The worst bug a SaaS can ship is one customer seeing another customer's data. If your isolation depends on every developer remembering .Where(x => x.TenantId == tenantId) on every query, it's not a matter of if .

Here's the pattern I use in ASP.NET Core and EF Core, taken from a production point of sale and now the foundation of my SaaS Starter Kit .

1. One base class for everything a tenant owns

public interface ITenantOwned { Guid TenantId { get ; } }

public abstract class Entity { public Guid Id { get ; set ; } public DateTimeOffset CreatedAt { get ; set ; } public DateTimeOffset UpdatedAt { get ; set ; } }

public abstract class TenantEntity : Entity , ITenantOwned { public Guid TenantId { get ; set ; } }

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