Rails 3 migrations: Adding reference column?

Question

If I create a new rails 3 migration with (for example)

rails g migration tester title:tester user:references

, everything works fine...however if I add a column with something along the lines of:

rails g migration add_user_to_tester user:references

the reference field is not recognised. In short, the question is: how do I add a referencing column to a rails migration from the command line?

Epileptic

Answer 1

If you are using the Rails 4.x you can now generate migrations with references, like this:

rails generate migration AddUserRefToProducts user:references

like you can see on rails guides

Submultiple

See section 2.1 from edgeguides.rubyonrails.org/active_record_migrations.html for example. – Respire

how do you specify a column name for the foreign key instead the auto generated name? – Dollhouse

@jwill you can use polymorphic: user:references{polymorphic}. – Submultiple

@PauloFidalgo Can you explain a bit about how to do that? may be some guide of links? (talking about polymorphic) – Experiment

@Anwar: here are the docs api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/… – Submultiple

Answer 2

EDIT: This is an outdated answer and should not be applied for Rails 4.x+

You don't need to add references when you can use an integer id to your referenced class.

I'd say the advantage of using references instead of a plain integer is that the model will be predefined with belongs_to and since the model is already created and will not be affected when you migrate something existing, the purpose is kind of lost.

So I would do like this instead:

rails g migration add_user_id_to_tester user_id:integer

And then manually add belongs_to :user in the Tester model

Cismontane

But that won't create the appropriate foreign key constraints on databases that support it, right? – Extravasate

No, afaik Rails never create foreign key restraints on the database unless you add plugins to do it for you. – Cismontane

just studying this post, pls how do i add the reference after all – Beastings

The way I showed does add a reference. But instead of it being a one line generator, it also requires you to add the reference code into your existing model, which is also included in the answer – Cismontane

remember to add the index with user:integer:index – Highbinder

This answer is really good, but I'm not sure I understand it fully due to the confusing grammar (multiple 'and' and 'since'. not sure where sentences end and start.) "the model will be predefined with belongs_to and since the model is already created and will not be affected when you migrate something existing, the purpose is kind of lost." – Initiation

What should i do if the user model is like, Subscription::User so how will i write the migration for this? – Expressway

Answer is dated, see @Paulo's answer for modern rails. – Ulises

@Extravasate with Rails > 4.2 there is now an add_foreign_key method which facilitates adding a fk constraint at the db level – Gasoline

Answer 3

Please note that you will most likely need an index on that column too.

class AddUserReferenceToTester < ActiveRecord::Migration
  def change
    add_column :testers, :user_id, :integer
    add_index  :testers, :user_id
  end
end

Sitting

Why? Is this true for most belongs_to relationships? – Initiation

It is indeed for performance reasons and comes in handy if you have a has_many/has_one on the other side of that belongs_to relation. If you are absolutely sure that you will not go through user.testers you can omit the index. – Undersized

The rails g migration ... generated add_reference :installs, :device, index: true which also creates the index. – Respire

Answer 4

With the two previous steps stated above, you're still missing the foreign key constraint. This should work:

  class AddUserReferenceToTester < ActiveRecord::Migration
      def change
          add_column :testers, :user_id, :integer, references: :users
      end
  end

Optimize

This is the only actual answer here. The foreign key is the most critical part here – Halla

this should be marked as the correct answer since the questions asks for rails 3 – Supernatant

Answer 5

You can use references in a change migration. This is valid Rails 3.2.13 code:

class AddUserToTester < ActiveRecord::Migration
  def change
    change_table :testers do |t|
      t.references :user, index: true 
    end
  end
  def down
    change_table :testers do |t|
      t.remove :user_id
    end
  end
end

c.f.: http://apidock.com/rails/ActiveRecord/ConnectionAdapters/SchemaStatements/change_table

Chiliasm

change and down methods? aren't up and down methods instead? – Potash

@Potash yes, and you can also just leave off the down method. – Raster

@Potash Without the down method, I got ActiveRecord::IrreversibleMigration when rolling back using Rails 3.2. I also had to change change to up. – Cooley

Answer 6

Running rails g migration AddUserRefToSponsors user:references will generate the following migration:

def change
  add_reference :sponsors, :user, index: true
end

Bimolecular

Which version of Rails is this for? – Cooley

Answer 7

When adding a column you need to make that column an integer and if possible stick with rails conventions. So for your case I am assuming you already have a Tester and User models, and testers and users tables.

To add the foreign key you need to create an integer column with the name user_id (convention):

add_column :tester, :user_id, :integer

Then add a belongs_to to the tester model:

class Tester < ActiveRecord::Base
  belongs_to :user
end

And you might also want to add an index for the foreign key (this is something the references already does for you):

add_index :tester, :user_id

Stereochrome

Answer 8

That will do the trick:

rails g migration add_user_to_tester user_id:integer:index

Devonne

I like that this also adds the index that you will most likely want. – Bael

Answer 9

You can add references to your model through command line in the following manner:

rails g migration add_column_to_tester user_id:integer

This will generate a migration file like :

class AddColumnToTesters < ActiveRecord::Migration
  def change
    add_column :testers, :user_id, :integer
  end
end

This works fine every time i use it..

Omnirange

Answer 10

For Rails 4

The generator accepts column type as references (also available as belongs_to).

This migration will create a user_id column and appropriate index:

$ rails g migration AddUserRefToProducts user:references 

generates:

class AddUserRefToProducts < ActiveRecord::Migration
  def change
    add_reference :products, :user, index: true
  end
end

http://guides.rubyonrails.org/active_record_migrations.html#creating-a-standalone-migration

For Rails 3

Helper is called references (also available as belongs_to).

This migration will create a category_id column of the appropriate type. Note that you pass the model name, not the column name. Active Record adds the _id for you.

change_table :products do |t|
  t.references :category
end

If you have polymorphic belongs_to associations then references will add both of the columns required:

change_table :products do |t|
  t.references :attachment, :polymorphic => {:default => 'Photo'}
end

Will add an attachment_id column and a string attachment_type column with a default value of Photo.

http://guides.rubyonrails.org/v3.2.21/migrations.html#creating-a-standalone-migration

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