Add a reference column to a table

To add a reference to a team to the users table, run this command:

$ rails generate migration AddTeamRefToUsers team:references

This generates the following migration:

class AddTeamRefToUsers < ActiveRecord::Migration[5.0]
  def change
    add_reference :users, :team, foreign_key: true
  end
end

That migration will create a team_id column in the users table.

If you want to add an appropriate index and foreign_key on the added column, change the command to rails generate migration AddTeamRefToUsers team:references:index. This will generate the following migration:

class AddTeamRefToUsers < ActiveRecord::Migration
  def change
    add_reference :users, :team, index: true
    add_foreign_key :users, :teams
  end
end

If you want to name your reference column other than what Rails auto generates, add the following to your migration: (E.g.: You might want to call the User who created the Post as Author in the Post table)

class AddAuthorRefToPosts < ActiveRecord::Migration
  def change
    add_reference :posts, :author, references: :users, index: true
  end
end