Use Hash#fetch when using Rails params in controllers

The parameters for a Rails action are stored in an ActionController::Parameters object which behaves quite a bit like a standard Ruby hash.

A Ruby hash is typically accessed using square brackets, like hash[key], but you can also use the method fetch, like hash.fetch(key).

Instead of…

…accessing required parameters from the params hash using the typical #[] method…

class ThingController < ApplicationController
  # … actions

  private

   def an_important_param
     params[:important_and_required]
   end
end

Use…

…the #fetch method to raise an error if the required parameter is missing.

class ThingController < ApplicationController
  # … actions

  private

   def an_important_param
     params.fetch(:important_and_required)
   end
end

But why?

Needing to access an item from the params object implies that the value is important for the correct functioning of your action.

If you attempt to access a value using the #[] method and the relevant key is missing, Ruby will return nil. This might lead to confusing NilClass errors if you try and use the result later on.

Using #fetch means the hash will not return nil in the case of a missing key. Instead it’ll raise a KeyError. You want the code to raise an exception at the point where the data is found to be missing, not when that value is used.

Why not?

You might take a view that you simply like the look of the regular #[] hash syntax. After all, it is shorter. But you must be willing to deal with the errors.

If you’re using a parameter in this way you might also consider changing the route to include it. You could make your search route /search/term rather than /search?q=term.

Note that you can use #fetch to provide a default value:

params.fetch(:important) { 'default' }

This would also help avoid lots of checking for nil in the code where you use the value.