find_by_sql in Rails, accessing the resulting array

Question

I'm trying to run a query in a very quick and dirty way in Rails, without putting the rest of the model in place. I know this is bad practice but I just need a quick result in a tight timeframe until I've got the whole solution in place.

I've got items that have a shipping price, based on weight. The weight is stored in the item, the price is stored in the table shipping_zone_prices, and all I currently do is look for the price relating to the first row where the weight is heavier than the item for sale:

class Item < ActiveRecord::Base
  def shipping_price
    item_id = self.id
    shipping_price = ShippingZonePrice.find_by_sql(
      "SELECT z.price as price
       FROM shipping_zone_prices z, items i
       WHERE i.id = '#{item_id}'
       AND z.weight_g > d.weight
       ORDER BY z.weight_g asc limit 1")    
  end
end

This sort of works. The SQL does the job, but when plugged into the app as follows:

 <%= @item.shipping_price %> Shipping

I get the following displayed:

[#<ShippingZonePrice price: 12>] Shipping

In this example, '12' is the price that is being pulled from the db, and is correct. @item.shipping_price.class returns 'Array'. Trying to access the array using [0] (or any other integer) returns a blank.

Is there another way to access this, or am I missing something fundamental?

Well spotted, that should be i.weight - I was just trying to make the data a bit generic by using 'item' and I didn't make all the correct changes — Christian Mayne
– Christian Mayne, Commented Sep 19, 2012 at 15:09

Kyle · Accepted Answer · 2012-09-19 14:59:31Z

7

Since you are defining an instance method, I think it should return the price if it exists or nil

Try something like this:

def shipping_price
  ShippingZonePrice.find_by_sql(
    "SELECT z.price as price
     FROM shipping_zone_prices z, items i
     WHERE i.id = '#{self.id}'
     AND z.weight_g > d.weight
     ORDER BY z.weight_g asc limit 1").first.try(:price)
end

Then this should work for you:

@item.shipping_price

The first.try(:price) part is needed because find_by_sql may return an empty array. If you tried to do something like first.price on an empty array, you would get an exception along the lines of NoMethodError: undefined method 'price' for nil:NilClass.

edited Sep 19, 2012 at 14:59

answered Sep 19, 2012 at 14:46

Kyle

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Comments

tadman · Accepted Answer · 2012-09-19 15:03:39Z

This is because find_by_sql returns a model, not data. If you want to do a direct fetch of the data in question, use something like this:

ShippingZonePrice.connection.select_value(query)

There are a number of direct-access utility methods available through connection that can fetch single values, a singular array, rows of arrays, or rows of hashes. Look at the documentation for ActiveRecord::ConnectionAdapters::DatabaseStatements.

As when writing an SQL directly, you should be very careful to not create SQL injection bugs. This is why it is usually best to encapsulate this method somewhere safe. Example:

class ShippingZonePrice < ActiveRecord::Base
  def self.price_for_item(item)
    self.connection.select_value(
      self.sanitize_sql(
        %Q[
          SELECT z.price as price
            FROM shipping_zone_prices z, items i
            WHERE i.id=?
              AND z.weight_g > d.weight
            ORDER BY z.weight_g asc limit 1
        ],
        item.id
      )
    )
  end
end

B Seven · Accepted Answer · 2012-09-20 00:11:25Z

3

@item.shipping_price.first.price

or

@item.shipping_price[0].price

Thanks Atastor for pointing that out!

When you use AS price in find_by_sql, price becomes a property of the result.

edited Sep 20, 2012 at 0:11

answered Sep 19, 2012 at 14:40

B Seven

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Comments

Atastor · Accepted Answer · 2012-09-19 14:45:48Z

1

If not for you saying that you tried and failed accessing [0] i'ld say you want to put

@item.shipping_price.first.price # I guess BSeven just forgot the .first. in his solution

into the view...strange

answered Sep 19, 2012 at 14:45

Atastor

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Comments

Caullyn · Accepted Answer · 2015-08-26 16:12:57Z

So, I had a hacky solution for this, but it works great. Create a table that has the same output as your function and reference it, then just call a function that does a find_by_sql to populate the model.

Create a dummy table:

CREATE TABLE report.compliance_year (
 id BIGSERIAL,
 year TIMESTAMP,
 compliance NUMERIC(20,2),
 fund_id INT);

Then, create a model that uses the empty table:

class Visualization::ComplianceByYear < ActiveRecord::Base
    self.table_name = 'report.compliance_year'
    def compliance_by_year(fund_id)
        Visualization::ComplianceByYear.find_by_sql(["
            SELECT year, compliance, fund_id
              FROM report.usp_compliance_year(ARRAY[?])", fund_id])
    end
end

In your controller, you can populate it:

def visualizations
  @compliancebyyear = Visualization::ComplianceByYear.new()
  @compliancefunds = @compliancebyyear.compliance_by_year(current_group.id)
  binding.pry
end

Then, you can see it populate with what you need:

[1] pry(#<Thing::ThingCustomController>)> @compliancefunds
[
[0] #<Visualization::ComplianceByYear:0x00000008f78458> {
          :year => Mon, 31 Dec 2012 19:00:00 EST -05:00,
    :compliance => 0.93,
       :fund_id => 1
},
[1] #<Visualization::ComplianceByYear:0x0000000a616a70> {
          :year => Tue, 31 Dec 2013 19:00:00 EST -05:00,
    :compliance => 0.93,
       :fund_id => 4129
},
[2] #<Visualization::ComplianceByYear:0x0000000a6162c8> {
          :year => Wed, 31 Dec 2014 19:00:00 EST -05:00,
    :compliance => 0.93,
       :fund_id => 4129
}
]

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