1 files changed, 50 insertions, 0 deletions
diff --git a/rubocop/cop/migration/add_column.rb b/rubocop/cop/migration/add_column.rb
new file mode 100644
index 00000000000..d5c8eeeea1f
--- /dev/null
+++ b/rubocop/cop/migration/add_column.rb
@@ -0,0 +1,50 @@
+module RuboCop
+  module Cop
+    module Migration
+      # Cop that checks if columns are added in a way that doesn't require
+      # downtime.
+      class AddColumn < RuboCop::Cop::Cop
+        include MigrationHelpers
+
+        WHITELISTED_TABLES = [:application_settings]
+
+        MSG = '`add_column` with a default value requires downtime, ' \
+          'use `add_column_with_default` instead'
+
+        def on_send(node)
+          return unless in_migration?(node)
+
+          name = node.children[1]
+
+          return unless name == :add_column
+
+          # Ignore whitelisted tables.
+          return if table_whitelisted?(node.children[2])
+
+          opts = node.children.last
+
+          return unless opts && opts.type == :hash
+
+          opts.each_node(:pair) do |pair|
+            if hash_key_type(pair) == :sym && hash_key_name(pair) == :default
+              add_offense(node, :selector)
+            end
+          end
+        end
+
+        def table_whitelisted?(symbol)
+          symbol && symbol.type == :sym &&
+            WHITELISTED_TABLES.include?(symbol.children[0])
+        end
+
+        def hash_key_type(pair)
+          pair.children[0].type
+        end
+
+        def hash_key_name(pair)
+          pair.children[0].children[0]
+        end
+      end
+    end
+  end
+end