Super polymorphic event schema - sql

Question

I want to be able to track every action a user takes on my site.
An action can originate from a visitor or a user (both of which are human).
An action can affect a subject (visitor or a user)
An action can have an object, which can be any of the other database tables
Some examples:
User A (actor) assigns User B (subject) to conversation (object)
User A (actor) creates team (object)
User B (actor) moved Visitor (subject) to group C (object)
In my application, I want a feed of all events, and for each event, show exactly what actor, subject (if any) and object it refers to
I am thinking something like
create table actors ( -- contains as many rows as there are people
int ID,
)
create table roles ( -- roles like for both human and object roles such as: Visitor, Team, User, Conversation, Group
int ID,
nvarchar(max) Name
)
create table actors_roles ( -- associates people with roles
int Actor_ID, -- FK to actors.ID
int Role_ID -- FK to roles.ID
)
create table objects ( -- associates objects with roles
int ID,
)
create table object_roles ( -- associates objects with roles
int Object_ID, -- FK to object.ID
int Role_ID -- FK to roles.ID
)
create table tEvent (
int ID,
int Type_ID,
int Actor_ID, -- FK to actors.ID
int Subject_ID -- FK to actors.ID
int Object_ID -- FK to objects.ID
)
Besides these tables, every record in roles will have a corresponding, separate table maintaining all the data related to the object with a foreign key.
I'd love to get some feedback on this structure and if it is scaleable, or perhaps, there is a better way to accomplish this?
Credit to Daniel A. Thompson for pushing me in this direction

Answer 1

Based on your requirements, I'd propose the following schema:
-- roles for both human and object roles such as:
-- Visitor, Team, User, Conversation, Group
CREATE TABLE tRole (
int ID,
nvarchar(max) Name
)
-- contains as many rows as there are people
CREATE TABLE tActor (
int ID,
int Role_ID -- FK to tRole.ID
)
-- contains as many rows as there are objects
CREATE TABLE tObject (
int ID,
int Role_ID -- FK to tRole.ID
)
CREATE TABLE tEvent (
int ID,
int Type_ID,
int Actor_ID, -- FK to tActor.ID
int Subject_ID -- FK to tActor.ID
int Object_ID -- FK to tObject.ID
)