ef core - multi tenancy using global filters

Question

OnModelCreating is called once per db context. This is a problem since the tenant Id is set per request.

How do I re-configure the global filter everytime I create an new instance of the dbcontext?

If I can't use global filter, what is the alternative way?

Update:

I needed to provide a generic filter with an expression like e => e.TenantId == _tenantId. I am using the following expression:

var p = Expression.Parameter(type, "e");
Expression.Lambda(
    Expression.Equal(
        Expression.Property(p, tenantIdProperty.PropertyInfo),
        Expression.Constant(_tenantId))
    p);

Since this is run once, _tenantId is fixed. So even if I update it, the first value is captured in the linq expression.

So my question, what is the proper way to set the right side of that equality.

Answer 1

With EF.Core you can actually use the following filter and syntax

protected void OnModelCreating(ModelBuilder modelBuilder)
{
    var entityConfiguration = modelBuilder.Entity<MyTenantAwareEntity>();
    entityConfiguration.ToTable("my_table")
               .HasQueryFilter(e => EF.Property<string>(e, "TenantId") == _tenantProvider.GetTenant())
    [...]

The _tenantProvider is the class responsible to get your tenant, in your case from the HttpRequest, to do it you can use HttpContextAccessor.

Answer 2

This is fixed with the following as the right expression

Expression.MakeMemberAccess(
    Expression.Constant(this, baseDbContextType),
    baseDbContextType.GetProperty("TenantId")

I use a base class for all my db contexts. GetProperty() works as is because TenantId is public property.