Add cop for use of remove_column

remove_column should only be used in the up (or change) step of a migration if
it's a post-deployment migration. Otherwise there will be downtime due to the
ActiveRecord column cache, which we can avoid by using the IgnorableColumn
concern in combination with a post-deployment migration.

3 files changed, 37 insertions, 0 deletions

diff --git a/rubocop/cop/migration/remove_column.rb b/rubocop/cop/migration/remove_column.rb
new file mode 100644
index 00000000000..e53eb2e07b2
--- /dev/null
+++ b/rubocop/cop/migration/remove_column.rb
@@ -0,0 +1,30 @@
+require_relative '../../migration_helpers'
+
+module RuboCop
+  module Cop
+    module Migration
+      # Cop that checks if remove_column is used in a regular (not
+      # post-deployment) migration.
+      class RemoveColumn < RuboCop::Cop::Cop
+        include MigrationHelpers
+
+        MSG = '`remove_column` must only be used in post-deployment migrations'.freeze
+
+        def on_def(node)
+          def_method = node.children[0]
+
+          return unless in_migration?(node) && !in_post_deployment_migration?(node)
+          return unless def_method == :change || def_method == :up
+
+          node.each_descendant(:send) do |send_node|
+            send_method = send_node.children[1]
+
+            if send_method == :remove_column
+              add_offense(send_node, :selector)
+            end
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/rubocop/migration_helpers.rb b/rubocop/migration_helpers.rb
index c3473771178..c066d424437 100644
--- a/rubocop/migration_helpers.rb
+++ b/rubocop/migration_helpers.rb
@@ -7,5 +7,11 @@ module RuboCop
 
       dirname.end_with?('db/migrate', 'db/post_migrate')
     end
+
+    def in_post_deployment_migration?(node)
+      dirname = File.dirname(node.location.expression.source_buffer.name)
+
+      dirname.end_with?('db/post_migrate')
+    end
   end
 end
diff --git a/rubocop/rubocop.rb b/rubocop/rubocop.rb
index 7621ea50da9..eb52be3d731 100644
--- a/rubocop/rubocop.rb
+++ b/rubocop/rubocop.rb
@@ -14,6 +14,7 @@ require_relative 'cop/migration/add_index'
 require_relative 'cop/migration/add_timestamps'
 require_relative 'cop/migration/datetime'
 require_relative 'cop/migration/hash_index'
+require_relative 'cop/migration/remove_column'
 require_relative 'cop/migration/remove_concurrent_index'
 require_relative 'cop/migration/remove_index'
 require_relative 'cop/migration/reversible_add_column_with_default'