postgres_enum.rb

module PostgresEnum
  class MissingEnumError < ActiveRecord::ActiveRecordError
  end

  class MethodDefinedError < ActiveRecord::ActiveRecordError
  end

  extend ActiveSupport::Concern

  class_methods do
    def postgres_enum field,
                      name:    field.to_s.pluralize,
                      method:  field.to_s.pluralize
                      # TODO: implement sometime when this should be full replacement for rails enum
                      # mark_methods: true, scopes: true, _suffix: false, _prefix: false

      @@postgres_enum ||= {}

      @@postgres_enum[field.to_sym] = {
        name:   name,
        values: postgres_enum_query_values(name)
      }

      # TODO: validate method existence.
      # For some unknow reason this does not work
      if self.respond_to?(method)
        raise MethodDefinedError, "enum list method '#{method}' already exists."
      end

      # Singleton method for list of values from given enum
      define_singleton_method method do
        @@postgres_enum[field.to_sym][:values]
      end

      # Setter for enum with value check
      define_method "#{field}=" do |value|
        values = self.class.send(method)

        unless values.index(value)
          raise ArgumentError, "'#{value}' is not a valid #{field} (available values: #{values.join(", ")})"
        end

        super(value)
      end
    rescue ActiveRecord::StatementInvalid => ex
      if ex.message.index("PG::UndefinedObject")
        raise MissingEnumError, "Enum '#{name}' does not exist (create with: \"CREATE TYPE #{name} AS ENUM ('value1', ..., 'valueN')\")"
      else
        raise ex
      end
    end

    private

    def postgres_enum_query_values(name)
      conn = self.connection

      quoted_enum_name = conn.quote_table_name(name)

      enum_values_sql = "SELECT unnest(enum_range(NULL::#{quoted_enum_name}))::text AS enum_value"

      result = conn.exec_query(enum_values_sql)

      result.rows.map(&:first)
    end
  end
end