read_metafield_map.rb

# For a given Metafield, this is how 
# you access the serialized JSON data
mf = {"id"=>10030144453,
 "namespace"=>"product",
 "key"=>"custom_attrs",
 "value"=>"{\"width\":11.2,\"height\":35.7}",
 "value_type"=>"string",
 "description"=>nil,
 "owner_id"=>1234567890,
 "created_at"=>"2015-11-20T16:48:03-05:00",
 "updated_at"=>"2015-11-20T16:48:03-05:00",
 "owner_resource"=>"product"}
attrs = JSON.parse(mf["value"])
=> {"width"=>11.2, "height"=>35.7}