Rails group and sum array of objects

Question

A production has_many :production_lines, production_line belongs_to :item, item has_one :recipe, recipe has_many :recipe_lines, recipe_line belongs_to :item,

production_line and recipe line have attribute quantity. I need to group recipe_lines for a production by item, with quantity that equals to production_line.quantity * recipe_line.quantity

  def item_quantities
        array = production_lines.map do |p|
          p.item.recipe.recipe_lines.map do |r|
            {
              item_id: r.item_id,
              item_name: r.item.name,
              quantity: r.quantity * p.quantity
            }
          end
        end
        array.flatten(1).group_by { |p| p[:item_id] }
             .transform_values { |vals| vals.sum { |val| val[:quantity] } }
          end

This returns:

item_quantities = {
1: 10,
2: 5
}

where key is item_id and value is quantity. Values are correct.

However I would like to return:

item_quantities = [
{
id: 1,
name: "Tomato",
quantity: 10,
},
{
id: 2, 
name: "Carrot",
quantity: 5
}
]

How should I change my solution to achieve that?

max pleaner · Accepted Answer · 2020-05-18 08:19:05Z

4

First of all, your nested map followed by flatten(1) can be simplified by making the first map into flat_map. If you do this you could remove the flatten(1).

From this point your code is most of the way there, but you could make the following changes to get the desired output:

you can group by multiple attributes, name and id. In another language you might use a tuple for this. Ruby doesn't have tuples, so we can just use a len-2 array:
```
.group_by { |p| [p[:item_id], p[:item_name]] }
.transform_values { |vals| vals.sum { |val| val[:quantity] } }
```
At this point you have a hash mapping [id,name] tuple to quantity:
```
{ [1,"foo"] => 123, [2, "bar"] => 456 }
```
and you can coerce this to the desired data type using reduce (or each_with_object, if you prefer):
```
.reduce([]) do |memo, ((id, name), quantity)|
  memo << {
    id: id,
    name: name,
    quantity: quantity
  }
end
```

The wierd looking ((id, name), quantity) is a kind of destructuring. See https://jsarbada.wordpress.com/2019/02/05/destructuring-with-ruby/ specifically the sections on "Destructuring Block Arguments" and "Destructuring Hashes".

edited May 18, 2020 at 8:19

answered May 18, 2020 at 6:38

max pleaner

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5 Comments

catmal Over a year ago

I think there is an error in the point 1 where you do he group_by. A } missing I guess.

max pleaner Over a year ago

My bad, I had an extra { actually

catmal Over a year ago

Ok so in the group_by, changing p[:item_id] to [p.item_id] returns undefined method `item_id' for #<Hash....

catmal Over a year ago

Before grouping, I have an array of objects. Each object looks like { "item_id": "aadc79b9-ff50-4012-b2cb-0deee88138a9", "item_name": "Pumpkins", "quantity": "600.0" }

max pleaner Over a year ago

Yeah my bad @Catmal it was another error on my part, updated answer it should be [p[:item_id], p[:item_name]]

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