Code First CTP4: Table with no primary key?

问题

With Entity Framework and Code First, is it possible to let it create and use a table with no primary keys? I can't get this setup to work:

public class Report
{
    public virtual int ReportId
    public virtual ICollection<ReportChanges> ReportChanges
}

public class ReportChanges
{
    public virtual Report Report
    public virtual string EditorName
    public virtual DateTime Changed
}

Note that I've excluded assigning a primary key in ReportChanges. But with this setup I get: "Unable to infer a key for entity type 'ReportChanges'".

Either I'm missing something, or Code First doesn't support tables with no primary keys. What is correct? Thanks.

回答1:

The EF can support a table with no PI if there is still a way to uniquely identify a row, but it can't infer the unique identifier without a proper PK. In other words, lie to the EF and say that there is a PK, say, on Report and Changed.

回答2:

EF needs a way to keep track of its own internal changes. Here's a code sample to help others who've Googled this:

public class YourDataContext : DbContext
{
    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);
        modelBuilder.Entity<ReportChanges>().HasKey(x => new {x.Report, x.Changed});
    }
}

The "new {x.Report, x.Changed}" creates a fake composite key allowing EF to uniquely point to a row internally.

来源：https://stackoverflow.com/questions/4068920/code-first-ctp4-table-with-no-primary-key