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 resul

Answer 1

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

Answer 2

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

Answer 3

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
}
]

Answer 4

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.

Answer 5

@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.