How to add a column with Rails

How to add a database column with Ruby-on-Rails ? Here is the short, and the long answer.


Let’s say you have a table named “books”, and an associated model named “Book”.

You want to add the column “author”.

Tools used in this tutorial : Rails 6.1.3, Ruby 3

Note that this article should work pretty well with any version of Rails or Ruby.

Short answer

For those who already know Rails, here is the short answer :

inside db/migrate/20210131201926_add_author_to_books.rb

class AddAuthorToBooks < ActiveRecord::Migration[6.1]
  def change
    add_column :books, :author, :string

And then run bin/rails db:migrate at the root of your project.

Full tutorial

Install new minimal app

For this tutorial we don’t need a full Rails app with bells and whistles, the bare minimum will suffice. From Rails 6.1, the –minimal flag is available when you create a new Rails app.

$> rails _6.1.3_ new myapp --minimal
$> cd myapp

By default, Rails will use an in-memory database named SQLite3.

Side note If you want to start with PostgreSQL, you can enter the following command :

$> rails _6.1.3_ new myapp --minimal --database=postgresql
$> cd myapp

Create the database

$/myapp> bin/rails db:create
Created database 'myapp_development'
Created database 'myapp_test'

Create a first model

$/myapp> bin/rails generate model Book title:string body:text

Note that “bin/rails” will use the installed rails binary for the current application, instead of the globally available one.

This will create many files :

      invoke  active_record
      create    db/migrate/20210331122059_create_books.rb
      create    app/models/book.rb
      invoke    test_unit
      create      test/models/book_test.rb
      create      test/fixtures/books.yml

You may not need everything. The interesting thing is the migration file created under db/migrate/20210131201925_create_books.rb (The timestamp represent the current time when file was created, of course you will have another than this example)

class CreateBooks < ActiveRecord::Migration[6.1]
  def change
    create_table :books do |t|
      t.string :title
      t.text :body


Then run the migration

$/myapp> bin/rails db:migrate
== 20210131201925 CreateBooks: migrating ======================================
-- create_table(:books)
   -> 0.0019s
== 20210131201925 CreateBooks: migrated (0.0020s) =============================

Add a migration file

Create a file named db/migrate/20210131201926_add_column_to_books.rb

Check the timestamp, it has to be a greater number than the one of 20210131201925_create_books.rb previously created. Or the migrations won’t be able to be run in the correct order.

class AddColumnToBooks < ActiveRecord::Migration[6.1]
  def change
    add_column :books, :author, :string

Migration file analyzed

We’ve already seen the timestamp. Now the name of the migration : it actually could be anything. But try to make the intent clear, for yourself or other developers : here I put add_column_to_books, a better name could have been add_author_to_books. Anyway, make sure that the filename (apart from the timestamp) is the same as the class name.

Now we inherit from ActiveRecord::Migration, for pretty obvious reasons : we want to migrate the data, which is a pretty common behavior, common enough to be hidden inside a standard Rails object, ActiveRecord::Migration.

The number indicates the 2 first numbers of the Rails version. Here I use Rails 6.1.3, thus the two numbers are [6.1]. It’s because features could be available in ActiveRecord::Migration[6.1] that were not available in ActiveRecord::Migration[6.0].

Then the “change” method. You cannot (ahem) change the name, because this method is automagically called when you run bin/rails db:migrate.

Finally, add_column is pretty clear : first arg is the name of the table, then the name of the column, and then the kind of column.

Migrate the data

bin/rails db:migrate Will actually add the column to your current database.

$/myapp> bin/rails db:migrate
== 20210131201926 AddColumnToBooks: migrating =================================
-- add_column(:books, :author, :string)
   -> 0.0021s
== 20210131201926 AddColumnToBooks: migrated (0.0022s) ========================

Check the schema.rb file

You can see that the column has been added inside your schema.rb file here :

ActiveRecord::Schema.define(version: 20210131201926) do

  create_table "books", force: :cascade do |t|
    t.string "title"
    t.text "body"
    t.datetime "created_at", precision: 6, null: false
    t.datetime "updated_at", precision: 6, null: false
    t.string "author"


The added column appear here : t.string "author"

The migration “appears” in another place : the timestamp. Look at ActiveRecord::Schema.define(version: 20210131201926) . The timestamp is the one of your migration files.

Bonus : Inspect created tables – if PostgreSQL is used

If you have used the default SQLite3, please read the next paragraph. If you have chosen PostgreSQL, this is the right paragraph 🙂

With Rails, you can inspect the actual database by entering the command :

$/myapp> bin/rails db
psql (13.1)
Type "help" for help.

Great ! You just entered the PostgreSQL command-line interface (CLI).

myapp_development=# \dt
List of relations
Schema | Name | Type  | Owner
public | ar_internal_metadata | table | shino
public | books                | table | shino
public | schema_migrations    | table | shino
(3 rows)

Great ! As you may notice, Rails creates 2 tables by default : ar_internal_metadata, and schema_migrations.

We are only concerned by the “books” table.

Let’s display it :

Inside Books Table
Inside Books Table

Bonus : Inspect created tables – if SQLite3 is used

This is the same paragraph as above, but the command will apply to a SQLite3 database only (which is the default one with Rails)

$/myapp> bin/rails db
SQLite version 3.28.0 2019-04-15 14:49:49
Enter ".help" for usage hints.

Then, to display tables

sqlite> .headers on
sqlite> .mode columns
sqlite> .table
ar_internal_metadata  books schema_migrations 

Then, to display the internals of the “Books” table :

sqlite> PRAGMA table_info('books');
cid         name        type        notnull     dflt_value  pk        
----------  ----------  ----------  ----------  ----------  ----------
0           id          integer     1                       1         
1           title       varchar     0                       0         
2           body        text        0                       0         
3           created_at  datetime(6  1                       0         
4           updated_at  datetime(6  1                       0         
5           author      varchar     0                       0         

The magic way : generate migration

Another possibility is to use the scaffolding, but in this case you have to pay extra attention to the naming of your file :

$/myapp> bin/rails generate migration AddAuthorToBooks author:string


$/myapp> bin/rails generate migration add_author_to_books author:string

And the migration file will be filled automagically with all correct values, timestamp included.

In everyday work, I don’t use this magic. I simply copy/paste other migrations, paying attention to the name of the file, and that’s it.

Options for add_column

You can see a full list of all available options in this article :


Adding a column with Rails is easy, I would say the important thing is to know what’s going on, and why.

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