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= 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 <tt>NoMethodError (undefined method `[]' for nil:NilClass)</tt>):
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.
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