Dig Methods

Ruby’s dig methods are useful for accessing nested data structures.

Consider this data:

item = {
  id: "0001",
  type: "donut",
  name: "Cake",
  ppu: 0.55,
  batters: {
    batter: [
      {id: "1001", type: "Regular"},
      {id: "1002", type: "Chocolate"},
      {id: "1003", type: "Blueberry"},
      {id: "1004", type: "Devil's Food"}
    ]
  },
  topping: [
    {id: "5001", type: "None"},
    {id: "5002", type: "Glazed"},
    {id: "5005", type: "Sugar"},
    {id: "5007", type: "Powdered Sugar"},
    {id: "5006", type: "Chocolate with Sprinkles"},
    {id: "5003", type: "Chocolate"},
    {id: "5004", type: "Maple"}
  ]
}

Without a dig method, you can write:

item[:batters][:batter][1][:type] # => "Chocolate"

With a dig method, you can write:

item.dig(:batters, :batter, 1, :type) # => "Chocolate"

Without a dig method, you can write, erroneously (raises NoMethodError (undefined method `[]' for nil:NilClass)):

item[:batters][:BATTER][1][:type]

With a dig method, you can write (still erroneously, but avoiding the exception):

item.dig(:batters, :BATTER, 1, :type) # => nil

Why Is dig Better?

• It has fewer syntactical elements (to get wrong).

• It reads better.

• It does not raise an exception if an item is not found.

How Does dig Work?

The call sequence is:

obj.dig(*identifiers)

The identifiers define a “path” into the nested data structures:

• For each identifier in identifiers, calls method #dig on a receiver with that identifier.

• The first receiver is self.

• Each successive receiver is the value returned by the previous call to dig.

• The value finally returned is the value returned by the last call to dig.

A dig method raises an exception if any receiver does not respond to #dig:

h = { foo: 1 }
# Raises TypeError (Integer does not have #dig method):
h.dig(:foo, :bar)

What Else?

The structure above has Hash objects and Array objects, both of which have instance method dig.

Altogether there are six built-in Ruby classes that have method dig, three in the core classes and three in the standard library.

In the core:

• Array#dig: the first argument is an Integer index.

• Hash#dig: the first argument is a key.

• Struct#dig: the first argument is a key.

In the standard library:

• OpenStruct#dig: the first argument is a String name.

• CSV::Table#dig: the first argument is an Integer index or a String header.

• CSV::Row#dig: the first argument is an Integer index or a String header.