Modify events table migration with plain SQL

db/migrate/20180501093846_create_events.rb

@@ -1,13 +1,40 @@
+# This migration does not use Rails ActiveRecord ORM DSL since
+# it doesn't provide data integrity (foreign key) for polymorphic models.
+#
+# The technique applied is called Exclusive Belongs To (AKA Exclusive Arc)
+#
+# Please read the following article for more details:
+# https://hashrocket.com/blog/posts/modeling-polymorphic-associations-in-a-relational-database
+#
 class CreateEvents < ActiveRecord::Migration
-  def change
-    create_table :events do |t|
-      t.string :action, :null => false
+  def up
+    execute <<-SQL
+      CREATE TYPE action_enum AS ENUM ('create', 'update');
 
-      t.integer :post_id
-      t.integer :member_id
-      t.integer :transfer_id
+      CREATE TABLE events (
+        id          serial PRIMARY KEY,
+        action      action_enum NOT NULL,
+        post_id     integer REFERENCES posts,
+        member_id   integer REFERENCES members,
+        transfer_id integer REFERENCES transfers,
+        created_at  timestamp without time zone,
+        updated_at  timestamp without time zone,
+        CHECK(
+          (
+            (post_id IS NOT NULL)::integer +
+            (member_id IS NOT NULL)::integer +
+            (transfer_id IS NOT NULL)::integer
+          ) = 1
+        )
+      );
 
-      t.timestamps
-    end
+      CREATE UNIQUE INDEX ON events (post_id) WHERE post_id IS NOT NULL;
+      CREATE UNIQUE INDEX ON events (member_id) WHERE member_id IS NOT NULL;
+      CREATE UNIQUE INDEX ON events (transfer_id) WHERE transfer_id IS NOT NULL;
+    SQL
+  end
+
+  def down
+    drop_table :events
   end
 end

db/schema.rb

@@ -76,14 +76,8 @@ ActiveRecord::Schema.define(version: 20180501093846) do
   add_index "documents", ["documentable_id", "documentable_type"], name: "index_documents_on_documentable_id_and_documentable_type", using: :btree
   add_index "documents", ["label"], name: "index_documents_on_label", using: :btree
 
-  create_table "events", force: :cascade do |t|
-    t.string   "action",      null: false
-    t.integer  "post_id"
-    t.integer  "member_id"
-    t.integer  "transfer_id"
-    t.datetime "created_at"
-    t.datetime "updated_at"
-  end
+# Could not dump table "events" because of following StandardError
+#   Unknown type 'action_enum' for column 'action'
 
   create_table "members", force: :cascade do |t|
     t.integer  "user_id"
@@ -207,4 +201,7 @@ ActiveRecord::Schema.define(version: 20180501093846) do
   add_index "users", ["email"], name: "index_users_on_email", using: :btree
 
   add_foreign_key "accounts", "organizations"
+  add_foreign_key "events", "members", name: "events_member_id_fkey"
+  add_foreign_key "events", "posts", name: "events_post_id_fkey"
+  add_foreign_key "events", "transfers", name: "events_transfer_id_fkey"
 end