Minor refactor for easier extensibility

In the previous version it was harder to extend as you could forget to
include your symbol in the query list. Also, the queries had some
duplication. For extensions this is the easier way forward and the
DRYest.

Also added the first tests for database monitoring.

lib/gitlab_monitor/database/row_count.rb

@@ -5,17 +5,21 @@ module GitLab
       # the row count
       # This class works under the assumption you do COUNT(*) queries
       class RowCountCollector < Base
-        QUERIES = [
-          :soft_deleted_projects,
-          :soft_deleted_namespaces,
-          :orphaned_projects
-        ].freeze
+        QUERIES = {
+          soft_deleted_projects: { select: :projects, where: "pending_delete=true" },
+          orphaned_projects: {
+            select: :projects,
+            joins: "LEFT JOIN namespaces ON projects.namespace_id = namespaces.id",
+            where: "namespaces.id IS NULL"
+          },
+          soft_deleted_namespaces: { select: :namespaces, where: "deleted_at IS NOT NULL" }
+        }.freeze
 
         def run
           results = Hash.new(0)
 
-          QUERIES.each do |query|
-            results[query] = execute(query)
+          QUERIES.each do |key, query|
+            results[key] = execute(query)
           end
 
           results
@@ -24,29 +28,15 @@ module GitLab
         private
 
         def execute(query)
-          connection.exec(send(query))[0]["count"]
+          connection.exec(construct_query(query))[0]["count"]
         end
 
-        def soft_deleted_projects
-          <<-SQL
-            SELECT COUNT(*) FROM projects
-            WHERE pending_delete=true;
-          SQL
-        end
-
-        def soft_deleted_namespaces
-          <<-SQL
-            SELECT COUNT(*) FROM namespaces
-            WHERE deleted_at IS NOT NULL;
-          SQL
-        end
-
-        def orphaned_projects
-          <<-SQL
-            SELECT COUNT(*) FROM projects
-            LEFT JOIN namespaces ON projects.namespace_id = namespaces.id
-            WHERE namespaces.id IS NULL;
-          SQL
+        # Not private so I can test it without meta programming tricks
+        def construct_query(query)
+          query_string = "SELECT COUNT(*) FROM #{query[:select]} "
+          query_string << "#{query[:joins]} "       if query[:joins]
+          query_string << "WHERE #{query[:where]}"  if query[:where]
+          query_string << ";"
         end
       end
 

spec/database/row_count_spec.rb

+require "spec_helper"
+require "gitlab_monitor/database/row_count"
+
+describe GitLab::Monitor::Database::RowCountCollector do
+  describe "#run" do
+    let(:query)     { { project_1: { select: :projects, where: "id=1" } } }
+    let(:collector) { described_class.new(connection_string: "host=localhost") }
+
+    before do
+      stub_const("GitLab::Monitor::Database::RowCountCollector::QUERIES", query)
+    end
+
+    it "executes the query" do
+      allow(collector).to receive(:execute).with(query[:project_1]).and_return(3)
+
+      expect(collector.run).to eq(project_1: 3)
+    end
+
+    describe "#construct_query" do
+      it "accepts a table and where clause" do
+        expect(collector.send(:construct_query, query[:project_1])).to eq "SELECT COUNT(*) FROM projects WHERE id=1;"
+      end
+    end
+  end
+end