ruby db result set to array in a hash in a hash

Question

I have a db query which returns results like:

db_result.each {|row| puts row}
{"IP"=>"1.2.3.4","Field1"=>"abc","Field2"=>"123"}
{"IP"=>"1.2.3.4","Field1"=>"abc","Field2"=>"234"}
{"IP"=>"1.2.3.4","Field1"=>"bcd","Field2"=>"345"}
{"IP"=>"3.4.5.6","Field1"=>"bcd","Field2"=>"456"}
{"IP"=>"3.4.5.6","Field1"=>"bcd","Field2"=>"567"}

And want to put it into a hash like:

{
  "1.2.3.4" => {
    "abc" => ["123", "234"],
    "bcd" => "345"
  },
  "3.4.5.6" => {
    "bcd" => ["456", "567"]
  }
}

What I am currently doing is:

result_hash = Hash.new { |h, k| h[k] = {} }
db_result.each do |row|
  result_hash[row["IP"]] = Hash.new { |h, k| h[k] = [] } unless result_hash.has_key? row["IP"]
  result_hash[row["IP"]][row["Field1"]] <<  row["Field2"]
end

Which works, however was wondering if there is a neater way.

There's nothing untidy about your method. You're doing a couple of operations, so it is more readable to leave it as you have it rather than try to cram it into a one-liner or obfuscate it down. — Michael Berkowski
– Michael Berkowski, Commented Aug 16, 2012 at 14:45
It depends on what you are using to handle the connection to the database. I think Sequel has the ability to map the results of a query into a hash of hashes based on a particular field. — the Tin Man
– the Tin Man, Commented Aug 16, 2012 at 15:01
Standard mysql2 client, so nothing fancy in there. Thanks for the saner formatting! Was just wondering if i was missing a trick. — Decado
– Decado, Commented Aug 16, 2012 at 15:06

the Tin Man · Accepted Answer · 2012-08-16 15:09:39Z

1

Consider this a peer-review. As a recommendation for processing and maintenance...

I'd recommend the data structure you want be a little more consistent.

Instead of:

{
  "1.2.3.4" => {
    "abc" => ["123", "234"],
    "bcd" => "345"
  },
  "3.4.5.6" => {
    "bcd" => ["456", "567"]
  }
}

I'd recommend:

{
  "1.2.3.4" => {
    "abc" => ["123", "234"],
    "bcd" => ["345"]
  },
  "3.4.5.6" => {
    "abc" => [],
    "bcd" => ["456", "567"]
  }
}

Keep the same keys in each sub-hash, and make the values all be arrays. The code for processing that overall hash will be more straightforward and easy to follow.

answered Aug 16, 2012 at 15:09

the Tin Man

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1 Comment

Decado Over a year ago

All the values should be arrays, that was my typo in copying out and obscuring the variable names. The number of sub-keys's is a decision making metric later on, so padding wont work. And I don't know what they'll be beforehand, so doing would be difficult anyhow.

Serge Balyuk · Accepted Answer · 2012-08-16 15:02:01Z

1

I agree with Michael, there is nothing wrong with your method. The intent behind the code can be easily seen.

If you want to get fancy, here's one (of many) ways to do it:

x = [
  {"IP"=>"1.2.3.4","Field1"=>"abc","Field2"=>"123"},
  {"IP"=>"1.2.3.4","Field1"=>"abc","Field2"=>"234"},
  {"IP"=>"1.2.3.4","Field1"=>"bcd","Field2"=>"345"},
  {"IP"=>"3.4.5.6","Field1"=>"bcd","Field2"=>"456"},
  {"IP"=>"3.4.5.6","Field1"=>"bcd","Field2"=>"567"}
]


y = x.inject({}) do |result, row|
  new_row = result[row["IP"]] ||= {}
  (new_row[row["Field1"]] ||= []) << row["Field2"]

  result
end

I think this should yield the same time complexity as your method.

answered Aug 16, 2012 at 15:02

Serge Balyuk

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1 Comment

Decado Over a year ago

Okay, that's pretty cool. Was wondering how to use the inject stuff.

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