[day-02] validate user input

Also improve migration to reflect requirements in validation done
through `Ecto.Changeset`.

lib/wabanex/user.ex

@@ -1,8 +1,12 @@
 defmodule Wabanex.User do
   use Ecto.Schema
 
+  import Ecto.Changeset
+
   @primary_key {:id, :binary_id, autogenerate: true}
 
+  @fields ~w(name email password)a
+
   schema "users" do
     field :name, :string
     field :email, :string
@@ -10,4 +14,14 @@ defmodule Wabanex.User do
 
     timestamps()
   end
+
+  def changeset(params) do
+    %__MODULE__{}
+    |> cast(params, @fields)
+    |> validate_required(@fields)
+    |> validate_length(:name, min: 2)
+    |> validate_length(:password, min: 6)
+    |> validate_format(:email, ~r/@/)
+    |> unique_constraint([:email])
+  end
 end