Ruby: Map array to hash creating an array inside the hash

Question

I'm creating a simple analytic chart and to avoid heavy query in realtime I need to save a cached result dataset in a table dedicated to these statistics.

Any user try some course, and I want to save the ids of the course using a status. So something like

{ "invited": [1,3,6], "done": [2,9] }

I write this function

u.courses.map { |w| [w.status, []<<w.id]  }.to_h

but of course every iteration my array is initialized so I have

{"invited"=>[5101]}

if I try with

u.courses.map { |w| [w.status, []<<w.id]  }

I obtain

[["invited", [1]], ["invited", [748]], ["invited", [1445]], ["invited", [2113]], ["invited", [2833]], ["invited", [6017]], ["invited", [4146]], ["invited", [5101]]]

How can I create the array on the first iteration keeping it inside my map?

Please read "minimal reproducible example". We need the minimal code that demonstrates the problem, the associated minimum input and your expected output. We don't know what u is, nor courses. We can guess but we shouldn't have to, you should show us. — the Tin Man
– the Tin Man, Commented Jul 14, 2016 at 23:24

souslov · Accepted Answer · 2016-07-14 21:04:03Z

2

You can try each_with_object:

u.courses.each_with_object({}){|w, o| (o[w.status] ||= []) << w.id}

or reduce/inject:

u.courses.reduce({}){|o, w| (o[w.status] ||= []) << w.id}

edited Jul 14, 2016 at 21:04

answered Jul 14, 2016 at 20:58

souslov

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Cary Swoveland · Accepted Answer · 2016-10-09 06:13:20Z

0

u.course.each_with_object({}) { |course, h|
  h.update(course.status=>[course.id]) { |_,o,n| o+n } }

This uses the form of Hash#update (aka merge!) that employs a block ({ |_,o,n| o+n }) to determine the values of keys that are present in both hashes being merged. The first block variable is the common key. Since that's not used in the block calculation, I've represented it with an underscore, which is common practice. See the doc for update for an explanation of the other two block variables.

edited Oct 9, 2016 at 6:13

answered Jul 14, 2016 at 23:54

Cary Swoveland

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