This allows you to specify an explicit order that you'd like records
returned in based on a SQL expression. By default, this will be accomplished
using a case statement, as in:

```ruby
Post.in_order_of(:id, [3, 5, 1])
```

will generate the SQL:

```sql
SELECT "posts".* FROM "posts" ORDER BY CASE "posts"."id" WHEN 3 THEN 1 WHEN 5 THEN 2 WHEN 1 THEN 3 ELSE 4 END ASC
```

However, because this functionality is built into MySQL in the form of the
`FIELD` function, that connection adapter will generate the following SQL
instead:

```sql
SELECT "posts".* FROM "posts" ORDER BY FIELD("posts"."id", 1, 5, 3) DESC
```

*Kevin Newton*