Rails - annotate method

The annotate method

is used to add an SQL comment to queries generated from the relation. Here is an example

Bullet.includes(:user).
  find_by_id(params[:id])

The above query uses include to eager-load the user table. We can use annotate to add a sql comment to explain the same like

Bullet.includes(:user).
  annotate('Eager loading user table').
  find_by_id(params[:id])

That's it.

Now when you see the rails server logs, you can find the sql comment which was added in the query explanation.

Processing by BulletsController#show as HTML
  Parameters: {"id"=>"5"}
  Bullet Load (0.1ms)  SELECT "bullets".* FROM "bullets" WHERE 
"bullets"."id" = ? /* Eager loading user table */ LIMIT ?
  [["id", 5], ["LIMIT", 1]]
  ↳ app/controllers/bullets_controller.rb:73:in `set_bullet'
  User Load (0.1ms)  SELECT "users".* FROM "users" WHERE
 "users"."id" = ?  [["id", 2]]

Caution: Some escaping is performed, however untrusted user input should not be used.

Source-Code