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POSlicing params hash for specific values
 

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    <h2>Summary</h2> <p>Given a Hash, what is the most efficient way to create a subset Hash based on a list of keys to use?</p> <pre><code>h1 = { a:1, b:2, c:3 } # Given a hash... p foo( h1, :a, :c, :d ) # ...create a method that... #=&gt; { :a=&gt;1, :c=&gt;3, :d=&gt;nil } # ...returns specified keys... #=&gt; { :a=&gt;1, :c=&gt;3 } # ...or perhaps only keys that exist </code></pre> <h2>Details</h2> <p>The <a href="http://sequel.rubyforge.org/index.html" rel="noreferrer">Sequel</a> database toolkit allows one to create or update a model instance by passing in a Hash:</p> <pre><code>foo = Product.create( hash_of_column_values ) foo.update( another_hash ) </code></pre> <p>The <a href="http://www.sinatrarb.com/" rel="noreferrer">Sinatra</a> web framework makes available a Hash named <code>params</code> that includes form variables, querystring parameters and also route matches.</p> <p>If I create a form holding only fields named the same as the database columns and post it to this route, everything works very conveniently:</p> <pre><code>post "/create_product" do new_product = Product.create params redirect "/product/#{new_product.id}" end </code></pre> <p>However, this is both fragile and dangerous. It's dangerous because a malicious hacker could post a form with columns not intended to be changed and have them updated. It's fragile because using the same form with this route will not work:</p> <pre><code>post "/update_product/:foo" do |prod_id| if product = Product[prod_id] product.update(params) #=&gt; &lt;Sequel::Error: method foo= doesn't exist or access is restricted to it&gt; end end </code></pre> <p>So, for robustness and security I want to be able to write this:</p> <pre><code>post "/update_product/:foo" do |prod_id| if product = Product[prod_id] # Only update two specific fields product.update(params.slice(:name,:description)) # The above assumes a Hash (or Sinatra params) monkeypatch # I will also accept standalone helper methods that perform the same end end </code></pre> <p>...instead of the more verbose and non-DRY option:</p> <pre><code>post "/update_product/:foo" do |prod_id| if product = Product[prod_id] # Only update two specific fields product.update({ name:params[:name], description:params[:description] }) end end </code></pre> <h2>Update: Benchmarks</h2> <p>Here are the results of benchmarking the (current) implementations:</p> <pre class="lang-none prettyprint-override"><code> user system total real sawa2 0.250000 0.000000 0.250000 ( 0.269027) phrogz2 0.280000 0.000000 0.280000 ( 0.275027) sawa1 0.297000 0.000000 0.297000 ( 0.293029) phrogz3 0.296000 0.000000 0.296000 ( 0.307031) phrogz1 0.328000 0.000000 0.328000 ( 0.319032) activesupport 0.639000 0.000000 0.639000 ( 0.657066) mladen 1.716000 0.000000 1.716000 ( 1.725172) </code></pre> <p>The second answer by @sawa is the fastest of all, a hair in front of my <code>tap</code>-based implementation (based on his first answer). Choosing to add the check for <code>has_key?</code> adds very little time, and is still more than twice as fast as ActiveSupport.</p> <p>Here is the benchmark code:</p> <pre><code>h1 = Hash[ ('a'..'z').zip(1..26) ] keys = %w[a z c d g A x] n = 60000 require 'benchmark' Benchmark.bmbm do |x| %w[ sawa2 phrogz2 sawa1 phrogz3 phrogz1 activesupport mladen ].each do |m| x.report(m){ n.times{ h1.send(m,*keys) } } end end </code></pre>
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    1. COYour example at the top doesn't seem to agree with the details? In the example you show that if you select a key that doesn't exist in the original hash you should get a nil value in the new hash. In your Sequel example it doesn't seem you need to create a new hash but really just a subset. What is the real requirement?
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    2. CO@Wes The two are not incompatible, I think. The real situation will be that I will never (knowingly) ask for a key that does not exist in the original. I included `:d` in the summary to clearly specify how the edge case should be handled. However, I am also amenable to solutions which do not include any missing-but-requested keys. (Indeed, Mladen's answer and ActiveSupport both do not include any keys not present in the original.)
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    3. COWow, good to hear the result. A possible lesson here; a naive implementation is faster than going too much into Rubyish way and fully using its function? Hope ruby implementation gets faster.
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