image by Mitchell Orr
Use Hash#fetch when using Rails params in controllers
In a URL, we often need to access parameters that aren’t part of the regular Rails routes. For example, the query parameter in /search?q=term
.
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.
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Last updated on April 29th, 2018