Make sure MySQL would not use CURRENT_TIMESTAMP

for timestamp columns magically. See:

https://gitlab.com/gitlab-org/gitlab-ce/issues/36405

config/initializers/active_record_mysql_timestamp.rb

+# Make sure that MySQL won't try to use CURRENT_TIMESTAMP when the timestamp
+# column is NOT NULL. See https://gitlab.com/gitlab-org/gitlab-ce/issues/36405
+# And also: https://bugs.mysql.com/bug.php?id=75098
+# This patch was based on:
+# https://github.com/rails/rails/blob/15ef55efb591e5379486ccf53dd3e13f416564f6/activerecord/lib/active_record/connection_adapters/mysql/schema_creation.rb#L34-L36
+
+if Gitlab::Database.mysql?
+  require 'active_record/connection_adapters/abstract/schema_creation'
+
+  module MySQLTimestampFix
+    def add_column_options!(sql, options)
+      # By default, TIMESTAMP columns are NOT NULL, cannot contain NULL values,
+      # and assigning NULL assigns the current timestamp. To permit a TIMESTAMP
+      # column to contain NULL, explicitly declare it with the NULL attribute.
+      # See http://dev.mysql.com/doc/refman/5.7/en/timestamp-initialization.html
+      if sql.end_with?('timestamp') && !options[:primary_key]
+        if options[:null] != false
+          sql << ' NULL'
+        elsif options[:column].default.nil?
+          sql << ' DEFAULT 0'
+        end
+      end
+
+      super
+    end
+  end
+
+  ActiveRecord::ConnectionAdapters::AbstractAdapter::SchemaCreation
+    .prepend(MySQLTimestampFix)
+end