[EASI-4635] Initial discussions API schema and migration

migrations/V195__Add_grb_review_discussions_table.sql

@@ -0,0 +1,43 @@
+CREATE TABLE IF NOT EXISTS system_intake_internal_grb_review_discussion_posts (
+  id UUID PRIMARY KEY NOT NULL,
+  content TEXT NOT NULL,
+  voting_role grb_reviewer_voting_role_type,
+  grb_role grb_reviewer_role_type,
+  system_intake_id UUID NOT NULL
+    REFERENCES system_intakes(id)
+    ON DELETE CASCADE
+    ON UPDATE CASCADE,
+  reply_to_id UUID
+    REFERENCES system_intake_internal_grb_review_discussion_posts (id)
+    ON DELETE CASCADE
+    ON UPDATE CASCADE,
+  created_by UUID NOT NULL REFERENCES user_account(id),
+  created_at TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  modified_by UUID REFERENCES user_account(id),
+  modified_at TIMESTAMP WITH TIME ZONE
+);
+
+CREATE OR REPLACE FUNCTION prevent_nested_replies_fn() RETURNS TRIGGER AS $$
+DECLARE
+    parent_reply_to_id UUID;
+BEGIN
+    -- If reply_to_id is not NULL, check if it points to a top-level post
+    IF NEW.reply_to_id IS NOT NULL THEN
+        -- Fetch the reply_to_id of the parent post
+        SELECT reply_to_id INTO parent_reply_to_id
+        FROM system_intake_internal_grb_review_discussion_posts
+        WHERE id = NEW.reply_to_id;
+
+        -- If parent_reply_to_id is NOT NULL, the parent post is a reply, so raise an error
+        IF parent_reply_to_id IS NOT NULL THEN
+            RAISE EXCEPTION 'Replies can only be made to top-level posts';
+        END IF;
+    END IF;
+
+    RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+
+CREATE TRIGGER prevent_nested_replies_trigger
+BEFORE INSERT OR UPDATE ON system_intake_internal_grb_review_discussion_posts
+FOR EACH ROW EXECUTE FUNCTION prevent_nested_replies_fn();