Rails - Building custom JSON using JBuilder

Question

I'm currently working on a personal Rails 4 project, and in one of the controllers, I have an array of three different objects, which I'm trying to convert into JSON. I'm using the find_by_sql method to query the DB with a custom MySQL query. The code is as follows:

The controller:

class ActivitiesController < ApplicationController
  def index
    user = current_user.id # using devise
    limit = 100

    likes = Like.find_by_sql(...)
    shares = Share.find_by_sql(...)
    comments = Comment.find_by_sql(...)

    @activities = [likes, shares, comments]
  end
end

Each of the three objects contain different fields. For example, the likes object contains fields such as post_id , first_name , last_name etc. The shares object contains fields such as post_id , share_count , shared_time etc.

This is the JSON I want to output:

{
    "likes": [
      { "post_id": "7", "first_name": "Yss", "last_name": "Salas", "profile_pic": "345943754_o.png" },
      { "post_id": "34", "first_name": "Jessica", "last_name": "Nigri", "profile_pic": "pikachu.png" }
    ],
    "shares": [
      { "post_id": "43", "share_count": 54, "shared_time": "2014-05-04 15:14:45" },
      { "post_id": "54", "share_count": 17, "shared_time": "2014-05-24 03:43:45" }
    ],
    "comments": [
      { "post_id": "34", "first_name": "Yss", "last_name": "Salas", "comment": "¡Me gustas mucho!" },
      { "post_id": "12", "first_name": "Jenna", "last_name": "Marbles", "comment": "Blah blah blah look at my ugly dog awwwww!" }
    ]
}

The index.json.jbuilder:

json.array! @activities do |subactivities|
  json.array! subactivities do |activity|
    if activity.class == Like
      json.likes.post_id activity.post_id #exception raised in this line
      json.likes.title activity.title
      json.likes.first_name activity.first_name
      json.likes.last_name activity.last_name
      json.likes.profile_pic activity.profile_pic
    elsif activity.class == Share
      json.shares.post_id activity.post_id
      json.shares.share_count activity.share_count
      json.shares.shared_time activity.shared_time
    else
      json.comments.post_id activity.post_id
      json.comments.first_name activity.first_name
      json.comments.last_name activity.last_name
      json.comments.comment activity.comment
    end
  end
end

The above raises an exception NoMethodError in Activities#index, undefined method 'post_id' for #<Object:0x00000003064250> . I'm very new to Rails, and I'm guessing that this is happening because there is no json.likes property to assign the values to. How do I go about building the above JSON structure with JBuilder?

Answer 1

You can do it without JBuilder, easier and faster.

def index
  user = current_user.id # using devise
  limit = 100

  likes = Like.find_by_sql(...).map(&:attributes) # this will return an array of JSON objects
  shares = Share.find_by_sql(...).map(&:attributes)
  comments = Comment.find_by_sql(...).map(&:attributes)

  @activities = {"likes" => likes, "shares" => shares, "comments" => comments}
end

Like this you have a hash with all the information you need. It may also be better to extract the logic of looking for likes, shares and comments to a separate class, so that you would call InformationCollector.collect and it will give you the response you need.

Answer 2

From Jbuilder's GitHub page , the solution is rather simple. All that needs to be done is pass the instance variables into a block, and assign the custom key/value pairs manually.

The below example would generate an array of key/value pairs for the @likes instance variable:

# activities/index.json.jbuilder

json.likes @likes do |like|
  json.post_id like.post_id
  json.first_name like.user.first_name
  json.last_name like.user.last_name
  json.profile_pic like.user.profile_pic
end

The above will output:

{
  "likes": [
    { "post_id": 7, "first_name": "Yss", "last_name": "Salas", "profile_pic": "345943754_o.png" },
    { "post_id": 34, "first_name": "Jessica", "last_name": "Nigri", "profile_pic": "pikachu.png" }
  ],
}