[Fix rubocop#952] Fix a false positive for Rails/NotNullColumn

Fixes rubocop#952.

This PR fixes a false positive for `Rails/NotNullColumn`
when using `null: false` for MySQL's TEXT type.

changelog/fix_a_false_positive_for_rails_not_null_column.md

@@ -0,0 +1 @@
+* [#952](https://github.com/rubocop/rubocop-rails/issues/952): Fix a false positive for `Rails/NotNullColumn` when using `null: false` for MySQL's TEXT type. ([@koic][])

config/default.yml

@@ -694,6 +694,10 @@ Rails/NotNullColumn:
   Enabled: true
   VersionAdded: '0.43'
   VersionChanged: '2.20'
+  Database: null
+  SupportedDatabases:
+    - mysql
+    - postgresql
   Include:
     - db/**/*.rb
 

lib/rubocop/cop/rails/not_null_column.rb

@@ -3,8 +3,13 @@
 module RuboCop
   module Cop
     module Rails
-      # Checks for add_column call with NOT NULL constraint
-      # in migration file.
+      # Checks for add_column call with NOT NULL constraint in migration file.
+      #
+      # `TEXT` can have default values in PostgreSQL, but not in MySQL.
+      # It will automatically detect an adapter from `development` environment
+      # in `config/database.yml` or the environment variable `DATABASE_URL`
+      # when the `Database` option is not set. If the database is MySQL,
+      # this cop ignores offenses for the `TEXT`.
       #
       # @example
       #   # bad
@@ -17,6 +22,8 @@ module Rails
       #   add_reference :products, :category
       #   add_reference :products, :category, null: false, default: 1
       class NotNullColumn < Base
+        include DatabaseTypeResolvable
+
         MSG = 'Do not add a NOT NULL column without a default value.'
         RESTRICT_ON_SEND = %i[add_column add_reference].freeze
 
@@ -45,7 +52,10 @@ def on_send(node)
 
         def check_add_column(node)
           add_not_null_column?(node) do |type, pairs|
-            return if type.respond_to?(:value) && (type.value == :virtual || type.value == 'virtual')
+            if type.respond_to?(:value)
+              return if type.value == :virtual || type.value == 'virtual'
+              return if type.value == :text && database == MYSQL
+            end
 
             check_pairs(pairs)
           end

spec/rubocop/cop/rails/not_null_column_spec.rb

@@ -113,4 +113,33 @@ def change
       end
     end
   end
+
+  context 'when database is MySQL' do
+    let(:cop_config) do
+      { 'Database' => 'mysql' }
+    end
+
+    it 'does not register an offense when using `null: false` for `:text` type' do
+      expect_no_offenses(<<~RUBY)
+        def change
+          add_column :articles, :content, :text, null: false
+        end
+      RUBY
+    end
+  end
+
+  context 'when database is PostgreSQL' do
+    let(:cop_config) do
+      { 'Database' => 'postgresql' }
+    end
+
+    it 'registers an offense when using `null: false` for `:text` type' do
+      expect_offense(<<~RUBY)
+        def change
+          add_column :articles, :content, :text, null: false
+                                                 ^^^^^^^^^^^ Do not add a NOT NULL column without a default value.
+        end
+      RUBY
+    end
+  end
 end