();

    BusinessUnit unit = new BusinessUnit();
    unit.CompanyName = name;
    unit .AccountsDataSourceModule = accountsModuleId; // this causes a problem
    unit .OptionalContactsDataSourceModule = contactsModuleId; // as does this
    unitOfWork.BusinessUnitRepository.Insert(unit);

    unitOfWork.Save();
    return unit;
}

Answer 1

You must attach the existing entities to the context:

BusinessUnit unit = new BusinessUnit();
unit.CompanyName = name;

unitOfWork.ExternalPluginRepository.Attach(accountsModuleId);
unitOfWork.ExternalPluginRepository.Attach(contactsModuleId);

unit.AccountsDataSourceModule = accountsModuleId;
unit.OptionalContactsDataSourceModule = contactsModuleId;
unitOfWork.BusinessUnitRepository.Insert(unit);

…where unitOfWork.ExternalPluginRepository.Attach(ExternalPlugin plugin) must do:

context.ExternalPlugins.Attach(plugin);

I expect that all repositories use the same context instance. Attach tells EF that the plugins already exist in the database and avoids an INSERT of those entities.

Edit

If you get the error message…

An entity object cannot be referenced by multiple instances of
IEntityChangeTracker.

…it means that you have an entity that is attached to more than one context instance at the same time. You can avoid that in most cases by disposing the context always when you don’t need it anymore. Your code sample does not follow this good practice. It rather should look like this:

public BusinessUnit NewBusinessUnit(string name,
    ExternalPlugin accountsModuleId = null,
    ExternalPlugin contactsModuleId = null)
{
    using (IUnitOfWork unitOfWork = UnitOfWorkFactory.CreateUnitOfWork())
    {
        BusinessUnit unit = new BusinessUnit();
        unit.CompanyName = name;

        unitOfWork.ExternalPluginRepository.Attach(accountsModuleId);
        unitOfWork.ExternalPluginRepository.Attach(contactsModuleId);

        unit.AccountsDataSourceModule = accountsModuleId;
        unit.OptionalContactsDataSourceModule = contactsModuleId;
        unitOfWork.BusinessUnitRepository.Insert(unit);

        unitOfWork.Save();

        return unit;
    }
}

At the end of the using block the Dispose method of the unitOfWork is called automatically. To get this working (and compiling as well) you need to derive your IUnitOfWork interface from IDisposable and implement it in the concrete UnitOfWork class:

public interface IUnitOfWork : IDisposable
{
    // ...
}

public class ConcreteUnitOfWork : IUnitOfWork, IDisposable
{
    private MyDbContext _context;
    // I assume that you have a member for the DbContext in this class
    // ...

    // implementation of IDisposable
    public void Dispose()
    {
        if (_context != null)
            _context.Dispose();
    }
}