How to issue a 'find' or 'where' that raises a RecordNotFound

For anyone coming across this question, now you have the method find_by! that does exactly what the OP asked for the find case.

example:

Foo::Bar.find_by!(name: "CocktailBar")

https://apidock.com/rails/v4.0.2/ActiveRecord/FinderMethods/find_by%21

In the last snippet of code you are actually fetching all records from DB and then doing select on a Ruby array. It has nothing to do with ActiveRecord, so you can do whatever you like to raise exception manually, either use the code suggested by Douglas, or if, or unless etc. But it seems that you don't quite understand what your code does. Your select {...} is not translated into SQL SELECT ... WHERE(...).

If you need an exception raised automatically by ActiveRecord query, use this:

Foo::Bar.where([ "lower(name) = ?", name.downcase ]).first!

The equivalent bang methods exist for find_by methods as well, for example Foo::Bar.find_by_name!(name)