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

@@ -18,8 +18,18 @@ class MissingNonNullConstraint < Base # :nodoc:
 
       private
 
+      TIMESTAMPS = ["created_at", "created_on", "updated_at", "updated_on"].freeze
+
       def message(column:, table:)
-        "add `NOT NULL` to #{table}.#{column} - models validates its presence but it's not non-NULL in the database"
+        if TIMESTAMPS.include?(column)
+          <<~WARN.squish
+            add `NOT NULL` to #{table}.#{column} - timestamp columns are set
+            automatically by Active Record and allowing NULL may lead to
+            inconsistencies introduced by bulk operations
+          WARN
+        else
+          "add `NOT NULL` to #{table}.#{column} - models validates its presence but it's not non-NULL in the database"
+        end
       end
 
       def detect
@@ -50,6 +60,8 @@ def sti_base_model?(model)
       end
 
       def non_null_needed?(model, column)
+        return true if TIMESTAMPS.include?(column.name)
+
         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 +310,27 @@ 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 - timestamp columns are set automatically by Active Record and allowing NULL may lead to inconsistencies introduced by bulk operations
+      add `NOT NULL` to users.updated_at - timestamp columns are set automatically by Active Record and allowing NULL may lead to inconsistencies introduced by bulk operations
+    OUTPUT
+  end
+
+  def test_created_on_updated_on_with_null_are_disallowed
+    Context.create_table(:users) do |t|
+      t.datetime :created_on
+      t.datetime :updated_on
+    end.define_model
+
+    assert_problems(<<~OUTPUT)
+      add `NOT NULL` to users.created_on - timestamp columns are set automatically by Active Record and allowing NULL may lead to inconsistencies introduced by bulk operations
+      add `NOT NULL` to users.updated_on - timestamp columns are set automatically by Active Record and allowing NULL may lead to inconsistencies introduced by bulk operations
+    OUTPUT
+  end
 end