references使ったらエラーが起こるんだけどどうすればいいの？

class CreateRelationCategories < ActiveRecord::Migration
  def self.up
    create_table :relation_categories do |t|
      t.references :post
      t.references :category
      t.integer :main

      t.timestamps
    end
    add_index :relation_categories, :post
    add_index :relation_categories, :category
  end

  def self.down
    drop_table :relation_categories
  end
end