In modern Rails versions you can also use ActiveRecord's pluck Show archive.org snapshot method.

>> User.active.pluck(:id)
=> [1, 5, 23, 42]

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If you are plucking from the id column in particular you can also say:

>> User.active.ids
=> [1, 5, 23, 42]

For a DISTINCT selection, use distinct on your scope (not the resulting array).

>> Article.distinct.pluck(:state)
# SELECT DISTINCT state FROM articles
=> ['draft', 'published']

In Rails 3 and 4 you must use uniq instead of distinct:

>> Article.uniq.pluck(:state)
# SELECT DISTINCT state FROM articles
=> ['draft', 'published']

Plucking columns in legacy Rails versions

For older Rails versions you can use the collect_column method from Edge Rider Show archive.org snapshot for the same purpose as pluck.