Rails-migrations.md

+## Adding a PostgreSQL extension
+
+When modifying Postgres extensions, these queries must be wrapped in an `EXISTS` condition. We do this because our development, integration, and production servers get their extensions added outside of the Rails migration system. Without checking for `EXISTS`, these migrations would fail.
+
+```ruby
+class EnableUnaccentExtension < ActiveRecord::Migration
+  def up
+    execute 'CREATE EXTENSION IF NOT EXISTS unaccent SCHEMA shared_extensions;'
+  end
+
+  def down
+    execute 'DROP EXTENSION IF EXISTS unaccent SCHEMA shared_extensions;'
+  end
+end
+```