Reshape hash of hashes into array of modified hashes without using nested each?

Question

My goal is to write more idiomatic ruby when working with nested data structures. More specifically, I need to break my habit of looping through everything with .each and nested .eaches. Here's a simple example that illustrates this:

I have the following data structure which cannot be changed:

structure_hash = { "website"=> { "base"=>"luma.demo", "custom_b2c_website"=>"homegoods.demo", }, "store_view"=> { "custom_store_view"=>"fr.homegoods.ca" } } 

I want to reshape this such that I return the data as an array of hashes like so:

structure_array = [ { :scope=>"website", :code=>"base", :url=>"luma.demo" }, { :scope=>"website", :code=>"custom_b2c_website", :url=>"homegoods.demo" }, { :scope=>"store", :code=>"custom_store_view", :url=>"fr.homegoods.ca" } ] 

I have achieved this with the following helper method with comments that illustrate the thought process I am trying to improve:

def get_vhost_data structure_hash = { ... } # The first hash above, etc. vhost_data = [] # We're returning an array, so initialize one # Loop through the containing hash. Each key will be our scope, so we need to # access that structure_hash.each do |scope, scope_hash| # We need to transform and add to the data in these hashes. scope_hash.each do |code, url| # Best to return a new hash to house the old values + the transformed ones demo_data = {} # The "scope" key from the outer hash needs to be changed conditionally demo_data[:scope] = scope == 'store_view' ? scope.gsub('store_view', 'store') : scope # The rest of the data is fine demo_data[:code] = code demo_data[:url] = url # Add the newly-created hash to the containing array vhost_data << demo_data end end # Return the containing array vhost_data end 

What Smells?

As best I can tell, the following things are fishy:

1. I shouldn't need to initialize an empty array -- surely .each_with_object?

2. Nested .each here seems tedious -- is there a better way to think about what I'm trying to do that would result in something more idiomatic? For example, instead of resorting to "Okay, we need to go through each hash and..." is it more idiomatic to say: "Since you're only manipulating one of the keys of the outer hash, use a select instead? (Just an example, not sure that select does what I want, although it could also take care of creating the containing array...)

3. Again, initializing the empty hash seems wrong -- .each_with_object again?

4. Looping through a hash to create a new hash from the existing hash's content and add to it. At first I thought map would be better somehow, but in my limited understanding, .map takes existing elements and transforms them -- it doesn't add additional elements...

What I've Tried

So far, I've tried the following to address the code smells above:

def get_vhost_data_refactor structure_hash = {...} structure_hash.each_with_object([]) do |(scope, scope_hash), vhost_arr| scope_hash.each_with_object({}) do |(code, url), data_hash| data_hash[:scope] = scope == 'store_view' ? scope.gsub('store_view', 'store') : scope data_hash[:code] = code data_hash[:url] = url vhost_arr << data_hash end end end 

which yields:

[ { :scope=>"website", :code=>"custom_b2c_website_3", :url=>"sierra.demo" }, { :scope=>"website", :code=>"custom_b2c_website_3", :url=>"sierra.demo" }, { :scope=>"store", :code=>"custom_store_view", :url=>"fr.homegoods.ca" } ] 

This is close, but obviously, the .each_with_object combination doesn't loop through each of the inner hashes properly, and more importantly, even if it did work, it's not idiomatic; it just replaces nested .each with the slightly more helpful each_with_object.

Any advice on how I can solve this "the Ruby way" and any tips for questions to ask in order to think "the Ruby way" would be greatly appreciated.

Answer 1

I would agree with your 1st and 3rd points - initializing intermediate arrays and hashes is almost always a smell. with_object is one solution as is reduce/inject. each is definitely not very idiomatic Ruby.

Your second solution is much better. The reason it gives an odd result is because the data_hash is the same object for each iteration of the inner loop and you are just modifying it so the result is the last iteration of the loop.

I would say that this is exactly what map is for - it is still a kind of transformation. I don't know of any ways to select into the inner loop or avoid the nesting but I would use flat_map which does the map and then flattens out the result (you can try with just map to see what I mean)

Something like this:

structure_hash.flat_map do |scope, scope_hash| scope = 'store' if scope == 'store_view' scope_hash.map do |code, url| { scope: scope, code: code, url: url, } end end