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]

3 files changed, 48 insertions, 0 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 57ec37c75b..2088b018d7 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,8 @@
+*   Add `#only_up` to database migrations for code that is only relevant when
+    migrating up, e.g. populating a new column.
+
+    *Rich Daley*
+
 *   Require raw SQL fragments to be explicitly marked when used in
     relation query methods.
 
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)
diff --git a/activerecord/test/cases/invertible_migration_test.rb b/activerecord/test/cases/invertible_migration_test.rb
index 60c628511f..e0f40710eb 100644
--- a/activerecord/test/cases/invertible_migration_test.rb
+++ b/activerecord/test/cases/invertible_migration_test.rb
@@ -161,6 +161,13 @@ module ActiveRecord
       end
     end
 
+    class UpOnlyMigration < SilentMigration
+      def change
+        add_column :horses, :oldie, :boolean, default: false
+        up_only { execute "update horses set oldie = 'true'" }
+      end
+    end
+
     setup do
       @verbose_was, ActiveRecord::Migration.verbose = ActiveRecord::Migration.verbose, false
     end
@@ -378,5 +385,23 @@ module ActiveRecord
               "horses_index_named index should not exist"
       end
     end
+
+    def test_up_only
+      InvertibleMigration.new.migrate(:up)
+      horse1 = Horse.create
+      # populates existing horses with oldie=true but new ones have default false
+      UpOnlyMigration.new.migrate(:up)
+      Horse.reset_column_information
+      horse1.reload
+      horse2 = Horse.create
+
+      assert horse1.oldie? # created before migration
+      assert !horse2.oldie? # created after migration
+
+      UpOnlyMigration.new.migrate(:down) # should be no error
+      connection = ActiveRecord::Base.connection
+      assert !connection.column_exists?(:horses, :oldie)
+      Horse.reset_column_information
+    end
   end
 end