Ruby try not handling exception - ruby-on-rails

Question

I have the following example ruby code:
def example_method(obj)
ExampleClass.new(color1: #theme.try(obj['color1'].to_sym))
end
Which should pass either the obj['color1'] as a symbol or nil to the class.
However if color1 is not passed I get the error: NoMethodError: undefined method 'to_sym' for nil:NilClass.
Shouldn't the try method be handling the exception?
Update: based on comments... I solved it by doing a ternary:
ExampleClass.new(color1: obj['color1'].present? ? #brand_theme.try(obj['color1'].try(:to_sym)) : nil)

Answer 1

You could write a helper method:
def theme_color(name)
return unless name
return unless #theme.respond_to?(name)
#theme.public_send(name)
end
def example_method(obj)
ExampleClass.new(color1: theme_color(obj['color1']))
end
theme_color returns nil if the argument is nil, i.e. obj['color1']. It also returns nil if theme does not respond to the given method. Otherwise, it invokes the method specified by name.
Note that respond_to? and public_send accept either a string or a symbol, so no to_sym is needed.
You could also define the helper method as an instance method of your #theme class:
class Theme
def color(name)
return unless name
return unless respond_to?(name)
public_send(name)
end
def red
'FF0000'
end
end
#theme = Theme.new
#theme.red #=> "FF0000"
#theme.color(:red) #=> "FF0000"
#theme.color('red') #=> "FF0000"
#theme.color('green') #=> nil
#theme.color(nil) #=> nil
And invoke it via:
def example_method(obj)
ExampleClass.new(color1: #theme.color(obj['color1']))
end
Keep in mind that these approaches (using public_send or try) allow you to invoke arbitrary methods on your #theme object. It might be safer to keep the colors in a hash.

Answer 2

From comments:
In this example obj['color1'] would be nil. So we'd be passing nil to the try.
Yes, that's an error. You can't call a method with no name. Technically, you could avoid the error by doing .try(obj['color'].to_s), but it's super-wrong.
I would check for presence explicitly and bail early if it's not there.
def example_method(obj)
return unless obj['color1'].present?
ExampleClass.new(color1: #theme.try(obj['color1']))
end

Answer 3

The try is called on the #theme object. The nil error is thrown because obj['color1'] returns nil and then to_sym is called on nil.
You'd have to alter the code to
ExampleClass.new(color1: #theme.try(obj['color1'].try(:to_sym) || ''))
to catch that.
And then you would have to prettify the code.
How the prettification works will depend on the use case, so I can only offer some general pointer. One way would be to have a default value to avoid having to deal with the null object
Instead of passing around nil, one simply returns a default value:
color_key = obj.fetch('color') { 'default_color' }.to_sym
ExampleClass.new(color1: #theme.send(color_key)))
This makes use of the fetch method which enables returning a default value. That way you will always have a value defined.