Many to many with implied and explicit relationships

问题

I have a standard many-to-many relationship in my database between Person and Widget. A Person in an administrative role has access to ALL Widgets. In my application, I want to see which Widgets a Person has access to.

I have two high level options:

1. Explicitly manage the relationships. When a Person becomes an administrator, relate that Person to all existing Widgets. When a Widget is created, relate that Widget to all existing administrators.

2. At run-time, if the Person is an administrator, assume they have access to ALL widgets and bypass the relationship table when loading Widgets.

Is one option better than the other? Is there a name for this scenario?

I have been trying to apply option 2 using NHibernate and I can't seem to figure out how to get it to bypass the relationship table when loading all Widgets for an entity (and even if I could, this would unnecessarily load alot of information unless I load Widgets separately from the Person entity and apply paging).

回答1:

I would map this by means of Roles.

Roles : Person = 1 : Many

So when you create a person, you also create a new Role, unless they are an Administrator in which case they use the existing Admin Role.

Then the problem is easy: You need a WidgetRole table.

When a new Widget is created, and entry is automatically added to the WidgetRole table for NewWidget, AdminRole

When a Person changes to an Admin Role, simply change their current Role.

imo this setup is logically simpler, than having a special Admin case.

回答2:

I had to share the final solution - should help anyone trying to force NH to load up a relationship that is not explicit in the DB.

I was already creating sub classes of person and NHibernate is smart enough to recognize Administrator : Person and instantiate that Person as an Administrator (where Administrator has a table with a PK/FK PersonId)

I just added a new mapping override for Administrator...

mapping.HasManyToMany(x => x.Widgets)
  .Table("AdministratorWidgetAccess")
  .Cascade.None();

And I added a view called AdministratorWidgetAccess...

SELECT a.PersonId as [AdministratorId], w.WidgetId as 
FROM dbo.Administrator AS a LEFT OUTER JOIN
  dbo.Widget AS w ON 1 = 1

When running, if the Person is an Administrator it loads up all Widgets based on the relationship in the view, otherwise it loads up Widgets based on the join table.

来源：https://stackoverflow.com/questions/6783317/many-to-many-with-implied-and-explicit-relationships