Rails ActiveRecord Query: Sum total article view count per category

I have articles that are able to be tagged in categories. I am struggling to create a query which will extract the sum of view count (tracked using impressionist gem) of articles within each category.

Schema:

  create_table "article_categories", force: :cascade do |t|
    t.integer "article_id"
    t.integer "category_id"
  end

  create_table "articles", force: :cascade do |t|
    t.string "title"
    t.text "description"
    t.bigint "user_id"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.integer "impressions_count", default: 0
  end

  create_table "categories", force: :cascade do |t|
    t.string "name"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
  end

Article model

class Article < ApplicationRecord
  is_impressionable :counter_cache => true, :unique => false
  belongs_to :user
  has_many :impressions, as: :impressionable
  has_many :article_categories
  has_many :categories, through: :article_categories
end

Category model

class Category < ApplicationRecord
  has_many :article_categories
  has_many :articles, through: :article_categories
end

ArticleCategory model

class ArticleCategory < ActiveRecord::Base
  belongs_to :article
  belongs_to :category
end

Here is what i have tried so far and the errors when i test in rails console:

cat_group = Article.joins(:article_categories).group_by(&:category_ids)
// this query results in an array of articles within each category

1) cat_group.sum("impressions_count")
//TypeError: no implicit conversion of Array into String

2) cat_group.select("sum(impressions_count) AS total_count")
//ArgumentError: wrong number of arguments (given 1, expected 0)

3) Article.joins(:article_categories).group(:category_id)

//ActiveRecord::StatementInvalid: PG::GroupingError: ERROR:  column "articles.id" must appear in the GROUP BY clause or be used in an aggregate function

If I understand you correctly and view count is impressions_count, here is query that you can use:

Article.joins(:categories).group('categories.name').sum(:impressions_count)