ruby on rails: merge! 'params' with a hash indifferently

When using a merge! with the params method key value pairs are not clobbered if the calling hash is using symbols as keys.

If params[:colors] contains:

params[:colors] = { "blue" => false, "green" => true }

and sym_hsh contains:

sym_hsh = { :blue => true, :red => false }

a merge! of the two will result in this:

sym_hsh.merge!(params[:colors])
=> { "blue"=> false, :blue => true, :red => false, "green" => true }

Notice the duplicate blue key and mixed key types. Some are symbols and some are strings.

Using rails you probably haven’t run into this problem much. Behind the scenes in most of rails hashes are made indifferent to key type. Indifference protects developers from running into problems with strings and symbols as hash keys.

The rails code base uses the class HashWithIndifferentAccess allowing hashes to use strings and hashes interchangeably.

When creating new hashes in a rails project it is best to avoid the problem alongside rails. You can do this by creating hashes two different ways:

hsh = HashWithIndifferentAccess.new( :blue => true, :red => false)

or this which does the above for you:

{ :blue => true, :red => false }.with_indifferent_access

Now the merge! will clobber :blue instead of appending another :blue key.

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