1 files changed, 46 insertions, 0 deletions
diff --git a/rubocop/cop/migration/add_index.rb b/rubocop/cop/migration/add_index.rb
new file mode 100644
index 00000000000..d9247a1f7ea
--- /dev/null
+++ b/rubocop/cop/migration/add_index.rb
@@ -0,0 +1,46 @@
+module RuboCop
+  module Cop
+    module Migration
+      # Cop that checks if indexes are added in a concurrent manner.
+      class AddIndex < RuboCop::Cop::Cop
+        include MigrationHelpers
+
+        MSG = 'add_index requires downtime, use add_concurrent_index instead'
+
+        def on_def(node)
+          return unless in_migration?(node)
+
+          new_tables = []
+
+          node.each_descendant(:send) do |send_node|
+            first_arg = first_argument(send_node)
+
+            # The first argument of "create_table" / "add_index" is the table
+            # name.
+            new_tables << first_arg if create_table?(send_node)
+
+            next if method_name(send_node) != :add_index
+
+            # Using "add_index" is fine for newly created tables as there's no
+            # data in these tables yet.
+            next if new_tables.include?(first_arg)
+
+            add_offense(send_node, :selector)
+          end
+        end
+
+        def create_table?(node)
+          method_name(node) == :create_table
+        end
+
+        def method_name(node)
+          node.children[1]
+        end
+
+        def first_argument(node)
+          node.children[2] ? node.children[0] : nil
+        end
+      end
+    end
+  end
+end