has_manyの第2引数にscopeを渡すよい

class User < ApplicationRecord
  has_many :published_articles, -> { where(state: 'published') }, class_name: Article
end

User.includes(:published_articles).published_articles

例

リファクタリング前

class User < ApplicationRecord
  has_many :articles
end

class Article < ApplicationRecord
  belongs_to :user
end

class UsersController < ApplicationController
  def index
    @users = User.includes(:articles)
  end
end

- @users.each do |user|
  - articles = user.articles.select { |article| article.state == 'published' }
  - articles.each do |article|
    p = article.title

リファクタリング後

class User < ApplicationRecord
  has_many :articles
  has_many :published_articles, -> { published }, class_name: Article
end

class Article < ApplicationRecord
  belongs_to :user
  scope :published, -> { where(state: 'published') }
end

class UsersController < ApplicationController
  def index
    @users = User.includes(:published_articles)
  end
end

- @users.each do |user|
  - user.published_articles.each do |article|
    p = article.title