Let’s say you have a model in Rails with certain attributes and columns. When you serialize it with to_json, by default Rails will include all columns. How can you add one more or remove some from now appearing there?

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to_json ? Use as_json instead

In the simplest way you can define the as_json method on your model. You can call super to get the standard behavior from ActiveRecord::Base and then add or remove more attributes on that result.

Imagine that you have Event class. And initially the JSON looks like.

{
  "id": 1,
  "title": "Ergonomic Granite Table course",
  "description": "You can’t just skip ahead to where you think...",
  "starts_at": "2017-10-10T00:00:00.000Z",
  "ends_at": "2017-10-13T00:00:00.000Z",
  "category": "Holiday",
  "state": "published",
  "image_url": "http://lorempixel.com/600/338/animals/1",
  "created_at": "2017-10-03T09:30:25.481Z",
  "updated_at": "2017-10-04T08:36:43.049Z",
}

You might want to remove created_at, updated_at and add one new field such as is_single_day_event.

You can do it that way:

class Event < ApplicationRecord
  def single_day_event?
    starts_at.to_date == ends_at.to_date
  end

  def as_json(*)
    super.except("created_at", "updated_at").tap do |hash|
      hash["is_single_day_event"] = single_day_event?
    end
  end
end

And now when you call event.to_json you are going to get

{
  "id": 1,
  "title": "Ergonomic Granite Table course",
  "description": "You can’t just skip ahead to where you think...",
  "starts_at": "2017-10-10T00:00:00.000Z",
  "ends_at": "2017-10-13T00:00:00.000Z",
  "category": "Holiday",
  "state": "published",
  "image_url": "http://lorempixel.com/600/338/animals/1",
  "is_single_day_event": true,
}

If the logic around serializing your objects gets more complex over time or you need multiple representations of the same model, I suggest you decouple JSON serialization from ActiveRecord. You can do it by using of the gems such as ActiveModel::Serializers or similar.

Potential gems for you to investigate

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