Add detector

Author: robinaugh

README.md

@@ -17,6 +17,7 @@ can detect:
 * 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)
+* undefined model references - [`active_record_doctor:undefined_model_references](#detecting-undefined-model-references)
 
 It can also:
 
@@ -664,6 +665,27 @@ Supported configuration options:
 - `enabled` - set to `false` to disable the detector altogether
 - `ignore_tables` - tables whose timestamp columns existence should not be checked
 
+### Detecting Undefined Model References
+
+When a table is no longer used by the application, it should usually be dropped from the database.
+
+Running the command below will list all tables without default timestamp columns:
+
+```
+bundle exec rake active_record_doctor:undefined_model_references
+```
+
+The output of the command looks like this:
+
+```
+The users table is not referenced by a Rails model. If you are in the process of migrating it away, temporarily ignore it by adding it to the `ignore_tables` configuration and then remove it after the ruby code no longer uses it. Remember, do not delete the table until your deployed application code no longer uses it.
+```
+
+Supported configuration options:
+
+- `enabled` - set to `false` to disable the detector altogether
+- `ignore_tables` - tables that are not represented by models but cannot yet be dropped from the database (i.e. because the deployed version of the code may still be reading it)
+
 ## Ruby and Rails Compatibility Policy
 
 The goal of the policy is to ensure proper functioning in reasonable

lib/active_record_doctor.rb

@@ -22,6 +22,7 @@
 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/detectors/undefined_model_references"
 require "active_record_doctor/errors"
 require "active_record_doctor/help"
 require "active_record_doctor/runner"

lib/active_record_doctor/config/default.rb

@@ -83,4 +83,8 @@
     enabled: true,
     ignore_tables: [],
     ignore_columns: []
+
+  detector :undefined_model_references,
+    enabled: true,
+    ignore_tables: []
 end

lib/active_record_doctor/detectors/undefined_model_references.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+require 'active_record_doctor/detectors/base'
+
+module ActiveRecordDoctor
+  module Detectors
+    class UndefinedModelReferences < Base # :nodoc:
+      @description = 'detect tables not referenced by any model'
+      @config = {
+        ignore_tables: {
+          description: 'tables whose corresponding models should not be checked for existence',
+          global: true
+        }
+      }
+
+      private
+
+      def message(table:, **)
+        "The #{table} table is not referenced by a Rails model. If you are in the process of migrating it away, temporarily ignore it " \
+          'by adding it to the `ignore_tables` configuration and then remove it after the ruby code no longer uses it. ' \
+          'Remember, do not delete the table until your deployed application code no longer uses it.'
+      end
+
+      def detect
+        each_table(except: config(:ignore_tables)) do |table|
+          matching_model = ActiveRecord::Base.descendants.find do |model|
+            model.table_name == table
+          end
+
+          problem!(table:) if matching_model.nil?
+        end
+      end
+    end
+  end
+end

test/active_record_doctor/detectors/undefined_model_references_test.rb

@@ -0,0 +1,47 @@
+# frozen_string_literal: true
+
+class ActiveRecordDoctor::Detectors::UndefinedModelReferencesTest < Minitest::Test
+  def test_table_with_model
+    Context.create_table(:users) do
+    end.define_model do
+    end
+
+    refute_problems
+  end
+
+  def test_table_without_model
+    Context.create_table(:users) do
+    end
+
+    assert_problems(<<~OUTPUT)
+      The users table is not referenced by a Rails model. If you are in the process of migrating it away, temporarily ignore it by adding it to the `ignore_tables` configuration and then remove it after the ruby code no longer uses it. Remember, do not delete the table until your deployed application code no longer uses it.
+    OUTPUT
+  end
+
+  def test_config_ignore_tables
+    Context.create_table(:users) do
+    end
+
+    config_file(<<-CONFIG)
+      ActiveRecordDoctor.configure do |config|
+        config.detector :undefined_model_references,
+          ignore_tables: ["users"]
+      end
+    CONFIG
+
+    refute_problems
+  end
+
+  def test_global_ignore_tables
+    Context.create_table(:users) do
+    end
+
+    config_file(<<-CONFIG)
+      ActiveRecordDoctor.configure do |config|
+        config.global :ignore_tables, ["users"]
+      end
+    CONFIG
+
+    refute_problems
+  end
+end