Question

class Box
  embeds_many :things
  after_init :add_default_things

  def add_default_things
    self.things.build(title: "Socks", value: 2)
    self.things.build(title: "Ring", value: 1)
  end
end

class Thing
  field :title
  field :value, type: Integer
end

all boxes have got some default things: Socks and a Ring. Everybody can add this or another things into a box. So now I need to order all boxes by count of socks:

Box.order_by(:thing_with_title_socks.value.desc) # ???

Answer 1

I am not good at ruby so I am going to try to explain it in java terms. The problem is how you design your data. For example you have n Box classes, each having a list of items in it:

public class Box {
  public List<Item> items;
}

public class Item {
  public String name;
  public int value;
}

to sort all boxes in this classes for a specific item value you need to loop through all items in boxes which is not good at all. so I would change my code to do it more efficiently:

public class Box {
  public Map<Integer, Item> items;
}

public class Item {
  public int id;
  public String name;
  public int value;
}

this way I would check if the item exists in box or not; access the items value with O(1) complexity, and I wouldn’t mistakenly put 2 different packets of socks in my box which would cause an error (which one would it sort for?).

your current data structure is like this :

{
  things : [ {"name" : "socks", "value" : 2} , {"name" : "ring", "value" : 5} ]
}

but if you are going to do sorting / direct access (using in query) on “known” object (like ring or socks) then your data should look like:

{
  things : { "socks": 2, "ring" : 5 }
}

if you have extra information attached to this items they can also look like :

{
  things : { "S01" : { "name" : "super socks", "value" : 1 }, "S02" : { "name" : "different socks", "value" : 2} }
}

this way you have direct access to the items in box.

hope this would help.

Edit : I thought it was obvious but just to make thing clear: you can’t efficiently query “listed” child data unless you know the exact position. (You can always use map/reduce though)