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Methods
D
Instance Protected methods
define_method_attribute(name)

We want to generate the methods via module_eval rather than define_method, because define_method is slower on dispatch. Evaluating many similar methods may use more memory as the instruction sequences are duplicated and cached (in MRI). define_method may be slower on dispatch, but if you're careful about the closure created, then define_method will consume much less memory.

But sometimes the database might return columns with characters that are not allowed in normal method names (like 'my_column(omg)'. So to work around this we first define with the __temp__ identifier, and then use alias method to rename it to what we want.

We are also defining a constant to hold the frozen string of the attribute name. Using a constant means that we do not have to allocate an object on each call to the attribute method. Making it frozen means that it doesn't get duped when used to key the @attributes in read_attribute.

# File activerecord/lib/active_record/attribute_methods/read.rb, line 27
          def define_method_attribute(name)
            safe_name = name.unpack("h*".freeze).first
            temp_method = "__temp__#{safe_name}"

            ActiveRecord::AttributeMethods::AttrNames.set_name_cache safe_name, name

            generated_attribute_methods.module_eval "            def #{temp_method}
              name = ::ActiveRecord::AttributeMethods::AttrNames::ATTR_#{safe_name}
              _read_attribute(name) { |n| missing_attribute(n, caller) }
            end
", __FILE__, __LINE__ + 1

            generated_attribute_methods.module_eval do
              alias_method name, temp_method
              undef_method temp_method
            end
          end