Possible Ruby Enum Fix (#6131)

modules/swagger-codegen/src/main/resources/ruby/partial_model_enum_class.mustache

@@ -1,4 +1,13 @@
   class {{classname}}
     {{#allowableValues}}{{#enumVars}}
     {{{name}}} = {{{value}}}.freeze{{/enumVars}}{{/allowableValues}}
+
+    # Builds the enum from string
+    # @param [String] The enum value in the form of the string
+    # @return [String] The enum value
+    def build_from_hash(value)
+      consantValues = {{classname}}.constants.select{|c| c.to_s == value}
+      raise "Invalid ENUM value #{value} for class #{{{classname}}}" if consantValues.empty?
+      value
+    end
   end