Related posts, before and after

related1.rb

def similar_posts(post)
  related_posts(post).tap do |posts|
    (posts << @posts.first(2)).flatten!
  end.first(2)
end

def related_posts(post)
  categories_ids = post.categories.map(&:id)

  @posts.select do |p|
    p != post && (p.categories.map(&:id) & categories_ids).present?
  end
end

related2.rb

# Limits the array to 2 elements and adds recent posts
# if there are not enough related posts
#
# @return [Array<Post>] `limit` items, somehow related to `post`
#
# @param [Post] post the object we need related posts for
# @param [Array<Post>] posts the collection to look for related posts
# @param [FixNum] limit The number of items to return
#
# @example
#   post = @posts.first
#   similar_posts post, @posts
def similar_posts(post, posts, limit=2)
  related_posts = related_posts(post, posts)
  completion    = (posts - [post]).first(limit)

  (related_posts | completion).first(limit)
end

# Returns an array of posts from the same category
#
# @return [Array<Post>] items somehow related to `post`
#
# @param [Post] post the object we need related posts for
# @param [Array<Post>] posts the collection to look for related posts
#
# @example
#   post = @posts.first
#   related_posts post, @posts
def related_posts(post, posts)
  category_ids = Array(post.categories).map(&:id)
  other_posts  = posts - [post]

  other_posts.select do |p|
    next if (other_categories = p.categories).nil?

    (other_categories.map(&:id) & category_ids).present?
  end
end