Assign variable only if not nil - ruby

Question

I have #obj.items_per_page, which is 20 at the beginning, and I want the method below to assign value to it only if many_items is not nil:
def fetch_it_baby (many_items = nil)
#obj.items_per_page = many_items
With the code above, even if many_items is nil, #obj.items_per_page remains at 20. Why? And is that "good" coding? Shouldn't I use something like
#obj.items_per_page = many_items || #obj.items_per_page
Or is there a third way? I don't feel completely comfortable with either way.

Answer 1

The style I generally see looks like this:
#obj.items_per_page = many_items if many_items
This uses the inline conditional, while avoiding negative or double-negative conditions.

Answer 2

I suggest the following as it makes it clear that you have a default value for the assignment in case the caller did not specify many_items in the call:
def function(argument = nil)
variable = argument || 20
...
end
However, since you specified that the assignment takes places only if the value is not nil then you'll need to check for the nil value otherwise you will miss the assignment if the value was false. If you really need that case then the solution is more long-winded:
def function(argument = nil)
variable = argument.nil? ? 20 : argument
...
end

Answer 3

You can use &&= (in the same way as ||= is used to assign only if nil or false)
> a = 20 # => 20
> a &&= 30 # => 30
> a # => 30
> a = nil # => nil
> a &&= 30 # => nil
> a = false # => false
> a &&= 30 # => false
> a = {} # => {}
> a &&= 30 # => 30
remember though
> a = 30 # => 30
> a &&= nil # => nil
> a &&= false # => nil
> b &&= 3 # => nil

Answer 4

Even if many_items is nil #obj.items_per_page remains at 20
That sounds like whatever class #obj is has a custom modifier method items_per_page= that only updates the value if the new value is not nil. This is not standard Ruby. For example, given this definition:
class Demo
attr_accessor :items_per_page
end
I get this behavior:
irb(main):005:0> demo = Demo.new #=> #<Demo:0x007fb7b2060240>
irb(main):006:0> demo.items_per_page = 20 #=> 20
irb(main):007:0> demo.items_per_page #=> 20
irb(main):008:0> demo.items_per_page = nil #=> nil
irb(main):009:0> demo.items_per_page #=> nil
As for your example, I would probably write it this way:
#obj.items_per_page = many_items unless many_items.nil?

Answer 5

For Rails you can also use presence as described here
region = params[:state].presence || params[:country].presence || 'US'

Answer 6

new-alexandria's answer is my go-to, but another "third way" would be to use a ternary:
class Demo
attr_accessor :items_per_page
end
many_items = 100
#obj = Demo.new
#obj.items_per_page = 20 #=> 20
#obj.items_per_page = !many_items.nil? ? 30 : nil #=> 30

Answer 7

I am using Rails and I have a similar need.
You can define a method on your model:
class Gift < ApplicationRecord
def safe_set(attribute, value)
return if value.nil?
send("#{attribute}=", value)
end
end
So you can do
g = Gift.new
g.colour = 'red'
g.safe_set(:colour, nil)
g.colour -> 'red'
g.safe_set(:colour, 'green')
g.colour -> 'green'

Answer 8

We have one more method in rails that can help to remove values that are nil.
compact
This method can be used with an array, hash. Returns the same data removing all values that are nil
Eg :
array.compact
Further reference:
https://apidock.com/ruby/v1_9_3_392/Array/compact

Answer 9

If many items is a variable the if/unless versions above are the best. But if many_items is actually a method you don't want to evaluate multiple times I find the following useful.
#obj.items_per_page = many_items.presence || #obj.items_per_page
This isn't quite what you want since it won't assign empty strings or other non-nil but non-present values but most of the time I want to do this kind of thing this works for me (it does create a self-assignment when the value is nil/non-present so only use it if your setters are idempotent).