sorting_by_category.rb

# you have your list of the user's posts,
# these are just in whatever order your DB gets them, which is probably (but not guaranteed) to be ordered by the post id:
@posts = user.posts
@posts.map { |p| [p.id, p.category_id] } # [[1,3], [2,5], [3,3], [4,1]]

# you can specify that you'd like the order explicitly like this,
# here we ask for them ASCending, you have to pick one, but it probably doesn't matter in your case:
@posts = user.posts.order('category_id ASC')
@posts.map { |p| [p.id, p.category_id] } # [[4,1], [3,3], [1,3], [2,5]]

# what I think you want is to actually group_by, note that this is a Ruby method: http://api.rubyonrails.org/classes/Enumerable.html#method-i-group_by
@posts = user.posts.order('category_id ASC').group_by(&:category)
@posts.map { |cat, ps| [cat.id, ps.map(&:id)] } # [[1, [4]], [3, [3,1]], [5, [2]]]