Collect KeyErrors from Ruby hash into array - ruby-on-rails

Question

I need to extract multiple fields from hash. But I respect my client and I want to gather all missed fields instead of returning it one by one. My idea was to use #fetch, intercept error with KeyError, put error.key into instance variable array and return proper error explanation with full list of missed keys.
Something like that
class Extractor
def initialize hash
#hash = hash
#missed_keys = []
end
def call
extract_values
return "Missed keys: #{#missed_keys.join(', ')}" if #missed_keys.present?
rescue KeyError => e
puts 'Field was missed'
#missed_keys << e.key
return 'Error'
end
private
def extract_values
{
value_1: #hash.fetch(:required_field_1),
value_2: #hash.fetch(:required_field_2),
value_3: #hash.fetch(:required_field_3)
}
end
end
When I try to process hash without required fields I got 'Error' after the first missed field:
pry(main)> Extractor.new(hash: {}).call
Field was missed
=> "Error"
Any clues?
DrySchema and other hash validators are not an option.

Answer 1

An issue with the provided solution is that the extracted values are never returned in the happy path (which presumably is important?). The call method is also stateful / non-idempotent. Subsequent calls to call will duplicate the missing-keys.
Finally - not sure how it's being used, but I don't love a method that returns either a hash or a string.
An alternative that attempts to follow a more functional pattern might look like:
class Extractor
attr_reader :hash, :missed_keys, :required_keys
def initialize hash
#hash = hash
#missed_keys = []
#required_keys = [:required_field_1, :required_field_2, :required_field_3]
end
def call
validate_keys_exist!
extract_values
end
private
def validate_keys_exist!
missed_keys = find_missing_keys
raise MissingKeysError, "Missed keys: #{missed_keys.join(', ')}" if missed_keys.any?
end
def find_missing_keys
required_keys - hash.keys
end
def extract_values
hash.slice(*required_keys)
# not sure if you need to map the keys to new values.
# if so you can iterate over a hash of `from: :to` pairs instead of the
# required_keys array.
end
end

Answer 2

Ok, I got it. The reason is in intercept level and method closures.
In aforementioned implementation Ruby tried to execute call method, got an error and exits.
If we rework it like that:
class Extractor
def initialize hash
#hash = hash
#missed_keys = []
end
def call
extract_values
return "Missed keys: #{#missed_keys.join(', ')}" if #missed_keys.present?
end
private
def extract_values
{
value_1: #hash.fetch(:required_field_1),
value_2: #hash.fetch(:required_field_2),
value_3: #hash.fetch(:required_field_3)
}
rescue KeyError => e
puts 'Field was missed'
#missed_keys << e.key
nil
end
end
it looks better, but still not what we wanted:
pry(main)> Extractor.new(hash: {}).call
Field was missed
=> "Missed keys: required_field_1"
This is because ruby tried to execute extract_values method, encounters first missed value and exits
So the solution as follow:
class Extractor
def initialize hash
#hash = hash
#missed_keys = []
end
def call
extract_values
return "Missed keys: #{#missed_keys.join(', ')}" if #missed_keys.present?
end
private
def extract_values
{
value_1: fetch_value(:required_field_1),
value_2: fetch_value(:required_field_2),
value_3: fetch_value(:required_field_3)
}
end
def fetch_value(key)
#hash.fetch(key)
rescue KeyError => e
puts 'Field was missed'
#missed_keys << e.key
nil
end
end
Extractor.new(hash: {}).call
Field was missed
Field was missed
Field was missed
=> "Missed keys: required_field_1, required_field_2, required_field_3"
Error interception is accomplished on the fetch_value level and Ruby skips required values one by one