perf: improve query performance for Metric.latest (#18)

Currently it pulls all records from the database but only uses the most recent amount value.

This is inefficient, particularly if the database is large. Now it only retrieves the amount from
the most recent record and ignores all the old ones.

Gemfile

@@ -1,7 +1,9 @@
 source 'https://rubygems.org'
 gemspec
+
+# Other dependencies are defined in the Appraisals file
 group :development do
   gem 'appraisal', '~> 2.4'
   gem 'rspec'
-  gem 'sqlite3'
+  gem 'sqlite3', '~> 1.4'
 end

README.md

@@ -227,3 +227,10 @@ compared_point.sum.percentage_change  # => The % change between 2019-01 and 2018
 # In addition to `sum`, there is also `count` and `last` as you expect from any
 # other point.
 ```
+
+## Run the tests
+
+To run the tests you can use the following commands from the [appraisal gem](https://github.com/thoughtbot/appraisal):
+
+- `bundle exec appraisal install`
+- `bundle exec appraisal rspec`

lib/metricks/models/metric.rb

@@ -67,7 +67,7 @@ def record(type, **options)
         # @return [Float]
         def latest(type, **options)
           scope = self.last(type, **options)
-          value = scope.pluck(:amount)&.first || 0.0
+          value = scope.select(:amount).first&.amount || 0.0
           type.transform_amount(value, options[:associations])
         end