With Ruby, there are some cases where you want to check if a given key is already in the hash.
If you’re new to hashes, I wrote a basic blog article about Ruby hashes.
Is checking the key of a hash already exists in the Ruby API ?
What if you want to check that a given hash has a given key ? Before writing any code, the first thing you should think about is “does this already exist somewhere in the API” ?
It’s very uncommon that a utility function is missing - excerpt may be for JavaScript, for historical reasons.
But not for Ruby, even more in a Ruby-on-Rails environment.
Ruby core : a one-liner way to check if key exists
For those who already know Ruby, here is the quick answer
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$> my_hash.key?("my_key")
Ruby : Check presence of key by casting to a boolean
Start the Ruby console (in a Rails project, enter “bin/rails console”).
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$> my_hash = {'a' => 40, a: 41, b: 'some_value'}
Note by the way that the String 'a' is considered as a different key than symbol a:
I want to check that the key belongs to my_hash, so a naive approach would be to use the double !! that will convert any value to a boolean. It’s a trick used in other languages like JavaScript.
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$> my_hash[:b]'some_value'$> my_hash[:c]nil$> !!my_hash[:b]false$> !!my_hash[:c]true
A more expressive way to check presence of a specific key of a Ruby hash
The Ruby hash has an expressive API for the most common use cases. You can use the .key? method instead of the !! seen above.
It’s a lot more readable for other developers.
Examples :
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$> my_hash[:b]'some_value'$> my_hash[:c]nil$> my_hash.key?[:b]false$> my_hash.key?[:c]true
Ruby-on-Rails : check if hash has key
You can use the .has_key? as well. I prefer this solution because it makes the intent a lot clearer. Since you expect a boolean, starting the method by is_ or has_/have_ is better.
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$> my_hash[:b]'some_value'$> my_hash[:c]nil$> my_hash.has_key?[:b]false$> my_hash.has_key?[:c]true
The most robust possible answer
So now what if you’re not quite sure about the hash you’re checking.
Case one : You’re not sure if the keys are symbols or not - think about a hash that is extracted from a remote API.
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$> my_hash.deep_symbolize_keys.try(:key?, :a)true
Case two : you’re not sure if it is actually a hash (could be nil or a wrong type)
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$> my_hash.try(:key?, :a)true
Note that this could be considered as over-engineering, depending on the context.
Conclusion
Checking if a hash has a key in Ruby is pretty straightforward, all you need is to pay a little attention in which context you’re writing this verification.
See also this SO question.
