Read database config from environment variable

changelog/change_read_database_config_from_environment.md

@@ -0,0 +1 @@
+* [#903](https://github.com/rubocop/rubocop-rails/pull/903): Read database config for `Rails/BulkChangeTable` from environment variable. ([@joergschiller][])

lib/rubocop/cop/rails/bulk_change_table.rb

@@ -12,7 +12,8 @@ module Rails
       # The `bulk` option is only supported on the MySQL and
       # the PostgreSQL (5.2 later) adapter; thus it will
       # automatically detect an adapter from `development` environment
-      # in `config/database.yml` when the `Database` option is not set.
+      # in `config/database.yml` or the environment variable `DATABASE_URL`
+      # when the `Database` option is not set.
       # If the adapter is not `mysql2` or `postgresql`,
       # this Cop ignores offenses.
       #
@@ -175,7 +176,7 @@ def include_bulk_options?(node)
         end
 
         def database
-          cop_config['Database'] || database_from_yaml
+          cop_config['Database'] || database_from_yaml || database_from_env
         end
 
         def database_from_yaml
@@ -211,6 +212,18 @@ def database_yaml
           nil
         end
 
+        def database_from_env
+          url = ENV['DATABASE_URL'].presence
+          return nil unless url
+
+          case url
+          when %r{\Amysql2://}
+            MYSQL
+          when %r{\Apostgres(ql)?://}
+            POSTGRESQL
+          end
+        end
+
         def support_bulk_alter?
           case database
           when MYSQL

spec/rubocop/cop/rails/bulk_change_table_spec.rb

@@ -529,4 +529,46 @@ def change
       it_behaves_like 'no offense'
     end
   end
+
+  context 'when `DATABASE_URL` is set' do
+    before do
+      allow(ENV).to receive(:[]).with('DATABASE_URL').and_return(database_url)
+    end
+
+    context 'mysql2' do
+      let(:database_url) { 'mysql2://localhost/my_database' }
+
+      it_behaves_like 'offense for mysql'
+    end
+
+    context 'postgres' do
+      let(:database_url) { 'postgres://localhost/my_database' }
+
+      context 'with Rails 5.2', :rails52 do
+        it_behaves_like 'offense for postgresql'
+      end
+
+      context 'with Rails 5.1', :rails51 do
+        it_behaves_like 'no offense for postgresql'
+      end
+    end
+
+    context 'postgresql' do
+      let(:database_url) { 'postgresql://localhost/my_database' }
+
+      context 'with Rails 5.2', :rails52 do
+        it_behaves_like 'offense for postgresql'
+      end
+
+      context 'with Rails 5.1', :rails51 do
+        it_behaves_like 'no offense for postgresql'
+      end
+    end
+
+    context 'unsupported (e.g. sqlserver)' do
+      let(:database_url) { 'sqlserver://localhost/my_database' }
+
+      it_behaves_like 'no offense'
+    end
+  end
 end