Ruby Hash to Object with OpenStruct and JSON

2011-02-11 ·

In Ruby you can convert a hash into an object using OpenStruct from the Ruby Standard Library. There is a substantial overhead when creating OpenStruct. While it may seem more convenient, it will decrease your application performance if there are many such objects being created.

require 'ostruct'

h = { foo: 1, bar: 2 }
obj = OpenStruct.new(h)

obj.foo   # => 1

To convert an OpenStruct object back to a hash use .marshal_dump method. Since Ruby 2.0 there's also a .to_h method.

obj.marshal_dump   # => {:foo=>1, :bar=>2}
obj.to_h           # => {:foo=>1, :bar=>2}

For given hash, OpenStruct constructor only converts its top level keys.

require 'ostruct'

h = { foo: { bar: 1 } }
obj = OpenStruct.new(h)
obj.foo       # => { bar: 1}
obj.foo.bar   # => NoMethodError: undefined method `bar' for {:bar=>1}:Hash

We can combine OpenStruct with JSON to convert nested keys as well.

require 'ostruct'
require 'json'

h = { foo: { bar: 1 } }

obj = JSON.parse(h.to_json, object_class: OpenStruct)
obj.foo       # => #<OpenStruct bar=1>
obj.foo.bar   # => 1

You can now monkey-patch Hash object as follows:

class Hash
  def to_o
    JSON.parse to_json, object_class: OpenStruct
  end
end

h = { foo: { bar: 1 } }
h.to_o.foo.bar   # => 1
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