In Ruby, group_by where I know there's only 1 element per group

Question

I have a CSV file where one column is a primary key. When I do this:

CSV.read(ARGV[0], headers: true).group_by {|r| r['myKey']}

I get a hash table from key to a list of rows, where the list is always length 1.

Is there a version of group_by which asserts that there's only a single value per key, and creates a hash from key to that single value?

Failing that, is there something like .first which asserts that there's exactly one element in the array/enumerable? I like my scripts to fail when my assumptions are wrong, rather than silently return the wrong thing.

Answer 1

If you use Rails you can use index_by method.