Add new checker for tables without default timestamp columns

README.md

@@ -16,6 +16,7 @@ can detect:
 * primary keys having short integer types - [`active_record_doctor:short_primary_key_type`](#detecting-primary-keys-having-short-integer-types)
 * mismatched foreign key types - [`active_record_doctor:mismatched_foreign_key_type`](#detecting-mismatched-foreign-key-types)
 * tables without primary keys - [`active_record_doctor:table_without_primary_key`](#detecting-tables-without-primary-keys)
+* tables without timestamps - [`active_record_doctor:table_without_timestamps`](#detecting-tables-without-timestamps)
 
 It can also:
 
@@ -642,6 +643,29 @@ Supported configuration options:
 - `enabled` - set to `false` to disable the detector altogether
 - `ignore_tables` - tables whose primary key existence should not be checked
 
+### Detecting Tables Without Timestamps
+
+Tables should have timestamp columns (`created_at`/`updated_at`). Otherwise, it becomes problematic
+to easily find when the record was created/updated, if the table is active or can be removed,
+automatic Rails cache expiration after record updates is not possible.
+
+Running the command below will list all tables without default timestamp columns:
+
+```
+bundle exec rake active_record_doctor:table_without_timestamps
+```
+
+The output of the command looks like this:
+
+```
+add a created_at column to companies
+```
+
+Supported configuration options:
+
+- `enabled` - set to `false` to disable the detector altogether
+- `ignore_tables` - tables whose timestamp columns existence should not be checked
+
 ## Ruby and Rails Compatibility Policy
 
 The goal of the policy is to ensure proper functioning in reasonable

lib/active_record_doctor.rb

@@ -21,6 +21,7 @@
 require "active_record_doctor/detectors/short_primary_key_type"
 require "active_record_doctor/detectors/mismatched_foreign_key_type"
 require "active_record_doctor/detectors/table_without_primary_key"
+require "active_record_doctor/detectors/table_without_timestamps"
 require "active_record_doctor/errors"
 require "active_record_doctor/help"
 require "active_record_doctor/runner"

lib/active_record_doctor/config/default.rb

@@ -64,6 +64,10 @@
     enabled: true,
     ignore_tables: []
 
+  detector :table_without_timestamps,
+    enabled: true,
+    ignore_tables: []
+
   detector :undefined_table_references,
     enabled: true,
     ignore_models: []

lib/active_record_doctor/detectors/table_without_timestamps.rb

@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+require "active_record_doctor/detectors/base"
+
+module ActiveRecordDoctor
+  module Detectors
+    class TableWithoutTimestamps < Base # :nodoc:
+      @description = "detect tables without created_at/updated_at columns"
+      @config = {
+        ignore_tables: {
+          description: "tables whose timestamp columns existence should not be checked",
+          global: true
+        }
+      }
+
+      private
+
+      TIMESTAMPS = {
+        "created_at" => "created_on",
+        "updated_at" => "updated_on"
+      }.freeze
+
+      def message(table:, column:)
+        "add a #{column} column to #{table}"
+      end
+
+      def detect
+        each_table(except: config(:ignore_tables)) do |table|
+          TIMESTAMPS.each do |column, alternative_column|
+            unless column(table, column) || column(table, alternative_column)
+              problem!(table: table, column: column)
+            end
+          end
+        end
+      end
+    end
+  end
+end

test/active_record_doctor/detectors/table_without_timestamps_test.rb

@@ -0,0 +1,54 @@
+# frozen_string_literal: true
+
+class ActiveRecordDoctor::Detectors::TableWithoutTimestampsTest < Minitest::Test
+  def test_table_without_timestamps_reported
+    Context.create_table(:companies) do |t|
+      t.string :name
+    end
+
+    assert_problems(<<~OUTPUT)
+      add a created_at column to companies
+      add a updated_at column to companies
+    OUTPUT
+  end
+
+  def test_table_with_timestamps_is_not_reported
+    Context.create_table(:companies) do |t|
+      t.timestamps
+    end
+    refute_problems
+  end
+
+  def test_table_with_alternative_timestamps_is_not_reported
+    Context.create_table(:companies) do |t|
+      t.timestamp :created_on
+      t.timestamp :updated_on
+    end
+    refute_problems
+  end
+
+  def test_config_ignore_tables
+    Context.create_table(:companies)
+
+    config_file(<<-CONFIG)
+      ActiveRecordDoctor.configure do |config|
+        config.detector :table_without_timestamps,
+          ignore_tables: ["companies"]
+      end
+    CONFIG
+
+    refute_problems
+  end
+
+  def test_global_ignore_tables
+    Context.create_table(:companies)
+
+    config_file(<<-CONFIG)
+      ActiveRecordDoctor.configure do |config|
+        config.global :ignore_tables, ["companies"]
+      end
+    CONFIG
+
+    refute_problems
+  end
+end