Using custom association names in Rails can be a great way of increasing understanding throughout an application. For example, it can allow for using a column called owner_id that references a User record. Let's dig into getting this setup for a migration.

This is the second post in a series

Previously, I’ve written about using add_reference and add_foreign_key in the above post. These are great and definitely accomplish the end goal, but recently I discovered an easier approach.

As a refresher, here’s the previous post’s example dataset:

Along with the corresponding migration:

# What the migration might look like
class AddCreatorToJournal < ActiveRecord::Migration
  def change

    # Notice how the index is for :creator but references users
    add_reference :journals, :creator, references: :users, index: true

    # Just like the belongs_to contained class_name: :User, the foreign key
    # also needs a specific custom column name as :creator_id
    add_foreign_key :journals, :users, column: :creator_id
  end
end

So to get started, let’s run the following Rails generator command:

# You can still use the :references syntax here
rails g model Journal creator:references

# The resulting database migration from the above command
class CreateJournals < ActiveRecord::Migration[5.0]
  def change
    create_table :journals do |t|
      t.references :creator, foreign_key: true

      t.timestamps
    end
  end
end

Making a column reference a different table

So how do we hook up the above :creator column to the User model without using add_reference? It’s actually pretty straight forward. First we need to let the reference know that it actually points at a table with a different name than the column. This is done via the references: :table_name syntax.

class CreateJournals < ActiveRecord::Migration[5.0]
  def change
    create_table :journals do |t|
      t.references :creator, references: :users, foreign_key: true

      t.timestamps
    end
  end
end

The above should look really familiar as in the last post we used: add_reference :journals, :creator, references: :users. Above we just added references: :users to the existing t.references line.

Hooking up a foreign key to a different table

Now that the table references the other one properly, we need to hook up its foreign key constraint. There’s a lesser known syntax for specifying the table that a foreign key points to called foreign_key: { to_table: :association }. You can find more documentation on in within the add_reference section of the rails api. This is exactly what we need here in order to make the migration work.

So let’s update the previous migration with the new to_table syntax.

class CreateJournals < ActiveRecord::Migration[5.0]
  def change
    create_table :journals do |t|
      t.references :creator, references: :users, foreign_key: { to_table: :users}

      t.timestamps
    end
  end
end

The result from running the above migration is a great looking schema that properly creates a new creator_id column, references the users table, and constrains it with a foreign key. See ya later add_reference and add_foreign_key

   Column   |            Type             | Collation | Nullable |               Default                
------------+-----------------------------+-----------+----------+--------------------------------------
 id         | integer                     |           | not null | nextval('journals_id_seq'::regclass)
 creator_id | integer                     |           |          | 
 created_at | timestamp without time zone |           | not null | 
 updated_at | timestamp without time zone |           | not null | 
Indexes:
    "journals_pkey" PRIMARY KEY, btree (id)
    "index_journals_on_creator_id" btree (creator_id)
Foreign-key constraints:
    "fk_rails_40d4de14e6" FOREIGN KEY (creator_id) REFERENCES users(id)

What’s the most helpful SQL migration trick that you know? Tell me about it by leaving a comment below. Thanks for reading.

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