Is there a clean way to avoid calling a method on nil in a nested params hash?

Question

I'm interested in getting the nested 'name' parameter of a params hash. Calling something like

params[:subject][:name]

throws an error when params[:subject] is empty. To avoid this error I usually write something like this:

if params[:subject] && params[:subject][:name]

Is there a cleaner way to implement this?

Answer 1

Check Ick's maybe. You don't need to significantly refactor your code, just intersperse maybe proxies when necessary:

params[:subject].maybe[:name]

The same author (raganwald) also wrote andand, with the same idea.

Answer 2

1. You can use #try, but I don't think it's much better:

   params[:subject].try(:[], :name)
   

2. Or use #fetch with default parameter:

   params.fetch(:subject, {}).fetch(:name, nil)
   

3. Or you can set #default= to new empty hash, but then don't try to modify values returned from this:

   params.default = {}
   params[:subject][:name]
   

   It also breaks all simple tests for existence, so you can't write:

   if params[:subject]
   

   because it will return empty hash, now you have to add #present? call to every test.

   Also this always returns hash when there is no value for key, even when you expect string.

But from what I see, you try to extract nested parameter, instead of assigning it to model and there placing your logic. If you have Subject model, then simply assigning:

@subject = Subject.new(params[:subject])

shuld extract all your parameters user filled in form. Then you try to save them, to see if user passed valid values.

If you're worrying about accessing fields which user should not set, then add attr_accessible whitelist for fields whoich should be allowed to set with mass assignment (as in my example, of with @subject.attributes = params[:subject] for update)

Answer 3

Ruby 2.3.0 makes this very easy to do with #dig

h = {foo: {bar: {baz: 1}}}

h.dig(:foo, :bar, :baz)           #=> 1
h.dig(:foo, :zot, :baz)           #=> nil

Answer 4

params[:subject].try(:[], :name) is the cleanest way

Answer 5

When I have same problem in coding, I sometimes use `rescue'.

name = params[:subject][:name] rescue ""
# => ""

This is not good manners, but I think it is simple way.

EDIT: I don't use this way often anymore. I recommend try or fetch.

Answer 6

Not really. You can try fetch or try (from ActiveSupport) but it's not much cleaner than what you already have.

More info here:

• Nested hash defined?()

UPDATE: Forgot about andand:

• http://andand.rubyforge.org/

andand lets you do:

params[:user].andand[:name] # nil guard is built-in

Similarly, you can use maybe from the Ick library per the answer above.

Answer 7

Or, add [] to it.

class NilClass; def [](*); nil end end
params[:subject][:name]

Answer 8

class Hash
  def fetch2(*keys)
    keys.inject(self) do |hash, key|
      hash.fetch(key, Hash.new)
    end
  end
end

e.g.

require 'minitest/autorun'

describe Hash do
  it "#fetch2" do
    { yo: :lo }.fetch2(:yo).must_equal :lo
    { yo: { lo: :mo } }.fetch2(:yo, :lo).must_equal :mo
  end
end

Answer 9

I cross posted this from my answer over here:

How to check if params[:some][:field] is nil?

I have been looking for a better solution too.

So I figured let's use try a different way to test for a nested key being set:

params[:some].try(:has_key?, :field)

It's not bad. You get nil vs. false if it's not set. You also get true if the param is set to nil.

Answer 10

I wrote Dottie for just this use case — reaching deep into a hash without first knowing whether the entire expected tree exists. The syntax is more succinct than using try (Rails) or maybe (Ick). For example:

# in a Rails request, assuming `params` contains:
{ 'person' => { 'email' => 'jon@example.com' } } # there is no 'subject'

# standard hash access (symbols will work here
# because params is a HashWithIndifferentAccess)
params[:person][:email] # => 'jon@example.com'
params[:subject][:name] # undefined method `[]' for nil:NilClass

# with Dottie
Dottie(params)['person.email'] # => 'jon@example.com'
Dottie(params)['subject.name'] # => nil

# with Dottie's optional class extensions loaded, this is even easier
dp = params.dottie
dp['person.email'] # => 'jon@example.com'
dp['subject.name'] # => nil
dp['some.other.deeply.nested.key'] # => nil

Check out the docs if you want to see more: https://github.com/nickpearson/dottie

Answer 11

I used:

params = {:subject => {:name => "Jack", :actions => {:peaceful => "use internet"}}}

def extract_params(params, param_chain)
  param_chain.inject(params){|r,e| r=((r.class.ancestors.include?(Hash)) ? r[e] : nil)}
end

extract_params(params, [:subject,:name])
extract_params(params, [:subject,:actions,:peaceful])
extract_params(params, [:subject,:actions,:foo,:bar,:baz,:qux])

gives:

=> "Jack"
=> "use internet"
=> nil

Answer 12

You can avoid the double hash access with an inline assignment:

my_param = subj_params = params[:subject] && subj_params[:name]