Common Rails Database Q&A

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Q&A on using ActiveRecord and Databases in Rails, based on popular StackOverflow questions. See Models and Data for a beginner's intro to Rails Models.

How can you duplicate a record?

Let's say you have a nice product record and you want to create a variant of it that will have some slight differences. You can copy it with .dup:

prod1 = Product.first
 => #<Product id: 1, name: "Cow", description: "Moos, eats grass.", price: 10, created_at: "2015-06-29 19:25:38", updated_at: "2015-06-29 19:25:38", category_id: 1, quantity: 5, image_url: nil> 
prod2 = prod1.dup
=> #<Product id: nil, name: "Cow", description: "Moos, eats grass.", price: 10, created_at: nil, updated_at: nil, category_id: 1, quantity: 5, image_url: nil> = "Zebra"
 => "Zebra"
=> true
=> #<Product id: 7, name: "Zebra", description: "Moos, eats grass.", price: 10, created_at: "2015-06-30 21:36:24", updated_at: "2015-06-30 21:36:24", category_id: 1, quantity: 5, image_url: nil> 

.dup copies the columns from the original, except for the id and timestamps (created_at and updated_at). It doesn't copy children elements, so prod1's likes will not be copied to prod2.

How can you change a column type?

Let's say your products table has an integer price column and you want to change it to a decimal column.

  • Generate a migration file, e.g. rails generate migration ChangePriceToDecimal
  • Use the following code to change the column type: change_column :products, :price, :decimal
  • Officially, your migrations should be reversible. To do this, replace the def change method in the migration file with def up and def down methods:


class ChangePriceToDecimal < ActiveRecord::Migration
  def self.up
    change_column :products, :price, :decimal
  def self.down
    change_column :products, :price, :integer

(See the Rails Migrations Guide and StackOverflow Answer for more info.)

What data types can you use in Rails columns?

Rails supports many different types of columns, as mentioned in the docs. The details of how each one is stored will depends on the database you're using, but you can see examples of each type below. (See also StackOverflow).

When you retrieve an item's data from a database, Rails will automatically set the correct class for it. For example, say you have a products table with string column name:

prod = Product.first
name =
name.class #String


type examples Ruby Class note
:integer 4368, -11 Fixnum store positive or negative integers
:float 2.000001 Float store decimal places
:decimal 3.01, 150.375 BigDecimal stores decimal places more precisely than float (e.g. for currency)


Both these types use the Ruby class String.

  • :text - general text column
  • :string - text that you know will never be longer than 256 characters. (See StackOverflow.)

Date & Time

Usually you can just use DateTime.

type example Ruby Class
:time 2015-06-30 21:15:16 Time
:date 2015-07-01 Date
:datetime 2015-06-30 21:15:16 DateTime
timestamp Fri, 03 Jul 2015 00:00:00 UTC +00:00 ActiveSupport::TimeWithZone


  • :binary - for storing binary data such as images or audio
    (note that such data should often be stored outside the database)
  • :boolean - true or false
  • :primary_key - unique key to identify each record in table. By default every table has an ID column that is used as the primary_key.

(You can also create a references column, though that isn't really its own datatype.)

Your database may also support additional datatypes.

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