You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: 015_create_review_mappings.rb
    path of file: ./repos/expertiza/db/migrate_save
    
    the code of the file until where you have to start completion: class CreateReviewMappings < ActiveRecord::Migration[4.2]
  # This table should have essentially the same format as review_of_review_mappings
  def self.up
    create_table :review_mappings do |t|
      t.column :author_id, :integer # if an individual is being reviewed, this fiel
    
  def self.up
    create_table :review_mappings do |t|
      t.column :author_id, :integer # if an individual is being reviewed, this field is non-null, otherwise is null
      t.column :team_id, :integer   # if a team is being reviewed, this field is non-null, otherwise is null
      t.column :reviewer_id, :integer
      t.column :assignment_id, :integer # assignment that is being reviewed
    end
    execute "alter table review_mappings
          add constraint fk_review_mapping_assignments
          foreign key (assignment_id) references assignments(id)"
  end