   subject_id uuid not null references sg_public.entity (entity_id), --  TODO check kind
   unique (user_id, subject_id)
 );
--- todo comment table/columns
+comment on table sg_public.user_subject
+  is 'The association between a user and a subject. This is a subject the learner is learning.';
+comment on column sg_public.user_subject.id
+  is 'The ID of the user subject.';
+comment on column sg_public.user_subject.created
+  is 'When the user created the association.';
+comment on column sg_public.user_subject.modified
+  is 'When the association last changed.';
+comment on column sg_public.user_subject.user_id
+  is 'Which user the association belongs to.';
+comment on column sg_public.user_subject.subject_id
+  is 'Which subject the association belongs to.';
 
 create table sg_public.response (
   id uuid primary key default uuid_generate_v4(),

  is 'Whenever card parameters changes, update the `modified` column.';
*/

------ Cards, Units, Subjects > Capabilities (TODO) ----------------------------

-- TODO Search per entity type


  'declined',
  'accepted'
);
comment on type sg_public.entity_status
  is 'The four statuses of entity versions.';

create type sg_public.card_kind as enum(
  'video',
