Best way to find a single record using ActiveRecord 3 / Arel?
I think the preferable way to return a single record would be along the lines of your second example, but you can omit the limit part:
Foo.where(:bar => 'a-value').first
This follows the new syntax and supports chaining if you want to add more conditions to the lookup.
Rails gives you a whole load of magic methods for this kind of thing:
Foo.find_by_bar('a-value')
You can also use multiple attributes:
Foo.find_by_bar_and_wibble('a foo value', 'a wibble value')
And appending a ! causes it to throw a RecordNotFound if nothing's found:
Foo.find_by_bar!('a-value')