alpenwerk-hr/db/migrations/20260724120000_employee_assignment_history.sql

-- Org-assignment history, so the Organigramm can be shown as of any date.
--
-- Until now `employees` carried only the *current* placement (manager_id,
-- team_id, division_id, job_title, is_lead, org_level) and every mutation
-- overwrote it in place. employee_history recorded that something happened,
-- but only as free text — no old/new values — so a past reporting line was
-- gone for good. Forward-looking dates already worked (pending_org_changes
-- holds not-yet-due changes structurally); it was the past that could not be
-- reconstructed. This migration adds the missing timeline.
--
-- Captured by a trigger rather than by editing the mutating RPCs: there are
-- ~70 `update employees` statements spread over fifteen migrations (several
-- of which redefine the same function repeatedly), so per-RPC bookkeeping
-- would miss paths today and again with every future RPC. A trigger on the
-- table catches all of them, including ones not written yet.

create table employee_assignments (
  id uuid primary key default gen_random_uuid(),
  employee_id uuid not null references employees(id) on delete cascade,
  manager_id uuid references employees(id) on delete set null,
  team_id uuid references teams(id),
  division_id uuid not null references divisions(id),
  job_title text not null,
  is_lead boolean not null default false,
  org_level int not null,
  valid_from date not null,
  -- Exclusive upper bound; null means "still in force". Note this tracks
  -- *placement*, not employment: the row of someone who has left stays open,
  -- because whether they were employed on a given date is derived separately
  -- from entry/exit/karenz dates. Keeping the two apart is what lets a
  -- rehire reuse the same open row instead of needing it reopened.
  valid_to date,
  created_at timestamptz not null default now(),
  constraint chk_assignment_range check (valid_to is null or valid_to > valid_from)
);

create index on employee_assignments (employee_id, valid_from desc);
create index on employee_assignments (valid_from);
-- At most one open interval per employee — the invariant the trigger relies
-- on when it looks up "the current row" to close.
create unique index employee_assignments_one_open on employee_assignments (employee_id) where valid_to is null;

alter table employee_assignments enable row level security;
create policy "employee_assignments_hr_read" on employee_assignments for select using (is_hr_user());

-- ── Backfill ───────────────────────────────────────────────────────
-- One open interval per employee, starting at their entry date, holding
-- today's placement. Changes made *before* this migration are not
-- recoverable — employee_history never stored structured old values — so a
-- Stichtag before today's date shows the placement as it stands now for
-- anyone whose assignment predates this table. Everything from here on is
-- exact.
insert into employee_assignments (employee_id, manager_id, team_id, division_id, job_title, is_lead, org_level, valid_from)
select id, manager_id, team_id, division_id, job_title, is_lead, org_level, entry_date
from employees;

-- ── Trigger ────────────────────────────────────────────────────────
-- The date a change takes effect is normally the day it is written: an RPC
-- with a future effective date does not touch `employees` at all (it queues
-- into pending_org_changes, and apply_due_pending_changes writes it on the
-- due date), and one dated today writes immediately. A *backdated* change is
-- the exception — it writes immediately although it should take effect
-- earlier — so callers may set `app.effective_date` for the transaction to
-- say so explicitly; unset, it falls back to the current date.
create or replace function fn_track_employee_assignment()
returns trigger
language plpgsql
security definer
set search_path = public
as $$
declare
  v_date date := coalesce(nullif(current_setting('app.effective_date', true), '')::date, current_date);
  v_open_from date;
begin
  if tg_op = 'INSERT' then
    insert into employee_assignments (employee_id, manager_id, team_id, division_id, job_title, is_lead, org_level, valid_from)
      values (new.id, new.manager_id, new.team_id, new.division_id, new.job_title, new.is_lead, new.org_level,
              coalesce(new.entry_date, v_date));
    return new;
  end if;

  if new.manager_id is not distinct from old.manager_id
     and new.team_id is not distinct from old.team_id
     and new.division_id is not distinct from old.division_id
     and new.job_title is not distinct from old.job_title
     and new.is_lead is not distinct from old.is_lead
     and new.org_level is not distinct from old.org_level then
    return new;
  end if;

  select valid_from into v_open_from
    from employee_assignments where employee_id = new.id and valid_to is null;

  if v_open_from is null then
    insert into employee_assignments (employee_id, manager_id, team_id, division_id, job_title, is_lead, org_level, valid_from)
      values (new.id, new.manager_id, new.team_id, new.division_id, new.job_title, new.is_lead, new.org_level, v_date);
  elsif v_date <= v_open_from then
    -- Two changes on the same day (or a backdate landing inside the open
    -- interval): overwrite in place, so no zero-length or inverted interval
    -- is ever stored.
    update employee_assignments
      set manager_id = new.manager_id, team_id = new.team_id, division_id = new.division_id,
          job_title = new.job_title, is_lead = new.is_lead, org_level = new.org_level
      where employee_id = new.id and valid_to is null;
  else
    update employee_assignments set valid_to = v_date where employee_id = new.id and valid_to is null;
    insert into employee_assignments (employee_id, manager_id, team_id, division_id, job_title, is_lead, org_level, valid_from)
      values (new.id, new.manager_id, new.team_id, new.division_id, new.job_title, new.is_lead, new.org_level, v_date);
  end if;

  return new;
end;
$$;

drop trigger if exists trg_track_employee_assignment on employees;
create trigger trg_track_employee_assignment
after insert or update on employees
for each row execute function fn_track_employee_assignment();

grant all on table employee_assignments to anon, authenticated, service_role;