Check timestamp columns (created_at, updated_at) are NOT NULL

Once the timestamps columns are established on a table, they might as
well be NOT NULL to enforce their existence. All ActiveRecord operations
set these values so they will always exist under normal usage.

Fixes #184

lib/active_record_doctor/detectors/missing_non_null_constraint.rb

@@ -19,7 +19,11 @@ class MissingNonNullConstraint < Base # :nodoc:
       private
 
       def message(column:, table:)
-        "add `NOT NULL` to #{table}.#{column} - models validates its presence but it's not non-NULL in the database"
+        message = "add `NOT NULL` to #{table}.#{column}"
+        unless ["created_at", "updated_at"].include?(column)
+          message = "#{messages} - models validates its presence but it's not non-NULL in the database"
+        end
+        message
       end
 
       def detect
@@ -50,6 +54,8 @@ def sti_base_model?(model)
       end
 
       def non_null_needed?(model, column)
+        return true if column.name == "created_at" || column.name == "updated_at"
+
         belongs_to = model.reflect_on_all_associations(:belongs_to).find do |reflection|
           reflection.foreign_key == column.name ||
             (reflection.polymorphic? && reflection.foreign_type == column.name)

test/active_record_doctor/detectors/missing_non_null_constraint_test.rb

@@ -310,4 +310,15 @@ def test_config_ignore_columns
 
     refute_problems
   end
+
+  def test_timestamps_with_null_are_disallowed
+    Context.create_table(:users) do |t|
+      t.timestamps null: true
+    end.define_model
+
+    assert_problems(<<~OUTPUT)
+      add `NOT NULL` to users.created_at
+      add `NOT NULL` to users.updated_at
+    OUTPUT
+  end
 end