Using Ruby 2.3's dig method and safe navigation operator to navigate nested objects like the Facebook Insights object

Ruby 2.3.0 was released on Christmas. Woohoo!

Actually, truth be told, since I started learning Ruby, beginning with 2.0.0-p353, I haven’t found many useful new methods or tricks. But! Here is 2.3.0 with at least one new operator and one new method that I’ll most likely be taking advantage of.

Let’s get to it.

The Dig Method

The #dig method, which I just learned about today, allows us to safely access nested data in Hash, Array, or Struct objects. The ruby-doc explanation is “Extracts the nested value specified by the sequence of idx objects by calling dig at each step, returning nil if any intermediate step is nil.”

André Barbosa’s post has a simple example illustrating how the method works. Basically if you’re looking to get a value out of nested hashes, in Ruby 2.2 and older you’d have something like:

name = post[:user][:name]  

But if you were iterating through a number of posts, and one of them did not have a :user defined, you’d hit an error and, without a rescue, your program would halt.

One way to prevent against this in Ruby 2.2 and older, as Barbosa notes, is to do some conditional work:

name = post[:user][:name] if post[:user]
# or
name = post[:user] && post[:name][:user]  

This is fine and good for this example, but as your hashes or arrays get more nested, this can get pretty messy. For example, when working with the Facebook Insights API, I’ve got Ruby 2.2 code that looks like this:

if insights[46] && insights[46]["values"][0] && insights[46]["values"][0]["value"]
  @link_clicks = insights[46]["values"][0]["value"]["link clicks"]
end

As you may have guessed, when you ask Facebook for the Insights for a given page post, it returns a large array called insights that has an element for roughly each metric about that post (and there are many). insights[46], for example, contains information about clicks on the post (when it exists!).

Here’s a look at just the first 3 elements of the Insights array for a recent CNN Facebook post (all numbers changed):

insights = [
 {"id"=>"5550296508_10154357616641509/insights/post_story_adds_unique/lifetime",
  "name"=>"post_story_adds_unique",
  "period"=>"lifetime",
  "values"=>[{"value"=>9000}],
  "title"=>"Lifetime Talking About This (Post)",
  "description"=>"Lifetime: The number of unique people who created a story by interacting with your Page post. (Unique Users)"},
 {"id"=>"5550296508_10154357616641509/insights/post_story_adds/lifetime",
  "name"=>"post_story_adds",
  "period"=>"lifetime",
  "values"=>[{"value"=>8000}],
  "title"=>"Lifetime Post Stories",
  "description"=>"Lifetime: The number of stories generated about your Page post. (Total Count)"},
 {"id"=>"5550296508_10154357616641509/insights/post_story_adds_by_action_type_unique/lifetime",
  "name"=>"post_story_adds_by_action_type_unique",
  "period"=>"lifetime",
  "values"=>[{"value"=>{"like"=>4000, "share"=>900, "comment"=>700}}],
  "title"=>"Lifetime Talking About This (Post) by action type",
  "description"=>"Lifetime: The number of unique people who created a story about your Page post by interacting with it. (Unique Users)"}
 # many more array elements here...
]

As you can see, if we wanted the number of shares on this post (which I’ve changed to 900), we’ve got some digging to do. Here’s how I have it using Ruby 2.2 and older:

shares_old_way = insights[2]["values"][0]["value"]["share"]

The big problem here is if, for whatever reason, any of the posts I’m looking at don’t have a hash inside insights[2]["values"][0] I’ll be calling ["value"] on nil and I’ll get a NoMethodError: undefined method error and my program will be halted. A similar fate will meet my little script if insights doesn’t have something at [46] or if any of the other links in the chain return a nil. Hence the conditional checks I outlined above.

However with the dig method, I can use this:

shares_using_dig = insights.dig(2, "values", 0, "value", "share")

Using dig, if any of links in the chain return nil, the dig call returns nil rather than erroring out. In this case, it’s pretty good to get shares set to nil if a given post doesn’t have shares for whatever reason. I could run something like shares = 0 if !shares afterward to set shares to 0 if none were detected, but that’s probably not necessary.

As with the #dig method, I learned about the safe navigation operator (a.k.a. the lonely operator) from a blog post that was posted to the Ruby subreddit. (Matz also mentions it in an interview on the Heroku blog.)

Similarly to the #dig method, the safe navigation operator .& makes it safer to string multiple methods together. Here’s the example Lasseigne gives:

Say you have this array:

some_array = [2,3,4]

We could check that the first element is positive by running:

some_array.first.positive?
# => true

(Lasseigne notes that the positive? method is also new in Ruby 2.3.0)

But similarly to our hash navigation problem earlier, if the array we’re checking is empty we’ll get a NoMethodError:

[].first.positive?
# => NoMethodError: undefined method `positive?' for nil:NilClass

Again, there’s a conditional fix here: [].first.positive? if [].first, but now we have the handy safe navigation operator!

some_array.first&.positive?

As Lasseigne writes, “If first returns nil then the entire expression returns nil. If not then we’ll continue down the method chain and call positive?.”

Lasseigne concludes his post by discussing whether this new safe navigation operator is more or less DRY than the previous alternative(s). I think it’s a worthwhile question, and I’m honestly not sure it will be in all cases.

Anyway, navigate on with confidence!

Captain Ron would use #dig

The Dig Method

The Safe Navigation Operator