Possibility of mapping enum values to string type

问题:

Enum attributes are great and I want to use them. But mapping enum values to integer would make it hard to maintain both code and database. Also my database would be highly coupled with my code which I think I should consider that a bad thing.

I know I can use a hash to organize an enum attribute with key/value pairs, but still it would be a lot better to be able to use an array and map to string values in database.

Is there any way to map enum to strings by default?

回答1:

As far as I know it's not possible with Active Record's built-in enum functionality. However, there are a few popular 3rd party gems that do this. The closest match to what comes with Active Record are probably enumerize and SimpleEnum.

However, if you're looking for something a little different, I'd recommend ClassyEnum (full disclosure: I wrote it). Here are some of my notes on the difference between enumerize and SimpleEnum vs ClassyEnum:

Class-less enums (enumerize, SimpleEnum) are great for simple use cases where you just need a field to represent one of a fixed set of values. The main issue I have with this solution is that it encourages conditionals scattered throughout your models, controllers and views. It's tempting to use these gems because they are the simplest to implement, but the long term payoff just isn't there IMO for anything but the simplest cases.

ClassyEnum was designed to solve the problem of having scattered conditional logic related to the different enums. You can still use it for simple collections that don't have logic, but when you do need to add logic (and you almost certainly will), you can push that into the individual enum classes and take advantage of polymorphism.

回答2:

Looking at the code for enum, you can do this (at least in 4.1+): https://github.com/rails/rails/blob/master/activerecord/lib/active_record/enum.rb#L96-98 by passing a hash, for example:

class Foo
  enum name: {
    foo: 'myfoo',
    bar: 'mybar'
  }

Altough with unexpected results when accessing it, see https://github.com/rails/rails/issues/16459

foo_instance.foo!
foo_instance.name
=> "foo"
foo_instance[:name]
=> "myfoo"

Update

This issue was fixed in Rails 5, see https://github.com/rails/rails/commit/c51f9b61ce1e167f5f58f07441adcfa117694301. Thanks Yuri.

回答3:

The short answer is no. You will need to use a gem (such as magic-enum) if you want to do anything but store integers.

In DHH's own words on this from the comments on the pull request that added this feature:

It's pretty inefficient to store enums as text. You're going to repeat the same text over and over and over again. I'd consider that an anti pattern. People are better off doing a migration to ints if they want to use this.

回答4:

It seems that with Rails 5 API only, an enum attribute of a model will be save in database as integer, but will be published in public API as a string (with ActiveModel::Serializer).

For example,

Article model:

class Article < ApplicationRecord
  enum status: [ :visible, :hidden ]
end

Article serializer:

class ArticleSerializer < ActiveModel::Serializer
  attributes :id, :status, :title, :body
end

Will publish the following json:

{
  "id": "1",
  "type": "articles",
  "attributes": {
    "status": "visible",
    "title": "Enums are passed as string in a json API render",
    "body": "Great!",
}

回答5:

How about:

class Foo < ApplicationRecord
  NAMES = [
    :foo,
    :bar
  ]

  enum names: NAMES.zip(NAMES).to_h
end