feat(felixible aggregation): Add documentation for expression evaluat…

…ion endpoint

lib/lago/api/resources/billable_metric.rb

@@ -12,6 +12,15 @@ def root_name
           'billable_metric'
         end
 
+        def evaluate_expression(params)
+          uri = URI("#{client.base_api_url}#{api_resource}/evaluate_expression")
+
+          payload = whitelist_evalute_expression_params(params)
+          response = connection.post(payload, uri)['expression_result']
+
+          JSON.parse(response.to_json, object_class: OpenStruct)
+        end
+
         def whitelist_params(params)
           {
             root_name => {
@@ -27,6 +36,19 @@ def whitelist_params(params)
             }.compact,
           }
         end
+
+        def whitelist_evalute_expression_params(params)
+          event = params[:event] || {}
+
+          {
+            expression: params[:expression],
+            event: {
+              code: event[:code],
+              timestamp: event[:timestamp],
+              properties: event[:properties],
+            }.compact,
+          }
+        end
       end
     end
   end

spec/fixtures/api/billable_metric_evaluate_expression.json

@@ -0,0 +1,5 @@
+{
+  "expression_result": {
+    "value": "2.0"
+  }
+}

spec/lago/api/resources/billable_metric_spec.rb

@@ -187,4 +187,36 @@
       end
     end
   end
+
+  describe '#evaluate_expression' do
+    let(:expression) { 'round(event.properties.value * event.properties.units)' }
+    let(:event) { { properties: { value: 10, units: 2 } } }
+
+    let(:evaluate_expression_response) { load_fixture('billable_metric_evaluate_expression') }
+
+    context 'when expression is successfully evaluated' do
+      before do
+        stub_request(:post, 'https://api.getlago.com/api/v1/billable_metrics/evaluate_expression')
+          .with(body: { expression: expression, event: event })
+          .to_return(body: evaluate_expression_response, status: 200)
+      end
+
+      it 'returns the evaluation result' do
+        evaluation_result = resource.evaluate_expression(event: event, expression: expression)
+
+        expect(evaluation_result.value).to eq('2.0')
+      end
+    end
+
+    context 'when there is an issue' do
+      before do
+        stub_request(:post, 'https://api.getlago.com/api/v1/billable_metrics/evaluate_expression')
+          .to_return(body: error_response, status: 422)
+      end
+
+      it 'raises an error' do
+        expect { resource.evaluate_expression(event: event, expression: expression) }.to raise_error Lago::Api::HttpError
+      end
+    end
+  end
 end