At the moment, multiple people can register with the same username, something that would inevitably cause a lot of problems in a real-world situation. Ecto does not allow us to verify the username's uniqueness without there being a database-level constraint check, so let's start by creating a migration to make usernames unique in the database. First, let's generate the migration:
$ mix ecto.gen.migration make_usernames_unique* creating priv/repo/migrations* creating priv/repo/migrations/20180201223131_make_usernames_unique.exs
Then, we'll give the migration the following code. Note that all we're doing here is telling Ecto to add a unique index to the users table on the username column:
defmodule Vocial.Repo.Migrations.MakeUsernamesUnique ...
