Using EF Core HasQueryFilter on navigation properties

Question

I'm trying to apply a filter to my queries for multi-tenancy but it doesn't allow me to apply the filter when the property is part of a navigation property:

modelBuilder.Entity<Level>().HasQueryFilter(lvl => lvl.SchoolYear.TenantId == _tenantProvider.TenantId);

Here, Level is the property I want filtered but the property to use for filtering which is TenantId is is inside Level.SchoolYear.

If its a top-level property it works fine. How can I apply a global filter when the property I need for filtering is a navigation property?

Answer 1

As said, these model level filters are not (yet) implemented for navigation properties. Which is sad, because people may not be aware of this restriction and count on it in a multi-tenant architecture. Everybody knows that making errors in a multi-tenant application may be the death of your company, so it's pretty vital to do this right.

Once you've chosen for tenant separation by TenantId (and not the safer schema-per-tenant or database-per-tenant), and knowing this current restriction in EF-core, you may want to build in a safeguard that at least you'll never mix up tenant data when saving changes.

This safeguard could be implemented by defining an interface:

public interface IHasTenantId
{
    int TenantId { get; }   
}

... to be implemented by each entity having a TenantId.

Then in an override of SaveChanges in the DbContext subclass there can be a check if all TenantIds in a change set are identical:

public override int SaveChanges()
{
    var distinctTenantIdsCount = this.ChangeTracker.Entries<IHasTenantId>()
                                     .Select(e => e.Entity.TenantId)
                                     .Distinct().Count();
    if(distinctTenantIdsCount > 1)
    {
        // Throw an exception and handle it.
    }
    return base.SaveChanges();
}

Answer 2

You can't. It's a documented limitation of the current (as of EF Core 2.0.1) Global Query Filters implementation:

Filters cannot contain references to navigation properties.

It's tracked by #8881: Allow to use navigations in model level entity filters enhancement request, but unfortunately has no concrete milestone assigned, which means we don't know if and when it will be fixed.