Add a #populate method to migrations (#31082)

* Add a #populate method to migrations

* Address rubocop issues

* Rename to #up_only and use #execute in the examples intead of the model

* Update CHANGELOG

[Rich Daley & Rafael Mendonça França]

1 files changed, 18 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/migration.rb b/activerecord/lib/active_record/migration.rb
index c13efa9d70..67c8c9fc08 100644
--- a/activerecord/lib/active_record/migration.rb
+++ b/activerecord/lib/active_record/migration.rb
@@ -734,6 +734,24 @@ module ActiveRecord
       execute_block { yield helper }
     end
 
+    # Used to specify an operation that is only run when migrating up
+    # (for example, populating a new column with its initial values).
+    #
+    # In the following example, the new column `published` will be given
+    # the value `true` for all existing records.
+    #
+    #    class AddPublishedToPosts < ActiveRecord::Migration[5.3]
+    #      def change
+    #        add_column :posts, :published, :boolean, default: false
+    #        up_only do
+    #          execute "update posts set published = 'true'"
+    #        end
+    #      end
+    #    end
+    def up_only
+      execute_block { yield } unless reverting?
+    end
+
     # Runs the given migration classes.
     # Last argument can specify options:
     # - :direction (default is :up)