This the existing table in migration folder.

class CreateArticles < ActiveRecord::Migration
  def change
    create_table :articles do |t|
      t.string :title
      t.text :text
      t.timestamps null: false
    end
  end
end

Now I have to add column or a new field "Picture" to the table Articles. so, i have to create migration by using this command in the terminal.

$ rails generate migration AddPictureToArticle picture:binary

Here, we are generating migration "Add" creates "change" method in Migration, and "ToArticle" indicates the table "articles."

So, I got a migration file, db/migrate/20160112054236_add_picture_to_article.rb whose contents is:

class AddPictureToArticle < ActiveRecord::Migration
  def change
    add_column :articles, :picture, :binary
  end
end

Now all we have to do is:

$ rake db:migrate