What exactly does (&:id) do in Product.all.map(&:id)

Question

Here's the line of code I'm trying to wrap my head around:

Category.all.map(&:id).each { |id| Category.reset_counters(id, :products) }

Hoping someone can help me understand what (&:id) is doing and how it impacts the rest of the line? I believe it turns the symbol :id into a proc that'll respond to id?!? But then it gets confusing...

Thanks in advance!

definitely a dupe. though hard to search for that... should have spelled it out "ampersand colon". (i can't delete; it has answers) — Meltemi, Aug 30 '13 at 01:53
I much prefer this alternative: `Category.all.each { |cat| Category.reset_counters(cat.id, :products) }` — Damien Roche, Aug 30 '13 at 01:56
@jvnill Why would you mark something as a duplicate of something that is itself a duplicate? — Andrew Marshall, Aug 30 '13 at 03:16
my bad. the first relevant SO result from google using "lambda operator shortcut rails" was that article. I didn't go through the page itself. just copied the url from the google result. i will have to be more careful next time. — jvnill, Sep 01 '13 at 02:46

score 3 · Accepted Answer · answered Aug 30 '13 at 01:51

Category.all.map(&:id)

is shorthand for

Category.all.map { |a| a.id }

as for how it affects the rest of the line, the above section returns all id values as a single Array. This Array of ids is then passed into another call to each, which iteratively passes each id into reset_counters.

What exactly does (&:id) do in Product.all.map(&:id)

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