How to Set a Unique Constraint to a Table Field

This article requires that you know: How to Generate a Migration File

If you already have a table and retroactively want to set a field to only allow unique values, you need to create a migration with:

create unique_index(:table_name, [:field_name])

This is a full example where a table named :groups has a field name :name set to a unique constraint.

defmodule App.Repo.Migrations.EnsureGroupNameIsUnique do
  use Ecto.Migration

  def change do
     create unique_index(:groups, [:name])
  end


end