Use exec_query for custom SQL execution

Also align how we use query Postgres from controllers.

The change is mainly due to regular (but not reproducible) issues with the `intervalstyle` defaulting to `postgres` (instead of `iso8601`) and thereby causing issues. We're just experimenting to see whether this change resolves the issue.

app/controllers/community_solutions_controller.rb

@@ -60,7 +60,7 @@ class CommunitySolutionsController < ApplicationController
 
     # Acquire lock here! This is expensive but required for synchronization
     @community_solution_lock = ActiveRecord::Base.transaction do
-      ActiveRecord::Base.connection.execute("LOCK #{CommunitySolutionLock.table_name} IN ACCESS EXCLUSIVE MODE")
+      ApplicationRecord.connection.exec_query("LOCK #{CommunitySolutionLock.table_name} IN ACCESS EXCLUSIVE MODE")
 
       lock = CommunitySolutionLock.where(user: current_user, community_solution: @community_solution).order(locked_until: :asc).last
 

app/controllers/concerns/redirect_behavior.rb

@@ -90,7 +90,7 @@ module RedirectBehavior
 
     # Acquire lock here! This is expensive but required for synchronization
     @community_solution_lock = ActiveRecord::Base.transaction do
-      ActiveRecord::Base.connection.execute("LOCK #{CommunitySolutionLock.table_name} IN ACCESS EXCLUSIVE MODE")
+      ApplicationRecord.connection.exec_query("LOCK #{CommunitySolutionLock.table_name} IN ACCESS EXCLUSIVE MODE")
 
       # This is returned
       CommunitySolutionLock.find_or_create_by(community_solution: @community_solution, locked_until: Time.zone.now...) do |lock|

app/controllers/execution_environments_controller.rb

@@ -107,7 +107,7 @@ class ExecutionEnvironmentsController < ApplicationController
     working_time_statistics = {}
     user_statistics = {}
 
-    ApplicationRecord.connection.execute(working_time_query).each do |tuple|
+    ApplicationRecord.connection.exec_query(working_time_query).each do |tuple|
       tuple = tuple.merge({
         'average_time' => format_time_difference(tuple['average_time']),
         'stddev_time' => format_time_difference(tuple['stddev_time']),
@@ -115,7 +115,7 @@ class ExecutionEnvironmentsController < ApplicationController
       working_time_statistics[tuple['exercise_id'].to_i] = tuple
     end
 
-    ApplicationRecord.connection.execute(user_query).each do |tuple|
+    ApplicationRecord.connection.exec_query(user_query).each do |tuple|
       user_statistics[tuple['exercise_id'].to_i] = tuple
     end
 

app/controllers/external_users_controller.rb

@@ -69,7 +69,7 @@ class ExternalUsersController < ApplicationController
 
     statistics = {}
 
-    ApplicationRecord.connection.execute(working_time_query(tag&.id)).each do |tuple|
+    ApplicationRecord.connection.exec_query(working_time_query(tag&.id)).each do |tuple|
       tuple = tuple.merge('working_time' => format_time_difference(tuple['working_time']))
       statistics[tuple['exercise_id'].to_i] = tuple
     end

app/controllers/ping_controller.rb

@@ -16,7 +16,7 @@ class PingController < ApplicationController
 
   def postgres_connected!
     # any unhandled exception leads to a HTTP 500 response.
-    return if ApplicationRecord.connection.execute('SELECT 1 as result').first['result'] == 1
+    return if ApplicationRecord.connection.exec_query('SELECT 1 as result').first['result'] == 1
 
     raise ActiveRecord::ConnectionNotEstablished
   end