arel_table.rb

require "bundler/inline"

gemfile(true) do
  gem "activerecord", "~> 5.2.0"
  gem "sqlite3"
end

require "active_record"

ActiveRecord::Base.establish_connection adapter: "sqlite3", database: ":memory:"

ActiveRecord::Schema.define do
  create_table :users, force: true do |t|
    t.string :name
  end

  create_table :posts, force: true do |t|
    t.belongs_to :user
  end

  create_table :comments, force: true do |t|
    t.belongs_to :post
    t.belongs_to :user
  end

  create_table :reactions, force: true do |t|
    t.belongs_to :post
    t.belongs_to :user
  end
end

class User < ActiveRecord::Base
  has_many :posts
  has_many :comments
  has_many :reactions
end

class Post < ActiveRecord::Base
  belongs_to :user
  has_many :comments
  has_many :reactions
end

class Comment < ActiveRecord::Base
  belongs_to :post
  belongs_to :user
end

class Reaction < ActiveRecord::Base
  belongs_to :post
  belongs_to :user
end

# Goal: query all Posts where a commenter was named FOO or a reacter was named BAR
arel_sql =
  Post.joins(comments: :user, reactions: :user)
  .where(Post._reflections["comments"].klass._reflections["user"].klass.arel_table[:name].eq("FOO"))
  .where(Post._reflections["reactions"].klass._reflections["user"].klass.arel_table[:name].eq("BAR"))
  .to_sql

puts arel_sql