Adding new column(reference) to existing table

Phoenix Forum Questions / Help
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1

I generated a migration using mix phx.gen.schema (Dogs)

I then created a new migration (Puppies). I would like to create a new migration and alter Puppies migration by adding a belongs_to relationship to Dogs.

I tried mix ecto.gen.migration puppies_add_dog_id_column. This creates a migration but how do I add reference to an existing table while migration? Or is this command even the right way to do it? Any suggestion is appreciated.

2

The command is fine, you have to write the actual migration yourself then, i.e. to add a foreign key referencing the dogs table you could do:

defmodule YourApp.Repo.Migrations.PuppiesAddDogIdColumn do
  use Ecto.Migration

  def change do
    alter table("puppies") do
      add :dog_id, references(:dogs)
    end

    # if you want the index -and why not wanting it? ;) 
    # pls note this is after the `alter` macro, and inside the `change` callback def
    create index(:puppies, [:dog_id])
  end
end

hth

Edit: you’ll find extensive informations on Ecto migrations in the offical docs:
Ecto.Migration — Ecto SQL v3.5.4

3 Likes
3

Great, thank you! That works!!