How to save a hash into a CSV

CSV can take a hash in any order, exclude elements, and omit a params not in the HEADERS

require "csv"
HEADERS = [
  'dog',
  'cat',
  'donkey'
]

def write_file

  CSV.open("data.csv", "wb", :headers => HEADERS, :write_headers => true) do |csv|
    csv << { 'dog' => 'canine', 'cat' => 'feline', 'donkey' => 'asinine' }
    csv << { 'dog' => 'canine'}
    csv << { 'cat' => 'feline', 'dog' => 'canine', 'donkey' => 'asinine' }
    csv << { 'dog' => 'canine', 'cat' => 'feline', 'donkey' => 'asinine', 'header not provided in the options to #open' => 'not included in output' }
  end
end

write_file # => 
# dog,cat,donkey
# canine,feline,asinine
# canine,,
# canine,feline,asinine
# canine,feline,asinine

This makes working with the CSV class more flexible and readable.

If you want column headers and you have multiple hashes:

require 'csv'
hashes = [{'a' => 'aaaa', 'b' => 'bbbb'}]
column_names = hashes.first.keys
s=CSV.generate do |csv|
  csv << column_names
  hashes.each do |x|
    csv << x.values
  end
end
File.write('the_file.csv', s)

(tested on Ruby 1.9.3-p429)

Try this:

require 'csv'
h = { 'dog' => 'canine', 'cat' => 'feline', 'donkey' => 'asinine' }
CSV.open("data.csv", "wb") {|csv| h.to_a.each {|elem| csv << elem} }

Will result:

1.9.2-p290:~$ cat data.csv 
dog,canine
cat,feline
donkey,asinine

I think the simplest solution to your original question:

def write_file
  h = { 'dog' => 'canine', 'cat' => 'feline', 'donkey' => 'asinine' }

  CSV.open("data.csv", "w", headers: h.keys) do |csv|
    csv << h.values
  end
end

With multiple hashes that all share the same keys:

def write_file
  hashes = [ { 'dog' => 'canine', 'cat' => 'feline', 'donkey' => 'asinine' },
             { 'dog' => 'rover', 'cat' => 'kitty', 'donkey' => 'ass' } ]

  CSV.open("data.csv", "w", headers: hashes.first.keys) do |csv|
    hashes.each do |h|
      csv << h.values
    end
  end
end