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83 lines
2.6 KiB
Text
83 lines
2.6 KiB
Text
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= Dig Methods
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Ruby's +dig+ methods are useful for accessing nested data structures.
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Consider this data:
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item = {
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id: "0001",
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type: "donut",
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name: "Cake",
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ppu: 0.55,
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batters: {
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batter: [
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{id: "1001", type: "Regular"},
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{id: "1002", type: "Chocolate"},
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{id: "1003", type: "Blueberry"},
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{id: "1004", type: "Devil's Food"}
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]
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},
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topping: [
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{id: "5001", type: "None"},
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{id: "5002", type: "Glazed"},
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{id: "5005", type: "Sugar"},
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{id: "5007", type: "Powdered Sugar"},
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{id: "5006", type: "Chocolate with Sprinkles"},
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{id: "5003", type: "Chocolate"},
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{id: "5004", type: "Maple"}
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]
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}
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Without a +dig+ method, you can write:
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item[:batters][:batter][1][:type] # => "Chocolate"
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With a +dig+ method, you can write:
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item.dig(:batters, :batter, 1, :type) # => "Chocolate"
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Without a +dig+ method, you can write, erroneously
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(raises <tt>NoMethodError (undefined method `[]' for nil:NilClass)</tt>):
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item[:batters][:BATTER][1][:type]
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With a +dig+ method, you can write (still erroneously, but avoiding the exception):
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item.dig(:batters, :BATTER, 1, :type) # => nil
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== Why Is +dig+ Better?
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- It has fewer syntactical elements (to get wrong).
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- It reads better.
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- It does not raise an exception if an item is not found.
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== How Does +dig+ Work?
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The call sequence is:
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obj.dig(*identifiers)
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The +identifiers+ define a "path" into the nested data structures:
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- For each identifier in +identifiers+, calls method \#dig on a receiver
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with that identifier.
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- The first receiver is +self+.
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- Each successive receiver is the value returned by the previous call to +dig+.
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- The value finally returned is the value returned by the last call to +dig+.
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A +dig+ method raises an exception if any receiver does not respond to \#dig:
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h = { foo: 1 }
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# Raises TypeError (Integer does not have #dig method):
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h.dig(:foo, :bar)
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== What Else?
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The structure above has \Hash objects and \Array objects,
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both of which have instance method +dig+.
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Altogether there are six built-in Ruby classes that have method +dig+,
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three in the core classes and three in the standard library.
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In the core:
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- Array#dig: the first argument is an \Integer index.
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- Hash#dig: the first argument is a key.
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- Struct#dig: the first argument is a key.
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In the standard library:
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- OpenStruct#dig: the first argument is a \String name.
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- CSV::Table#dig: the first argument is an \Integer index or a \String header.
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- CSV::Row#dig: the first argument is an \Integer index or a \String header.
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